[API] Propose contextual DI

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Victor Woeltjen 2015-11-19 13:02:03 -08:00
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@ -1032,6 +1032,9 @@ Incompatible with proposal to
(expose no third-party APIs)[#expose-no-third-party-apis]; Angular (expose no third-party APIs)[#expose-no-third-party-apis]; Angular
API would be ubiquitously exposed. API would be ubiquitously exposed.
This is a more specific variant of
(Bundle Declarations in JavaScript)[#bundle-declarations-in-javascript].
### Benefits ### Benefits
* Removes a whole category of API (bundle definitions), reducing * Removes a whole category of API (bundle definitions), reducing
@ -1048,3 +1051,83 @@ API would be ubiquitously exposed.
* Increases depth of understanding required of Angular. * Increases depth of understanding required of Angular.
* Increases amount of boilerplate (since a lot of this has * Increases amount of boilerplate (since a lot of this has
been short-handed by existing framework layer.) been short-handed by existing framework layer.)
## Contextual Injection
For extensions that depend on certain instance-level run-time
properties (e.g. actions or views which use objects and/or specific
capabilities of those objects), declare these features as dependencies
and expose them via dependency injection. (AngularJS does this for
`$scope` in the context of controllers, for example.)
A sketch of an implementation for this might look like:
```js
function ExtensionRegistry($injector, extensions, getLocals) {
this.$injector = $injector;
this.extensions = extensions;
this.getLocals = getLocals;
}
ExtensionRegistry.prototype.get = function () {
var $injector = this.$injector,
locals = this.getLocals.apply(null, arguments);
return this.extensions.filter(function (extension) {
return depsSatisfiable(extension, $injector, locals);
}).map(function (extension) {
return $injector.instantiate(extension, locals);
});
};
function ExtensionRegistryProvider(getLocals) {
this.getLocals = getLocals || function () { return {}; };
this.extensions = [];
}
ExtensionRegistryProvider.prototype.register = function (extension) {
this.extensions.push(extension);
};
ExtensionRegistryProvider.prototype.$get = ['$injector', function ($injector) {
return new ExtensionRegistry($injector, this.extensions, this.getLocals);
}];
```
Extension registries which need to behave context-sensitively could
subclass this to describe how these contextual dependencies are satisfied
(for instance, by returning various capability properties in `getLocals`).
Specific extensions could then declare dependencies as appropriate to the
registry they are using:
```js
app.config(['actionRegistryProvider', function (arp) {
arp.register(['contextCapability', 'domainObject', RemoveAction]);
}]);
```
### Benefits
* Allows contextual dependencies to be fulfilled in the same (or similar)
manner as global dependencies, increasing overall consistency of API.
* Clarifies dependencies of individual extensions (currently, extensions
themselves or policies generally need to imperatively describe what
dependencies will be used in order to filter down to applicable
extensions.)
* Factors out some redundant code from relevant extensions; actions,
for instance, no longer need to interpret an `ActionContext` object.
Instead, their constructors take inputs that are more relevant to
their behavior.
* Removes need for partial construction, as any arguments which would
normally be appended after partialization can instead be declared as
dependencies. Constructors in general become much less bound to the
specifics of the platform.
### Detriments
* Slightly increases dependency on Angular; other dependency injectors
may not offer comparable ways to specificy dependencies non-globally.
* Not clear (or will take effort to make clear) which dependencies are
available for which extensions. Could be mitigated by standardizing
descriptions of context across actions and views, but that may offer
its own difficulties.
* May seem counter-intuitive coming from "vanilla" AngularJS, where
`$scope` is the only commonly-used context-sensitive dependency.