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TodoFilter to that string. As we can see in the line app.

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Finally, we create an instance of our router and call Backbone. The latest version of the full app can be viewed online at any time and the sources are readily available via TodoMVC. While our first application gave us a good taste of how Backbone. In this exercise, we will build a library application for managing digital books using Backbone. For each book, we will store the title, author, date of release, and some keywords.

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First, we need to create a folder structure for our project. To keep the front-end and back-end separate, we will create a folder called site for our client in the project root. Within it, we will create css, img, and js directories. As with the last example, we will split our JavaScript files by their function, so under the js directory create folders named lib, models, collections, and views. Your directory hierarchy should look like this:.

We need a placeholder image for the book covers. We should also add in the HTML for the user interface. To see what this will look like with some data in it, go ahead and add a manually filled-in book to the books div. Not so great. This is not a CSS tutorial, but we still need to do some formatting.

Create a file named screen. So this is what we want the final result to look like, but with more books. Go ahead and copy the bookContainer div a few more times if you would like to see what it looks like. Now, we are ready to start developing the actual application. These are both very simple, with the model only declaring some defaults:. Note that in the initialize function, we accept an array of data that we pass to the app.

Library constructor. Finally, we have the entry point for our code, along with the sample data:. Our app just passes the sample data to a new instance of app. LibraryView that it creates. When the user clicks the add button, we want to take the data in the form and use it to create a new model.

In the LibraryView, we need to add an event handler for the click event:. Since we used the same names for ids in our form as the keys on our Book model, we can simply store them directly in the formData object. We then create a new Book from the data and add it to the collection. We skip fields without a value so that the defaults will be applied. Backbone passes an event object as a parameter to the event-handling function. Adding a call to preventDefault on the event in the addBook function takes care of this for us.

Now, we need to make a small detour and set up a server with a REST api. Since this is a JavaScript book, we will use JavaScript to create the server using node. Download and install node. The node package manager npm will be installed as well. Download, install, and run MongoDB from mongodb. There are detailed installation guides on the website. Amongst other things, this file tells npm what the dependencies are for our project.

On the command line, from the root of your project, type:. You should see npm fetch the dependencies that we listed in our package. We start off by loading the modules required for this project: Express for creating the HTTP server, Path for dealing with file paths, and mongoose for connecting with the database. We then create an Express server and configure it using an anonymous function. Finally, we start the server by running the listen function. The port number used, in this case , could be any free port on your system. I simply used since it is unlikely to have been used by anything else.

We are now ready to run our first server:. This is where we left off in Part 2, but we are now running on a server instead of directly from the files. Great job! We can now start defining routes URLs that the server should react to. Let us go back to server. The get function takes a URL as the first parameter and a function as the second. The function will be called with request and response objects.

Now, you can restart node and go to our specified URL:. Now, since we want to store our data in MongoDB, we need to define a schema. Add this to server. As you can see, schema definitions are quite straight forward. They can be more advanced, but this will do for us. I also extracted a model BookModel from Mongo.

This is what we will be working with. The find function of Model is defined like this: The callback will be called with an error object and an array of found objects. If there was no error, we return the array of objects to the client using the send function of the response object. Otherwise, we log the error to the console. Restart node and go to localhost: Open up the JavaScript console.

Most other browsers will have a similar console. In the console type the following:. The returned array is obviously empty, since we have not put anything into the database yet. We start by creating a new BookModel, passing an object with title, author, and releaseDate attributes.

The data are collected from request. Actually, the caller can omit any or all attributes since we have not made any of them mandatory. We then call the save function on the BookModel passing in a callback in the same way as with the previous get route. Finally, we return the saved BookModel. Restart node and go back to the console and type:. The object we get back, however, contains a JavaScript Date object. Here, we use colon notation: We also use the findById function on BookModel to get a single result.

If you restart node, you can get a single book by adding the id previously returned to the URL like this:. This is a little larger than previous ones, but is also pretty straight forward — we find a book by id, update its properties, save it, and send it back to the client. To test this, we need to use the more general jQuery ajax function. Again, in these examples, you will need to replace the id property with one that matches an item in your own database:. Well, until now I have left out the keywords part of our books.

