Encapsulation in Enyo

Jim Tang edited this page Sep 4, 2012 · 2 revisions

Encapsulation in Enyo

One of the most important features that Enyo brings to Web development is the ability to encapsulate useful blocks of presentation code and associated logic into reusable components with defined interfaces. While you are completely free to structure your code as you see best, we have prepared a set of recommended practices that we think will help you make the most of Enyo's encapsulation model.

Send events up and call functions down

In order to provide good functional boundaries that encourage reuse, you should generally follow the rule that children of a component should have no a priori knowledge of their parents. For example, a function call like this.owner.doSomething() is strongly discouraged, since it would tightly bind the child to a specific owner, restricting its ability to be reused in new scenarios.

Instead, Enyo encourages the use of events as the mechanism for children to communicate with their parents. By declaring events such as onSomethingHappened (sent by calling doSomethingHappened()), children bubble events up to their owner, allowing the owner to decide whether handling onSomethingHappened is important to its current use case.

Conversely, parents always have a priori knowledge of their children--they have to create them, after all, usually by declaring them in their components block. Thus, tight binding from the parent to the child is normal. Parents should communicate with their children by calling functions defined in their API. Although JavaScript lacks a clean way to enforce the separation between public API and private functions, we use the special //* @public comment before function definitions to designate public APIs. These comments serve as a guide for the API viewer tool when displaying the public API.

Finally, you should never make assumptions about how a component you are using is built internally. For example, say you are using a widget that includes a title and descriptive text, and you want to change the color of the title. Calling this.$.myWidget.$.title.applyStyle("color", "red") would clearly break encapsulation, since you are assuming a lot about the construction of myWidget (i.e., that its title component is called title and that applying a style on that component will change the color of the text). Instead, myWidget should provide an API that its owner may call to make this change, such as this.$.myWidget.setTitleColor("red").

In summary: "Send events up and call functions down."

Keep related code close

Because writing an app usually involves refactoring code as the app grows, you will often find yourself creating useful chunks of functionality that can be reused in other apps. We find it helpful to organize code into self-contained units that are easy to move around as whole, useful chunks (including whatever view, model, control, CSS, etc. is required for that chunk to do its job), as opposed to organizing files by function.

As an analogy, think of all the parts that make up a chest of drawers that you might buy from Ikea--panels, screws, washers, handles, and so on. There are two ways in which Ikea could distribute the parts. They could give you all of the screws in one bag, all the washers in another, and so on, and to build a new drawer you would take some materials from each bag. Alternatively, they could give you one bag per drawer, with each bag containing everything needed to build the drawer. We find this latter model to be better for code refactoring and reuse, as it makes it easier to identify one drawer's worth of material and move it to any location where you need to make a new drawer.

In more concrete terms, let's say you are developing an app with a login screen, a friend list, and a friend detail view. Although you could organize your code like this...

css/                                // Not preferred

...we tend to prefer a structure like this:

Login/                              // Preferred

With the latter structure, if you find a use for a FriendDetails view in another app you are making, it's a trivial amount of work to separate that piece from the others and use it in the new app.

Separate into packages for reuse

Enyo provides a higher level of encapsulation at the file level as well, via package.js files and the enyo.depends() loader mechanism. This is especially useful for third-party components and libraries, since it allows the author to specify all the JavaScript and CSS files a component or library depends on. As a result, the user does not have to worry about the library's internal structure, but can simply include the path to its top-level package.js and then use the library's public kinds and APIs.

As your app grows larger, you may find it useful to split it into multiple packages. Rather than maintain one top-level package.js file that references all the source and CSS in your app, you can separate chunks of functionality into individual packages, which are then referenced by the top-level package.js. While this is ultimately a matter of personal preference, a good rule of thumb is to create separate packages for independently useful sets of kinds, such as FriendList or FriendDetails in the example above.

Additional Reading