ANGULAR 2 BITS: UNIFIED DEPENDENCY INJECTION

Angular 1.x has two APIs for injecting dependencies into a directive. These APIs are not interchangeable, so depending on what you are injecting, you need to use one or the other. Angular 2 unifies the two APIs, making the code easier to understand and test.

ANGULAR 1.X

Let’s start with a simple example: a directive autocompleting the user input.

    <input name="title" autocomplete="movie-title">

The autocomplete directive takes the user input, and using the autocompleter service object, gets matching movie titles from the backend.

    module.directive('autocomplete', ["autocompleter", function(autocompleter) {
      return {
        restrict: 'A',
        link: function (scope, element, attrs) {
          //...
        }
      }
    }]);

Note, we have to use two mechanisms to inject the autocompleter and the element.

• The autocompleter service is injected into the directive factory function, and it is done by name.
• The element and the attributes are injected into the link function. This is done by position, which forces us to pass in scope, even though we may not need it.

ANGULAR 2

Now, contrast it with the Angular 2 way of defining the same directive.

    @Decorator({selector: '[autocomplete]'})
    class Autocomplete {
        constructor(autocompleter:Autocompleter, el:NgElement, attrs:NgAttributes){
            //...
        }
    }

Or if you prefer ES5

    function Autocomplete(autocompleter, el, attrs) {
        //...
    }
    Autocomplete.annotations = [new Decorator({selector: '[autocomplete]'})];
    Autocomplete.parameters = [[Autocompleter], [NgElement], [NgAttributes]];

In Angular 2 the autocompleter, the element, and the attributes are injected into the constructor by name.

HIERARCHICAL INJECTORS

How is it possible? Should not every instance of the directive get its own element? It should. To enable that the framework builds a tree of injectors that matches the DOM.

    <div>
        <input name="title" autocomplete="movie-title">
        <input name="actor" autocomplete="actor-name">
    </div>

The matching injector tree:

    Injector Matching Div
        |
        |__Injector Matching Movie Title
        |
        |__Injector Matching Actor Name

Since there is an injector for every DOM element, the framework can provide element-specific information, such as an element, attributes, or a shadow root.

SUMMARY

In Angular 2 there is a single way to inject dependencies into a directive, regardless of what you inject: user-defined services, framework services, elements, attributes, or other directives.

• paper writing service • 5 months ago

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  Robert Penner • a month ago

  Looks like a typo: Aucotomplete

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    Victor Savkin Mod Robert Penner • a month ago

    thanks. Fixed it.

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  Serg de Adelantado • 2 months ago

  Hi!
  You wrote: "To enable that the framework builds a tree of injectors that matches the DOM."
  Does it mean that every time we making changes in a DOM(for example, with ng-if, or dynamic ng-include), it will lead to creation of a new Injector tree or re-scan of injections?

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    Victor Savkin Mod Serg de Adelantado • 2 months ago

    This is a good question.

    It works as follows:

    * In Angular2 the view is a chunk of DOM that can be added or removed.
    * The view has a tree of injectors associated with it.
    * We create prototypes for Views and Injectors. This happens during the compilation phase (once per component).
    * NgIf contains either a single View or nothing.
    * When NgIf evaluates to true, we use the prototypes to very efficiently create the required view and injectors.
    * In addition, we also have a view pool that allows us to reuse views and injectors. This is done to reduce GC pressure.

    The answer is:

    The mental model is that we do create the tree every time. But behind the scenes we use optimizations to make it cheap.

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  Sekib Omazic • 5 months ago

  Cool. Do you have a small app showing all this stuff? I'd like to play with it, but example in ng2 repo (todo app) shows not much.

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    Victor Savkin Mod Sekib Omazic • 5 months ago

    Currently no, but we are working hard on it. I'll write a blog post when it is ready.