To do that, we need another schema. Add a Keywords schema right above our Book schema:. As we mentioned in chapter 3 Backbone Basics , we can retrieve models from a server using collection. This results in the default implementation of Backbone. To have our application retrieve the Book models from the server on page load, we need to update the LibraryView. The Backbone documentation recommends inserting all models when the page is generated on the server side, rather than fetching them from the client side once the page is loaded. Since this chapter is trying to give you a more complete picture of how to communicate with a server, we will go ahead and ignore that recommendation.

Go to the LibraryView declaration and update the initialize function as follows:. Now that we are populating our Library from the database using this. We have also added a listener on the reset event. We need to do this since the models are fetched asynchronously after the page is rendered. When the fetch completes, Backbone fires the reset event, as requested by the reset: If you reload the page now, you should see all books that are stored on the server:. As you can see, the date and keywords look a bit weird.

The date delivered from the server is converted into a JavaScript Date object and when applied to the underscore template, it will use the toString function to display it. Update the book template so that the date is displayed with:. Now, the date on the page should look a bit better.

How about the keywords? Since we are receiving the keywords in an array we need to execute some code that generates a string of separated keywords. Here I iterate over the keywords array using the Underscore each function and print out every single keyword. This will display the keywords with spaces between them. Now go ahead and delete a book and then reload the page: Not cool, why is this?

To fix this, we can use the parse function of Backbone. The parse function lets you edit the server response before it is passed to the Model constructor. Add a parse method to the Book model:. If you reload the page, you will see that models are actually deleted on the server when you press the delete button. Another, simpler way of making Backbone recognize id as its unique identifier is to set the idAttribute of the model to id.

This is because Backbone. Now, newly created books will get persisted. Go ahead and create a custom jQuery UI download containing datepicker from here. Link to them in index. Finally, we have to make sure that the form input is properly transformed into our storage format. Change the addBook function in LibraryView to:. Our change adds two checks to the form input fields. We then use the getTime function on that to get the time in milliseconds. We also looked at some problems that might occur when serializing and deserializing data and their solutions.

We looked at the dateFormat and the datepicker jQuery plugins and how to do some more advanced things in our Underscore templates. The code is available here. Backbone is flexible, simple, and powerful. However, you may find that the complexity of the application you are working on requires more than what it provides out of the box. Take for example Views, which provide a default render method which does nothing and produces no real results when called, despite most implementations using it to generate the HTML that the view manages. Also, Models and Collections have no built-in way of handling nested hierarchies - if you require this functionality, you need to write it yourself or use a plugin.

In these cases, there are many existing Backbone plugins which can provide advanced solutions for large-scale Backbone apps. A fairly complete list of plugins and frameworks available can be found on the Backbone wiki. Using these add-ons, there is enough for applications of most sizes to be completed successfully. In this section of the book we will look at two popular Backbone add-ons: MarionetteJS and Thorax. It gives us the core constructs that are needed to build small to mid-sized apps, organize jQuery DOM events, or create single page apps that support mobile devices and large scale enterprise needs.

MarionetteJS a. Marionette provides many of the features that the non-trivial application developer needs, above what Backbone itself provides. It is a composite application library that aims to simplify the construction of large scale applications.

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It does this by providing a collection of common design and implementation patterns found in the applications that the creator, Derick Bailey , and many other contributors have been using to build Backbone apps. Marionette follows a similar philosophy to Backbone in that it provides a suite of components that can be used independently of each other, or used together to create significant advantages for us as developers. But it steps above the structural components of Backbone and provides an application layer, with more than a dozen components and building blocks.

Its core components include various and specialized view types that take the boilerplate out of rendering common Backbone. Much like Backbone itself, you can pick and choose which features you want to use and when. This allows you to work with other Backbone frameworks and plugins very easily. It also means that you are not required to engage in an all-or-nothing migration to begin using Marionette.

Consider the code that it typically requires to render a view with Backbone and Underscore template. We need a template to render, which can be placed in the DOM directly, and we need the JavaScript that defines a view that uses the template and populates it with data from a model. Once this is in place, you need to create an instance of your view and pass your model into it. This is a standard set up for defining, building, rendering, and displaying a view with Backbone. It gets to be tedious and repetitious very quickly. View - include a built-in render method that handles the core rendering logic for you.

