# Dependency Injection (DI): Documentation

This document describes in detail how the DI module works in Angular 2.

## Core Abstractions

The library is built on top of the following core abstractions: `Injector`, `Binding`, and `Dependency`.

* An injector is created from a set of bindings.
* An injector resolves dependencies and creates objects.
* A binding maps a token, such as a string or class, to a factory function and a list of dependencies. So a binding defines how to create an object.
* A dependency points to a token and contains extra information on how the object corresponding to that token should be injected.

```
[Injector]
    |
    |
    |*
[Binding]
   |----------|-----------------|
   |          |                 |*
[Token]    [FactoryFn]     [Dependency]
                               |---------|
                               |         |
                            [Token]   [Flags]
```

## Example

```
class Engine {
}

class Car {
	constructor(@Inject(Engine) engine) {
	}
}

var inj = Injector.resolveAndCreate([
	bind(Car).toClass(Car),
	bind(Engine).toClass(Engine)
]);
var car = inj.get(Car);
```

In this example we create two bindings: one for Car and one for Engine. `@Inject(Engine)` declares a dependency on Engine.

## Injector

An injector instantiates objects lazily, only when asked for, and then caches them.

Compare

```
var car = inj.get(Car); //instantiates both an Engine and a Car
```

with

```
var engine = inj.get(Engine); //instantiates an Engine
var car = inj.get(Car); //instantiates a Car (reuses Engine)
```

and with

```
var car = inj.get(Car); //instantiates both an Engine and a Car
var engine = inj.get(Engine); //reads the Engine from the cache
```

To avoid bugs make sure the registered objects have side-effect-free constructors. In this case, an injector acts like a hash map, where the order in which the objects got created does not matter.



## Child Injectors and Dependencies

Injectors are hierarchical.

```
var parent = Injector.resolveAndCreate([
	bind(Engine).toClass(TurboEngine)
]);
var child = parent.resolveAndCreateChild([Car]);

var car = child.get(Car); // uses the Car binding from the child injector and Engine from the parent injector.
```

Injectors form a tree.

```
	GrandParentInjector
	 /              \
Parent1Injector  Parent2Injector
  |
ChildInjector
```

The dependency resolution algorithm works as follows:

```
// this is pseudocode.
var inj = this;
while (inj) {
  if (inj.hasKey(requestedKey)) {
		return inj.get(requestedKey);
	} else {
		inj = inj.parent;
	}
}
throw new NoBindingError(requestedKey);
```

So in the following example

```
class Car {
  constructor(e: Engine){}
}
```

DI will start resolving `Engine` in the same injector where the `Car` binding is defined. It will check whether that injector has the `Engine` binding. If it is the case, it will return that instance. If not, the injector will ask its parent whether it has an instance of `Engine`. The process continues until either an instance of `Engine` has been found, or we have reached the root of the injector tree.

### Constraints

You can put upper and lower bound constraints on a dependency. For instance, the `@Self` decorator tells DI to look for `Engine` only in the same injector where `Car` is defined. So it will not walk up the tree.

```
class Car {
  constructor(@Self() e: Engine){}
}
```

A more realistic example is having two bindings that have to be provided together (e.g., NgModel and NgRequiredValidator.)

The `@Host` decorator tells DI to look for `Engine` in this injector, its parent, until it reaches a host (see the section on hosts.)

```
class Car {
  constructor(@Host() e: Engine){}
}
```

The `@SkipSelf` decorator tells DI to look for `Engine` in the whole tree starting from the parent injector.

```
class Car {
  constructor(@SkipSelf() e: Engine){}
}
```

### DI Does Not Walk Down


Dependency resolution only walks up the tree. The following will throw because DI will look for an instance of `Engine` starting from `parent`.

```
var parent = Injector.resolveAndCreate([Car]);
var child = injector.resolveAndCreateChild([
  bind(Engine).toClass(TurboEngine)
]);

parent.get(Car); // will throw NoBindingError
```


## Bindings

You can bind to a class, a value, or a factory. It is also possible to alias existing bindings.

```
var inj = Injector.resolveAndCreate([
	bind(Car).toClass(Car),
	bind(Engine).toClass(Engine)
]);

var inj = Injector.resolveAndCreate([
	Car,  // syntax sugar for bind(Car).toClass(Car)
	Engine
]);

var inj = Injector.resolveAndCreate([
	bind(Car).toValue(new Car(new Engine()))
]);

var inj = Injector.resolveAndCreate([
	bind(Car).toFactory((e) => new Car(e), [Engine]),
	bind(Engine).toFactory(() => new Engine())
]);
```

You can bind any token.

```
var inj = Injector.resolveAndCreate([
	bind(Car).toFactory((e) => new Car(), ["engine!"]),
	bind("engine!").toClass(Engine)
]);
```

If you want to alias an existing binding, you can do so using `toAlias`:

```
var inj = Injector.resolveAndCreate([
	bind(Engine).toClass(Engine),
	bind("engine!").toAlias(Engine)
]);
```
which implies `inj.get(Engine) === inj.get("engine!")`.

Note that tokens and factory functions are decoupled.

```
bind("some token").toFactory(someFactory);
```

The `someFactory` function does not have to know that it creates an object for `some token`.

### Resolved Bindings

When DI receives `bind(Car).toClass(Car)`, it needs to do a few things before before it can create an instance of `Car`:

- It needs to reflect on `Car` to create a factory function.
- It needs to normalize the dependencies (e.g., calculate lower and upper bounds).

The result of these two operations is a `ResolvedBinding`.

The `resolveAndCreate` and `resolveAndCreateChild` functions resolve passed-in bindings before creating an injector. But you can resolve bindings yourself using `Injector.resolve([bind(Car).toClass(Car)])`. Creating an injector from pre-resolved bindings is faster, and may be needed for performance sensitive areas.

You can create an injector using a list of resolved bindings.

```
var listOfResolvingBindings = Injector.resolve([Binding1, Binding2]);
var inj = Injector.fromResolvedBindings(listOfResolvingBindings);
inj.createChildFromResolvedBindings(listOfResolvedBindings);
```


### Transient Dependencies

An injector has only one instance created by each registered binding.

```
inj.get(MyClass) === inj.get(MyClass); //always holds
```

If we need a transient dependency, something that we want a new instance of every single time, we have two options.

We can create a child injector for each new instance:

```
var child = inj.resolveAndCreateChild([MyClass]);
child.get(MyClass);
```

Or we can register a factory function:

```
var inj = Injector.resolveAndCreate([
  bind('MyClassFactory').toFactory(dep => () => new MyClass(dep), [SomeDependency])
]);

var factory = inj.get('MyClassFactory');
var instance1 = factory(), instance2 = factory();
// Depends on the implementation of MyClass, but generally holds.
expect(instance1).not.toBe(instance2);
```
