详解Angular 4.x Injector

在介绍 Angular Injector (注入器) 之前，我们先要了解 Dependency Injection，即依赖注入的概念。

依赖注入允许程序设计遵从依赖倒置原则 (简单的说就是要求对抽象进行编程，不要对实现进行编程，这样就降低了客户端与实现模块间的耦合) 调用者只需知道服务的接口，具体服务的查找和创建由注入器 (Injector) 负责处理并提供给调用者，这样就分离了服务和调用者的依赖，符合低耦合的程序设计原则。

从上述的内容可知，依赖注入中包含三种角色：调用者、服务和注入器 (Injector)。现在我们开始介绍 Injector，在 Angular 中 Injector (注入器) 用来管理服务对象的创建和获取。接下来我们先来看一下 Injector 抽象类：

Injector 抽象类

// angular2\packages\core\src\di\injector.ts
export abstract class Injector {
 static THROW_IF_NOT_FOUND = _THROW_IF_NOT_FOUND;
 static NULL: Injector = new _NullInjector();

 /**
 * 用于根据给定的Token从注入器中获取相应的对象。
 * 如果没有找到相应的对象，将返回notFoundValue设置的值。若notFoundValue的值与
 * _THROW_IF_NOT_FOUND相等，则会抛出异常。
 */
 abstract get<T>(token: Type<T>|InjectionToken<T>, notFoundValue?: T): T;
}

const _THROW_IF_NOT_FOUND = new Object();

Injector 抽象类中定义了一个 get() 抽象方法，该方法用于根据给定的 Token 从注入器中获取相应的对象，每个Injector 抽象类的子类都必须实现该方法。在 Angular 中常见的 Injector 抽象类子类有：

1. _NullInjector
2. ReflectiveInjector

下面我们来依次介绍它们：

_NullInjector 类

_NullInjector 类的实例用于表示空的注入器。

// angular2\packages\core\src\di\injector.ts
class _NullInjector implements Injector {
 get(token: any, notFoundValue: any = _THROW_IF_NOT_FOUND): any {
 if (notFoundValue === _THROW_IF_NOT_FOUND) {
  throw new Error(`No provider for ${stringify(token)}!`);
 }
 return notFoundValue;
 }
}

ReflectiveInjector 抽象类

ReflectiveInjector 表示一个依赖注入容器，用于实例化对象和解析依赖。

ReflectiveInjector 使用示例

@Injectable()
class Engine {}

@Injectable()
class Car {
 constructor(public engine:Engine) {}
}

var injector = ReflectiveInjector.resolveAndCreate([Car, Engine]);
var car = injector.get(Car);
expect(car instanceof Car).toBe(true);
expect(car.engine instanceof Engine).toBe(true);

上面示例中，我们通过调用 ReflectiveInjector 抽象类的 resolveAndCreate() 方法，创建注入器。然后通过调用注入器的 get() 方法，获取 Token 对应的对象。该抽象类除了 resolveAndCreate() 静态方法外，还含有以下静态方法：

1. resolve() - 解析 Provider 列表为 ResolvedReflectiveProvider 列表
2. fromResolvedProviders() - 基于 ResolvedReflectiveProvider 列表创建 ReflectiveInjector 对象

接下来我们来分析上述的静态方法：

resolveAndCreate()

static resolveAndCreate(providers: Provider[], parent?: Injector): ReflectiveInjector {
 const ResolvedReflectiveProviders = ReflectiveInjector.resolve(providers);
 return ReflectiveInjector.fromResolvedProviders(ResolvedReflectiveProviders, parent);
}

从上面代码中，我们可以看出 resolveAndCreate() 方法内部是通过调用 ReflectiveInjector.resolve() 方法和 ReflectiveInjector.fromResolvedProviders() 方法来创建 ReflectiveInjector 对象。

resolve()

该方法用于把 Provider 数组解析为 ResolvedReflectiveProvider 数组。

static resolve(providers: Provider[]): ResolvedReflectiveProvider[] {
 return resolveReflectiveProviders(providers);
}

resolve() 使用示例

@Injectable()
class Engine {}

@Injectable()
class Car {
 constructor(public engine:Engine) {}
}

var providers = ReflectiveInjector.resolve([Car, [[Engine]]]);
expect(providers.length).toEqual(2);

expect(providers[0] instanceof ResolvedReflectiveProvider).toBe(true);
expect(providers[0].key.displayName).toBe("Car");
expect(providers[1].key.displayName).toBe("Engine");

resolve() 解析图示

Provider 类型

export type Provider =
 TypeProvider | ValueProvider | ClassProvider | ExistingProvider | FactoryProvider | any[];