We can take advantage of this by changing the MyView instance to inherit from one of these rather than Backbone. Instead of having to provide our own render method for the view, we can let Marionette render it for us. Thus, we can reduce the amount of code needed for this view. Just replace Backbone. You can still create the view instance with a model , call the render method on the view instance, and display the view in the DOM the same way that we did before. But the view definition has been reduced to a single line of configuration for the template.

In addition to the reduction of code needed to define a view, Marionette includes some advanced memory management in all of its views, making the job of cleaning up a view instance and its event handlers easy. If we create two instances of this view using the same variable name for both instances, and then change a value in the model, how many times will we see the alert box?

Whenever we pass this. Since the model is now holding a reference to the view instance, replacing the zombieView variable with a new view instance is not going to let the original view fall out of scope. The model still has a reference, therefore the view is still in scope. Since the original view is still in scope, and the second view instance is also in scope, changing data on the model will cause both view instances to respond.

Fixing this is easy, though. You just need to call stopListening when the view is done with its work and ready to be closed. To do this, add a close method to the view. Then call close on the first instance when it is no longer needed, and only one view instance will remain alive. Rather than having to manually remove these event handlers, though, we can let Marionette do it for us. Notice in this case we are using a method called listenTo. This method comes from Backbone. Events, and is available in all objects that mix in Backbone. Events - including most Marionette objects.

The listenTo method signature is similar to that of the on method, with the exception of passing the object that triggers the event as the first parameter. This means we no longer need to define a close method directly, and when we use the listenTo method, we know that our events will be removed and our views will not turn into zombies. But how do we automate the call to close on a view, in the real application? When and where do we call that? Enter the Marionette. Region - an object that manages the lifecycle of an individual view.

After a view is created, it typically needs to be placed in the DOM so that it becomes visible. This is usually done with a jQuery selector and setting the html of the resulting object:. This, again, is boilerplate code. To solve these problems, Marionette provides a Region object - an object that manages the lifecycle of individual views, displayed in a particular DOM element. There are several things to note, here. When we use a region to manage the lifecycle of our views, and display the views in the DOM, the region itself handles these concerns.

By passing a view instance into the show method of the region, it will call the render method on the view for us. It will then take the resulting el of the view and populate the DOM element. The next time we call the show method of the region, the region remembers that it is currently displaying a view. Regions are not limited to just Marionette views, though.

Any valid Backbone. View can be managed by a Marionette. If your view happens to have a close method, it will be called when the view is closed. If not, the Backbone. View built-in method, remove , will be called instead. Our final implementation will be visually and functionally equivalent to the original app, as seen below. First, we define an application object representing our base TodoMVC app. This will contain initialization code and define the default layout regions for our app. We supply a jQuery selector for each region to manage e.

Once the application object has been initialized, we call Backbone. Next, we define our Layouts. A layout is a specialized type of view that directly extends Marionette. One of the main differences between a Layout and an ItemView is that the layout contains regions. When defining a Layout, we supply it with both a template and the regions that the template contains. After rendering the layout, we can display other views within the layout using the regions that were defined.

This captures some of the view logic that was previously in our AppView and TodoView. Note that Marionette modules such as the below offer a simple module system which is used to create privacy and encapsulation in Marionette apps. Next, we tackle application routing and workflow, such as controlling Layouts in the page which can be shown or hidden.

Recall how Backbone routes trigger methods within the Router as shown below in our original Workspace router from our first exercise:. Marionette uses the concept of an AppRouter to simplify routing. This reduces the boilerplate for handling route events and allows routers to be configured to call methods on an object directly. The TodoList Controller, also found in this next code block, handles some of the remaining visibility logic originally found in AppView and TodoView , albeit using very readable Layouts.

This inevitably leads to mashing every possible combination of code into the router methods - view creation, model loading, coordinating different parts of the app, etc. Developers such as Derick view this as a violation of the single-responsibility principle SRP and separation of concerns. This allows the application to be used with or without a router. Derick has written extensively about his thoughts on this topic, which you can read more about on his blog:. Our next task is defining the actual views for individual Todo items and lists of items in our TodoMVC application.

