Angular里useExisting和useClass的使用场景StackOverflow上的一个帖子.从上图能看出，

StackOverflow上的一个帖子.

从上图能看出，所有注入的示例都通过factory方法返回，只是factory方法的实现有所差异。

As we can see in the picture above all of providers can be presented similar useFactory. When it’s the time to get an instance of provider angular just calls factory function.

一些例子：

{provide: Class1, useClass: Class1}

等价于：

Class1

而：

{provide: Class1, useClass: Class3}

you can configure, that when a constructor requests Class1 Angular DI creates an instance of Class3 and passes it to the constructor.

当构造函数的参数需要传入Class1时，Angular DI传入一个Class3的框实例到构造函数里。

而下面这个例子：

{provide: Class2, useExisting: Class2}

doesn’t result in an instance being created, but you can see this rather than an alias. If a constructor requests Class2, Angular DI looks for another provider for key Class2 and injects the instance from this Class2 provider. You can see useExisting like a reference to another provider or an alias.

当构造函数需要Class2时，Angular DI从另一个key为Class2的provider里查找，取出对应的实例进行注入。

Injectable and Provider are 2 different things. The injectable is a class DI can create an instance of, or a provided value that might be passed to a constructor. A provider is an entry in a registry of providers that DI maintains and that allows to lookup up providers by key (key is the type of a constructor parameter, a string or an InjectToken). The provider also holds information about how to “create” the injectable value/instance. DI looks up a provider, the provider provides the value, and DI passes the value (instance of an injectable or a value provided as-is) to a constructor.

Injectable和Provider是两个不同的概念。Injectable是一个class，Angular DI可以基于该class创建实例，或者DI可以提供一个值，能够传递到constructor里。DI内部维护了一个provider注册表，该注册表支持根据key查询provider.

Key is the type of a constructor parameter, a string or an InjectToken)

key是构造函数的参数，一个字符串或者InjectToken. Provider有足够的knowledge知道如何去创建一个value或者实例。

DI询问provider，provider提供值，DI将值传递到构造函数里。

If you register MyClass as provider, this is the short form of { provide: MyClass, useClass: MyValue } where provide: MyClass is the key a provider can be found with, and useClass: MyValue is a strategy passed to the provider that informs the provider what value it should provide for this key.

provide: MyClass是provider的key，Angular DI注册表里根据这个key找到provider.

useClass: MyValue, 一个传递给provider的strategy，告诉provider对于指定的key，应该提供何种value.