// ApiService
export interface TypeProvider extends Type<any> {}

// { provide: ApiService, useClass: ApiService }
export interface ClassProvider {
 // 用于设置与依赖对象关联的Token值，Token值可能是Type、InjectionToken、OpaqueToken的实例或字符串
 provide: any;
 useClass: Type<any>;
 // 用于标识是否multiple providers，若是multiple类型，则返回与Token关联的依赖对象列表
 multi?: boolean;
}

// { provide: 'API_URL', useValue: 'http://my.api.com/v1' }
export interface ValueProvider {
 provide: any;
 useValue: any;
 multi?: boolean;
}

// { provide: 'ApiServiceAlias', useExisting: ApiService }
export interface ExistingProvider {
 provide: any;
 useExisting: any;
 multi?: boolean;
}

// { provide: APP_INITIALIZER, useFactory: configFactory, deps: [AppConfig], multi: true }
export interface FactoryProvider {
 provide: any;
 useFactory: Function;
 deps?: any[]; // 用于设置工厂函数的依赖对象
 multi?: boolean;
}

ResolvedReflectiveProvider 接口

export interface ResolvedReflectiveProvider {
 // 唯一的对象用来从ReflectiveInjector中获取对象
 key: ReflectiveKey;
 // 工厂函数用于创建key相关的依赖对象
 resolvedFactories: ResolvedReflectiveFactory[];
 // 标识当前的provider是否为multi-provider
 multiProvider: boolean;
}

ResolvedReflectiveFactory 类

export class ResolvedReflectiveFactory {
 constructor(
  public factory: Function,
  public dependencies: ReflectiveDependency[]) {}
}

ReflectiveDependency 类

export class ReflectiveDependency {
 constructor(
  public key: ReflectiveKey,
  public optional: boolean,
  public visibility: Self|SkipSelf|null) {}

 static fromKey(key: ReflectiveKey): ReflectiveDependency {
 return new ReflectiveDependency(key, false, null);
 }
}

ReflectiveKey 类

ReflectiveKey 对象中包含两个属性：系统范围内唯一的id 和 token。系统范围内唯一的id，允许注入器以更高效的方式存储已创建的对象。另外我们不能手动的创建 ReflectiveKey，当 ReflectiveInjector 对象解析 providers 的时候会自动创建 ReflectiveKey 对象。

export class ReflectiveKey {
 constructor(public token: Object, public id: number) {
 if (!token) {
  throw new Error('Token must be defined!');
 }
 }

 // 返回序列化的token
 get displayName(): string { return stringify(this.token); }

 // 获取token对应的ReflectiveKey
 static get(token: Object): ReflectiveKey {
 return _globalKeyRegistry.get(resolveForwardRef(token));
 }

 // 获取系统中已注册ReflectiveKey的个数
 static get numberOfKeys(): number { return _globalKeyRegistry.numberOfKeys; }
}

const _globalKeyRegistry = new KeyRegistry(); // 创建Key仓库

export class KeyRegistry {
 private _allKeys = new Map<Object, ReflectiveKey>();

 /**
 * 若token是ReflectiveKey类的实例，则直接返回。若_allKeys对象中包含token属性
 * 则返回token对应的ReflectiveKey对象。否则创建一个新的ReflectiveKey对象，并
 * 保存到_allKeys对象中
 */
 get(token: Object): ReflectiveKey {
 if (token instanceof ReflectiveKey) return token;

 if (this._allKeys.has(token)) {
  return this._allKeys.get(token) !;
 }

 const newKey = new ReflectiveKey(token, ReflectiveKey.numberOfKeys);
 this._allKeys.set(token, newKey);
 return newKey;
 }

 // 获取已保存ReflectiveKey的个数
 get numberOfKeys(): number { return this._allKeys.size; }
}

分析完 resolve() 方法的输入参数和返回类型，我们来看一下该方法内部的具体实现：

export function resolveReflectiveProviders(providers: Provider[])
 : ResolvedReflectiveProvider[] {
  const normalized = _normalizeProviders(providers, []); // 步骤一
  const resolved = normalized.map(resolveReflectiveProvider); // 步骤二
  const resolvedProviderMap = mergeResolvedReflectiveProviders(resolved, new Map()); // 步骤三
  return Array.from(resolvedProviderMap.values()); // 步骤四
}

步骤一 —— 规范化Provider

const normalized = _normalizeProviders(providers, []);

// 规范化Providers
function _normalizeProviders(providers: Provider[], res: Provider[]): Provider[] {
 providers.forEach(b => {
  // providers: [Type] => providers: [{provide: Type, useClass: Type }]
  if (b instanceof Type) {
   res.push({provide: b, useClass: b});
  } else if (b && typeof b == 'object' && (b as any).provide !== undefined) {
   res.push(b as NormalizedProvider);
  } else if (b instanceof Array) { // 若b是数组，则递归调用_normalizeProviders()方法
   _normalizeProviders(b, res);
  } else {
   throw invalidProviderError(b);
  }
 });
 return res;
}

interface NormalizedProvider extends TypeProvider, ValueProvider, ClassProvider,
  ExistingProvider, FactoryProvider {}