The idea behind a CompositeView is that it represents a visualization of a composite or hierarchical structure of leaves or nodes and branches. Think of these views as being a hierarchy of parent-child models, and recursive by default. The same CompositeView type will be used to render each item in a collection that is handled by the composite view. For non-recursive hierarchies, we are able to override the item view by defining an itemView attribute. At the end of the last code block, you will also notice an event handler using vent.

This is an event aggregator that allows us to handle filterItem triggers from our TodoList controller. Finally, we define the model and collection for representing our Todo items. We finally kick-start everything off in our application index file, by calling start on our main application object:. Derick feels that maintainability largely comes down to modularity, separating responsibilities Single Responsibility Principle and Separation of Concerns by using patterns to keep concerns from being mixed together.

It can, however, be difficult to simply extract things into separate modules for the sake of extraction, abstraction, or dividing the concept down into its simplest parts. The Single Responsibility Principle SRP tells us quite the opposite - that we need to understand the context in which things change. What parts always change together, in this system? What parts can change independently? The way Derick organizes his apps into modules is by creating a breakdown of concepts at each level.

A higher level module is a higher level of concern - an aggregation of responsibilities. Each responsibility is broken down into an expressive API set that is implemented by lower level modules Dependency Inversion Principle. These are coordinated through a mediator - which he typically refers to as the Controller in a module. The way Derick organizes his files also plays directly into maintainability and he has also written posts about the importance of keeping a sane application folder structure that I recommend reading:.

Marionette is a flexible framework, much like Backbone itself. The flexibility and versatility in Marionette is easiest to understand by examining three variations of TodoMVC implemented with it that have been created for comparison purposes:. The simple version: The objects that are created are added directly to the global namespace and are fairly straightforward. This is a great example of how Marionette can be used to augment existing code without having to re-write everything around Marionette. The RequireJS version: Using Marionette with RequireJS helps to create a modularized application architecture - a tremendously important concept in scaling JavaScript applications.

RequireJS provides a powerful set of tools that can be leveraged to great advantage, making Marionette even more flexible than it already is. The Marionette module version: For those that wish to build applications in modules and namespaces, Marionette provides a built-in module and namespacing structure. Marionette certainly provides its share of opinions on how a Backbone application should be architected. The combination of modules, view types, event aggregator, application objects, and more, can be used to create a very powerful and flexible architecture based on these opinions.

It provides many elements of an application foundation that can be mixed and matched with other architectural styles, such as AMD or namespacing, or provide simple augmentation to existing projects by reducing boilerplate code for rendering views. There is far more functionality, more features, and more flexibility and customizability that can be put to use in both of these objects.

Then we have the other dozen or so components that Marionette provides, each with their own set of behaviors built in, customization and extension points, and more. Thorax makes an opinionated decision to use Handlebars as its templating solution. Some of the patterns found in Marionette are found in Thorax as well. Marionette exposes most of these patterns as JavaScript APIs while in Thorax they are often exposed as template helpers. This chapter assumes the reader has knowledge of Handlebars. To learn more about other features implemented in Thorax and to download boilerplate projects visit the Thorax website.

In Backbone, when creating a new view, options passed are merged into any default options already present on a view and are exposed via this. View differs from Backbone. View in that there is no options object. All arguments passed to the constructor become properties of the view, which in turn become available to the template:.

In most examples in this chapter a template property will be specified. In larger projects including the boilerplate projects provided on the Thorax website a name property would instead be used and a template of the same file name in your project would automatically be assigned to the view. The view helper allows you to embed other views within a view.

Child views can be specified as properties of the view:. Or the name of a child view to initialize as well as any optional properties you wish to pass. In this case the child view must have previously been created with extend and given a name property:. The view helper may also be used as a block helper, in which case the block will be assigned as the template property of the child view:. Handlebars is string based, while Backbone. View instances have a DOM el. Since we are mixing metaphors, the embedding of views works via a placeholder mechanism where the view helper in this case adds the view passed to the helper to a hash of children , then injects placeholder HTML into the template such as:.

One of the most useful constructs in Thorax is Handlebars.

