on 04 November 2013 by in
In the lessons learned post, I talked about how Aura was born of the idea that we could extract independent decoupled packages from Solar, and how in doing so, we discovered that some of those extracted packages themsleves could be further split into independent pieces.
Today, I’m going to talk about Aura.Dispatcher as a combined extraction from three separate packages.
At its core, a dispatcher uses a set of parameters to determine what logic should be invoked. That set of parameters is generally the result of parsing a URL with a router, but it could come from anywhere.
In Aura v1, we actually have two layers of dispatchers. First, there is a front controller at the Aura.Framework level to determine what page controller class should be instantiated. Then, at the Aura.Web page controller level, there is an internal dispatcher that picks what method will be invoked. (Some dispatchers work in only one pass to pick both the class and the method; others, such as micro-framework dispatchers, pick only a closure to execute.)
In addition to the web dispatching, the Aura v1 framework also has a CLI equivalent of a front controller. It picks the right command class based on the CLI input, but the method is always the same.
Finally, it’s not enough to know what object should get instantiated; something has to actually instantiate it. As such, the web and CLI front controllers need factories to create the objects they are dispatching to.
It turns out that dispatching at the framework level, and at the web and CLI levels, is all remarkably similar. After realizing that, we extracted the dispatching logic to its own independent package, without any dependendcies on any other packages.
The Aura.Dispatcher package lets you define named objects that will get instantiated only as they are dispatched to (i.e., lazy loading). It picks which named object to instantiate, and optionally which method to invoke, based on an array of router values (or any other array you wish to pass). It also works as a micro-framework dispatcher; instead of using an object factory proper, you can add a named closure and that will be invoked.
Additionally, if you want a two-stage invocation where the dispatcher picks an object, and the object picks its own method, Aura.Dispatcher comes with a trait that lets you pass named parameters to any method you like. You can use that trait in any object to pick a method and invoke it with the router (or other) parameters.
Aura.Dispatcher is built with the idea that some developers may begin with a micro-framework architecture, and evolve over time toward a full-stack architecture. Here’s an outline of the full description; note that the logic used to invoke the dispatcher never changes; the only thing that changes are the params being passed and the object being dispatched to.
At first, the developer uses closures embedded in the routers parameters passed to the dispatcher; this is a typical micro-framework approach.
After adding several closure-based controllers to a router, the developer is likely to want to keep the routing configurations separate from the controller actions. At this point the developer may start putting the controller actions into the dispatcher as named closures.
As the number and complexity of controllers continues to grow, the developer may wish to convert the controller closures into separately invokable classes, lazy-loading along the way.
Finally, the developer may collect several action methods into a single controller, keeping related functionality in the same class.
With that, we have a clear path from closures in a router, to closures in the dispatcher, to invokable classes factoried by the dispatcher, and finally to classes of multiple methods factoried by the dispatcher. This means you can modify your application architecture and not have to worry about swapping dispatcher systems.
Because we have extracted the dispatcher behaviors from both the Aura.Web and Aura.Cli packages, it means that those packages can become further reduced in size and more focused. More on the Aura.Web v2 package next time!
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