步骤二 —— 转化NormalizedProvider为ResolvedReflectiveProvider

const resolved = normalized.map(resolveReflectiveProvider);

// 解析NormalizedProvider为ResolvedReflectiveProvider
function resolveReflectiveProvider(provider: NormalizedProvider): ResolvedReflectiveProvider {
 return new ResolvedReflectiveProvider_(
   ReflectiveKey.get(provider.provide), [resolveReflectiveFactory(provider)],
   provider.multi || false);
}

// 用于创建已解析的Provider实例
export class ResolvedReflectiveProvider_ implements ResolvedReflectiveProvider {
 constructor(
   public key: ReflectiveKey,
   public resolvedFactories: ResolvedReflectiveFactory[],
   public multiProvider: boolean) {}

 get resolvedFactory(): ResolvedReflectiveFactory { return this.resolvedFactories[0]; }
}

// 解析NormalizedProvider对象，创建ResolvedReflectiveFactory对象
function resolveReflectiveFactory(provider: NormalizedProvider): ResolvedReflectiveFactory {
 let factoryFn: Function;
 let resolvedDeps: ReflectiveDependency[];
 if (provider.useClass) {
  // { provide: ApiService, useClass: ApiService }
  const useClass = resolveForwardRef(provider.useClass);
  factoryFn = reflector.factory(useClass);
  resolvedDeps = _dependenciesFor(useClass);
 } else if (provider.useExisting) {
  // { provide: 'ApiServiceAlias', useExisting: ApiService }
  factoryFn = (aliasInstance: any) => aliasInstance;
  resolvedDeps = [ReflectiveDependency.fromKey(ReflectiveKey.get(provider.useExisting))];
 } else if (provider.useFactory) {
  // { provide: APP_INITIALIZER, useFactory: configFactory, deps: [AppConfig],
  //   multi: true }
  factoryFn = provider.useFactory;
  resolvedDeps = constructDependencies(provider.useFactory, provider.deps);
 } else {
  // { provide: 'API_URL', useValue: 'http://my.api.com/v1' }
  factoryFn = () => provider.useValue;
  // const _EMPTY_LIST: any[] = [];
  resolvedDeps = _EMPTY_LIST;
 }
 return new ResolvedReflectiveFactory(factoryFn, resolvedDeps);
}

步骤三 —— 合并已解析的Provider

const resolvedProviderMap = mergeResolvedReflectiveProviders(resolved, new Map());

export function mergeResolvedReflectiveProviders(
  providers: ResolvedReflectiveProvider[],
  normalizedProvidersMap: Map<number, ResolvedReflectiveProvider>):
  Map<number, ResolvedReflectiveProvider> {
  for (let i = 0; i < providers.length; i++) {
  const provider = providers[i];
   // 从normalizedProvidersMap对象中获取key.id对应的ResolvedReflectiveProvider对象
  const existing = normalizedProvidersMap.get(provider.key.id);
  if (existing) {
    // 如果当前的provider不是multi provider，则抛出异常
   if (provider.multiProvider !== existing.multiProvider) {
    throw mixingMultiProvidersWithRegularProvidersError(existing, provider);
   }
   // 如果当前的provider是multi provider，则把当前provider的resolvedFactories
   // 列表中的每一项添加到已存在的provider对象的resolvedFactories列表中。
   if (provider.multiProvider) {
    for (let j = 0; j < provider.resolvedFactories.length; j++) {
     existing.resolvedFactories.push(provider.resolvedFactories[j]);
    }
   } else {
    // 如果当前的provider不是multi provider，则覆盖已存在的provider
    normalizedProvidersMap.set(provider.key.id, provider);
   }
  } else {
   let resolvedProvider: ResolvedReflectiveProvider;
   // 如果当前的provider是multi provider，则创建一个新的ResolvedReflectiveProvider对象
   if (provider.multiProvider) {
    resolvedProvider = new ResolvedReflectiveProvider_(
      provider.key, provider.resolvedFactories.slice(), provider.multiProvider);
   } else {
    resolvedProvider = provider;
   }
   // 在normalizedProvidersMap中保存已解析的ResolvedReflectiveProvider对象
   normalizedProvidersMap.set(provider.key.id, resolvedProvider);
  }
 }
 return normalizedProvidersMap;
}

步骤四 —— 生成ResolvedReflectiveProvider[]

// resolvedProviderMap的values，创建ResolvedReflectiveProvider[]
Array.from(resolvedProviderMap.values());