Book Details

This method will register a new block helper that will create and embed a HelperView instance with its template set to the captured block. Like other child views it will have a parent property set to that of the declaring view. Many of the built-in helpers in Thorax including the collection helper are created in this manner. An example use of this would be to have a counter that would increment each time a button was clicked.

This example makes use of the button helper in Thorax which simply makes a button that triggers a view event when clicked:. The collection helper creates and embeds a CollectionView instance, creating a view for each item in a collection, updating when items are added, removed, or changed in the collection. The simplest usage of the helper would look like:. The block in this case will be assigned as the template for each item view created, and the context will be the attributes of the given model.

This helper accepts options that can be arbitrary HTML attributes, a tag option to specify the type of tag containing the collection, or any of the following:. Options and blocks can be used in combination, in this case creating a KittenView class with a template set to the captured block for each kitten in the collection:. Note that multiple collections can be used per view, and collections can be nested. This is useful when there are models that contain collections that contain models that contain….

Thorax makes heavy use of custom HTML data attributes to operate. While some make sense only within the context of Thorax, several are quite useful to have in any Backbone project for writing other functions against, or for general debugging. In order to add some to your views in non-Thorax projects, override the setElement method in your base view class:. In Thorax, this can particularly useful in conjunction with the collection helper which generates a view class with a model property for each model in the collection. An example template:. A common anti-pattern in Backbone applications is to assign a className to a single view class.

Consider using the data-view-name attribute as a CSS selector instead, saving CSS classes for things that will be used multiple times:. No Backbone related tutorial would be complete without a todo application. A Thorax implementation of TodoMVC is available, in addition to this far simpler example composed of this single Handlebars template:. Perhaps the most frequent of these questions surround how to do more with Views.

If you are interested in discovering how to work with nested Views, learn about view disposal and inheritance, this section will hopefully have you covered. Since pages are composed of nested elements and Backbone views correspond to elements within the page, nesting views is an intuitive approach to managing a hierarchy of elements. Beginners sometimes also try using setElement to solve this problem, however keep in mind that using this method is an easy way to shoot yourself in the foot.

Avoid using this approach when the first solution is a viable option:. Note that setElement returns the view, allowing us to chain the call to render. This works and has some positive qualities: Note that InnerView needs to call View. In the template, use a custom HTML attribute named data-view-cid to create placeholder elements for each view to embed.

The use of cid s client ids here is useful because it illustrates separating a model and its views by having views referenced by their instances and not their attributes. That is, unless you specify additional attributes that separate duplicates. Using cid s solves this problem as it allows for direct references to views. The Backbone extensions Marionette and Thorax provide logic for nesting views, and rendering collections where each item has an associated view.

We will examine both of these in an upcoming chapter. Thanks to Lukas and Ian Taylor for these tips. This way you can traverse the nesting first up to the parent and then down to any siblings that you know of. So, assuming we have models modelA, modelB and modelC:. This allows you to reach the parent model in any child model function through this. Now, we have already discussed a few options for how to construct nested Views using Backbone. For the sake of simplicity, let us imagine that we are creating a new child view ViewB from within the initialize method of ViewA below.

ViewB can reach out over the ViewA model and listen out for changes on any of its nested models. In a scenario where you have a view containing another view, such as a photo gallery containing a larger view modal, you may find that you need to render or re-render the parent view from the child. The good news is that solving this problem is quite straight-forward.

If however inversion of control is desired, events may be used to provide an equally valid solution. Say we wish to begin rendering when a particular event has occurred. The parent view can bind notifications on the child view to know when the event has occurred. It can then render itself. Thanks to Tal Bereznitskey for this tip. Where your application is setup with multiple Parent and Child Views, it is also common to desire removing any DOM elements associated with such views as well as unbinding any event handlers tied to child elements when you no longer require them.

The solution in the last question should be enough to handle this use case, but if you require a more explicit example that handles children, we can see one below:. Here, a close method for views is implemented which disposes of a view when it is no longer needed or needs to be reset. In most cases, the view removal should not affect any associated models.

For example, if you are working on a blogging application and you remove a view with comments, perhaps another view in your app shows a selection of comments and resetting the collection would affect those views as well. You may also be interested in reading about the Marionette Composite Views in the Extensions part of the book. Let us say you have a Collection, where each item in the Collection could itself be a Collection. You can render each item in the Collection, and indeed can render any items which themselves are Collections.

The problem you might have is how to render HTML that reflects the hierarchical nature of the data structure. In this framework is a type of view called a CompositeView. The basic idea of a CompositeView is that it can render a model and a collection within the same view. It can render a single model with a template. It can also take a collection from that model and for each model in that collection, render a view. There is a working demo of this in action available online.

And you can get the source code and documentation for Marionette too. How do I work around this? Take for example a Building model that contains many Room models which could sit in a Rooms collection. You could expose a this. There are also a number of Backbone plugins which can help with nested data structures, such as Backbone Relational. This plugin handles one-to-one, one-to-many and many-to-one relations between models for Backbone and has some excellent documentation.

As we learned earlier in the book, the validate method on a Model is called by set when the validate option is set and save. It is passed the model attributes updated with the values passed to these methods. We have a form where a user focuses and blurs first name, last name, and email HTML input boxes without entering any data. Basic validation that could be written using the current Backbone validate method to work with this form could be implemented using something like:.

Unfortunately, this method would trigger a firstname error each time any of the fields were blurred and only an error message next to the first name field would be presented. This can be adapted into a solution that defines a Field model for each input in our form and works within the parameters of our use case as follows:. This works fine as the solution checks the validation for each attribute individually and sets the message for the correct blurred field. A demo of the above by [ braddunbar] http: Unfortunately, this solution does perform validation on all fields every time, even though we are only displaying errors for the field that has changed.

If we have multiple client-side validation methods, we may not want to have to call each validation method on every attribute every time, so this solution might not be ideal for everyone. A potentially better alternative to the above is to use [ gfranko] http: Here is how we would setup a partial User Model and validate method using this plugin for our use case:. We can simply define a new Model instance and then set the data on our model using the validateAll option to use the behavior defined by the plugin:. Both solutions presented in this section should work fine however.

Developing Applications -

Often developers run into the issue of implementing this validation as nested ifs and elses, which can become unmaintainable when things get complicated. Another helpful plugin for Backbone called Backbone. Validation attempts to solve this problem by offering an extensible way to declare validation rules on the model and overrides the validate method behind the scenes.

One of the useful methods this plugin includes is pseudo live validation via a preValidate method. This can be used to check on key-press if the input for a model is valid without changing the model itself. You can run any validators for a model attribute by calling the preValidate method, passing it the name of the attribute along with the value you would like validated. That said, the most optimal solution to this problem may not be to stick validation in your model attributes. Instead, you could have a function specifically designed for validating a specific form and there are many good JavaScript form validation libraries out there that can help with this.

For more information on validation plugins available for Backbone, see the Backbone wiki. In instances out of your control, you may have to work around having more than one version of Backbone in the same page. How do you work around this without causing conflicts?

Several things happen during this configuration stage. A Backbone namespace is created, and multiple versions of Backbone on the same page are supported through the noConflict mode:. Multiple versions of Backbone can be used on the same page by calling noConflict like this:. How does inheritance work with Backbone?

How can I share code between similar models and views? How can I call methods that have been overridden? For its inheritance, Backbone internally uses an inherits function inspired by goog. We can take this further and also apply it to View inheritance. The following is an example of how to extend one View using another:. Rather than using Backbone. Using an Event Aggregator. Design patterns often differ only in semantics and intent.

That is, the language used to describe the pattern is what sets it apart, more than an implementation of that specific pattern. It often comes down to squares vs rectangles vs polygons. You can create the same end result with all three, given the constraints of a square are still met — or you can use polygons to create an infinitely larger and more complex set of things. When it comes to the Mediator and Event Aggregator patterns, there are some times where it may look like the patterns are interchangeable due to implementation similarities.

However, the semantics and intent of these patterns are very different.

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  • And even if the implementations both use some of the same core constructs, I believe there is a distinct difference between them. I also believe they should not be interchanged or confused in communication because of the differences. The easiest event aggregator to show is that of Backbone. In this example, the first view is triggering an event when a DOM element is clicked.

    Did you know that jQuery has a built-in event aggregator? A Mediator is an object that coordinates interactions logic and behavior between multiple objects. It makes decisions on when to call which objects, based on the actions or inaction of other objects and input. Yes, of course this is just an object literal in JavaScript.

    This example shows a very basic implementation of a mediator object with Backbone-based objects that can trigger and subscribe to events. It is an object that handles the workflow between many other objects, aggregating the responsibility of that workflow knowledge into a single object. The result is workflow that is easier to understand and maintain. The similarities boil down to two primary items: These differences are superficial at best, though. When we dig into the intent of the pattern and see that the implementations can be dramatically different, the nature of the patterns become more apparent.

    Both the event aggregator and mediator use events, in the above examples. The mediator only uses events because it makes life easy when dealing with Backbone, though.

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    There is nothing that says a mediator must be built with events. You can build a mediator with callback methods, by handing the mediator reference to the child object, or by any of a number of other means. The difference, then, is why these two patterns are both using events. The event aggregator, as a pattern, is designed to deal with events. Both the event aggregator and mediator, by design, use a third-party object to facilitate things.

    The event aggregator itself is a third-party to the event publisher and the event subscriber. It acts as a central hub for events to pass through. The mediator is also a third party to other objects, though. So where is the difference? The answer largely comes down to where the application logic and workflow is coded. In the case of an event aggregator, the third party object is there only to facilitate the pass-through of events from an unknown number of sources to an unknown number of handlers. All workflow and business logic that needs to be kicked off is put directly into the object that triggers the events and the objects that handle the events.

    In the case of the mediator, though, the business logic and workflow is aggregated into the mediator itself. The mediator decides when an object should have its methods called and attributes updated based on factors that the mediator knows about. It encapsulates the workflow and process, coordinating multiple objects to produce the desired system behaviour. The individual objects involved in this workflow each know how to perform their own task.

    It just fires the event and moves on. A mediator pays attention to a known set of input or activities so that it can facilitate and coordinate additional behavior with a known set of actors objects. Understanding the similarities and differences between an event aggregator and mediator is important for semantic reasons. The basic semantics and intent of the patterns does inform the question of when, but actual experience in using the patterns will help you understand the more subtle points and nuanced decisions that have to be made.

    In general, an event aggregator is used when you either have too many objects to listen to directly, or you have objects that are entirely unrelated. When two objects have a direct relationship already — say, a parent view and child view — then there might be little benefit in using an event aggregator. Have the child view trigger an event and the parent view can handle the event.

    A Collection often uses model events to modify the state of itself or other models. This could quickly deteriorate performance of the application and user experience. Indirect relationships are also a great time to use event aggregators. In Backbone applications, it is very common to have multiple view objects that need to communicate, but have no direct relationship. For example, a menu system might have a view that handles the menu item clicks.

    Having the content and menu coupled together would make the code very difficult to maintain, in the long run. A mediator is best applied when two or more objects have an indirect working relationship, and business logic or workflow needs to dictate the interactions and coordination of these objects. There are multiple views that facilitate the entire workflow of the wizard.

    Rather than tightly coupling the view together by having them reference each other directly, we can decouple them and more explicitly model the workflow between them by introducing a mediator. The mediator extracts the workflow from the implementation details and creates a more natural abstraction at a higher level, showing us at a much faster glance what that workflow is.

    We no longer have to dig into the details of each view in the workflow, to see what the workflow actually is. The crux of the difference between an event aggregator and a mediator, and why these pattern names should not be interchanged with each other, is illustrated best by showing how they can be used together. The menu example for an event aggregator is the perfect place to introduce a mediator as well.

    Clicking a menu item may trigger a series of changes throughout an application. Some of these changes will be independent of others, and using an event aggregator for this makes sense. Some of these changes may be internally related to each other, though, and may use a mediator to enact those changes. A mediator, then, could be set up to listen to the event aggregator. It could run its logic and process to facilitate and coordinate many objects that are related to each other, but unrelated to the original event source.

    An event aggregator and a mediator have been combined to create a much more meaningful experience in both the code and the application itself. We now have a clean separation between the menu and the workflow through an event aggregator and we are still keeping the workflow itself clean and maintainable through the use of a mediator. There is one overriding point to make in all of this discussion: Communicating intent and semantics through the use of named patterns is only viable and only valid when all parties in a communication medium understand the language in the same way.

    Am I talking about a fruit? Or am I talking about a technology and consumer products company? As Sharon Cichelli says: As you probably know, loose coupling facilitates easier maintainability of apps by removing dependencies where possible. Instead, developers are left to fall back on variations of the module or object literal patterns combined with script tags or a script loader.

    Whilst native solutions to these problems may be arriving via ES6 the next version of the official JavaScript specification modules proposal , the good news is that writing modular JavaScript has never been easier and you can start doing it today. Amongst other things RequireJS helps you to load multiple script files, define modules with or without dependencies, and load in non-script dependencies such as text files.

    You might be thinking that there is little benefit to RequireJS. However, doing it that way has a lot of drawbacks, including increasing the HTTP overhead. It has to make a new HTTP request for each file you want to load, which causes problems. What tools like RequireJS do is load scripts asynchronously. Dependency management is a challenging subject, in particular when writing JavaScript in the browser. This is not a good approach. Being able to load code on an as-needed basis is something RequireJS is very good at. Rather than load all our JavaScript code in during initial page load, a better approach is to dynamically load modules when that code is required.

    This avoids loading all the code when the user first hits your application, consequently speeding up initial load times. Think about the GMail web client for a moment. Through dynamic dependency loading, Google could load up the chat module at that time, rather than forcing all users to load it when the page first initializes. This can improve performance and load times and can definitely prove useful when building larger applications. As the codebase for an application grows this becomes even more important.

    Something with good debugging characteristics that did not require server-specific tooling to get started. As discussed above, the overall goal for the AMD format is to provide a solution for modular JavaScript that developers can use today. The two key concepts you need to be aware of when using it with a script-loader are the define method for defining modules and the require method for loading dependencies. The dependencies argument is an array representing all of the other modules that this module depends on and the third argument is a factory that can either be a function that should be executed to instantiate the module or an object.

    A barebones module compatible with RequireJS could be defined using define as follows:. As such, this extension is generally omitted when specifying dependencies. There is also a sugared version of define available that allows you to declare your dependencies as local variables using require. Here is the previous snippet using the alternate syntax:. The require method is typically used to load code in a top-level JavaScript file or within a module should you wish to dynamically fetch dependencies.

    An example of its usage is:. Defining and using modules will be covered in this book shortly when we look at more structured examples of using RequireJS. The first thing to do is to Download RequireJS. You do this by adding in a data-main attribute to the script tag:. In the main JavaScript file that you load with the data-main attribute you can configure how RequireJS loads the rest of your application. This is done by calling require. To see other configuration options available to you, I recommend checking out the RequireJS documentation.

    That is, it uses the define method to define the library as a module. Shims are very simple to implement:. Note that when specifying paths for RequireJS you should omit the. The important line here is exports: We can set up a shim for Backbone too:. This means whenever this:. Is run, it will first make sure the dependencies are met, and then pass the global Backbone object into the callback function.

    For example, jQuery does support it, as of jQuery 1. RequireJS lets us set up custom paths in our configuration object. At its simplest, a Backbone model may just require Backbone and Underscore. These are dependencies, so we can define those when defining the new modules. Note that the following examples presume you have configured RequireJS to shim Backbone and Underscore, as discussed previously. Note how we alias Underscore. For a view which might require other dependencies such as jQuery, this can similarly be done as follows:.

    Doing it this way makes it easy to organize your Backbone application as you like. For example, individual folders for models, collections, views and so on. It is a stock application for a manager of a shop.

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    They can add new items and filter down the items based on price, but nothing more. At the base of this application is the Item model, which describes a single item in the stock. Its implementation is very straight forward:. Converting an individual model, collection, view or similar into an AMD, RequireJS compliant one is typically very straight forward.

    This time we will need to reference the Item model, so we add it as a dependency:. As the Item model is the second dependency, I can bind the variable Item to it by declaring it as the second argument to the callback function. I can then refer to this within my collection implementation. Jeff Siarto.

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