/**
 * 基于一个类似数组或可迭代对象创建一个新的数组实例
 *
 * arrayLike：转换成真实数组的类数组对象或可遍历对象。
 * mapFn(可选)：如果指定了该参数，则最后生成的数组会经过该函数的加工处理后再返回。
 * thisArg(可选)：执行mapFn函数时this的值。
 */
Array.from(arrayLike[, mapFn[, thisArg]])

fromResolvedProviders()

该方法用于基于已解析的 providers 创建注入器。

static fromResolvedProviders(providers: ResolvedReflectiveProvider[], parent?: Injector):
 ReflectiveInjector {
  return new ReflectiveInjector_(providers, parent);
}

fromResolvedProviders() 使用示例

@Injectable()
class Engine {}

@Injectable()
class Car {
 constructor(public engine:Engine) {}
}

var providers = ReflectiveInjector.resolve([Car, Engine]);
var injector = ReflectiveInjector.fromResolvedProviders(providers);
expect(injector.get(Car) instanceof Car).toBe(true);

了解完 fromResolvedProviders() 方法的使用方式，接下来我们来重点分析一下 ReflectiveInjector_ 类。

ReflectiveInjector_ 类

ReflectiveInjector_ 类的属性

// 构造次数
_constructionCounter: number = 0;

// ResolvedReflectiveProvider列表
 public _providers: ResolvedReflectiveProvider[];

// 父级注入器
 public _parent: Injector|null;

// ReflectiveKey id列表
 keyIds: number[];

// 依赖对象列表
 objs: any[];

ReflectiveInjector_ 构造函数

export class ReflectiveInjector_ implements ReflectiveInjector {
 constructor(_providers: ResolvedReflectiveProvider[], _parent?: Injector) {
   this._providers = _providers;
   // 设置父级注入器
   this._parent = _parent || null;

   const len = _providers.length;

   this.keyIds = new Array(len);
   this.objs = new Array(len);

   // 初始化keyIds列表和objs对象列表
   for (let i = 0; i < len; i++) {
    this.keyIds[i] = _providers[i].key.id;
    this.objs[i] = UNDEFINED;
   }
 }
}

const UNDEFINED = new Object();

ReflectiveInjector_ 类的方法

ReflectiveInjector_ 类中的方法较多，我们只分析其中比较重要的方法，首先先根据方法的实现的功能进行分类：

1. 用于创建ReflectiveInjector注入器
2. 用于获取对象
3. 用于创建对象
4. 用于获取工厂函数依赖对象

用于创建ReflectiveInjector注入器

// 基于Provider列表并创建子注入器
resolveAndCreateChild(providers: Provider[]): ReflectiveInjector {
  const ResolvedReflectiveProviders = ReflectiveInjector.resolve(providers);
  return this.createChildFromResolved(ResolvedReflectiveProviders);
}

// 基于已解析的ResolvedReflectiveProvider列表，创建子注入器
createChildFromResolved(providers: ResolvedReflectiveProvider[]): ReflectiveInjector {
  const inj = new ReflectiveInjector_(providers);
  inj._parent = this;
  return inj;
}

用于获取对象

// 获取当前注入器的父级注入器
get parent(): Injector|null { return this._parent; }

// 获取token对应的依赖对象
get(token: any, notFoundValue: any = THROW_IF_NOT_FOUND): any {
  return this._getByKey(ReflectiveKey.get(token), null, notFoundValue);
}

// 根据ReflectiveKey及visibility可见性，获取对应的依赖对象
private _getByKey(key: ReflectiveKey, visibility: Self|SkipSelf|null, notFoundValue: any): any {
  // const INJECTOR_KEY = ReflectiveKey.get(Injector);
  if (key === INJECTOR_KEY) {
   return this;
  }

  // 判断该依赖对象是否使用@Self装饰器定义，表示从本级注入器获取依赖对象
  if (visibility instanceof Self) {
   return this._getByKeySelf(key, notFoundValue);

  } else {
   // 使用默认的方式获取依赖对象
   return this._getByKeyDefault(key, notFoundValue, visibility);
  }
}

// 从本级注入器获取依赖对象
 _getByKeySelf(key: ReflectiveKey, notFoundValue: any): any {
  const obj = this._getObjByKeyId(key.id);
  return (obj !== UNDEFINED) ? obj : this._throwOrNull(key, notFoundValue);
}

// 使用默认的方式获取依赖对象
_getByKeyDefault(key: ReflectiveKey, notFoundValue: any,
  visibility: Self|SkipSelf|null): any {
  let inj: Injector|null;

  // 判断该依赖对象是否使用@SkipSelf装饰器定义，表示不从本级注入器获取依赖对象
  if (visibility instanceof SkipSelf) {
   inj = this._parent;
  } else {
   inj = this;
  }

  // 从本级注入器获取依赖对象，若本级获取不到，则从父级注入器中查找
  while (inj instanceof ReflectiveInjector_) {
   const inj_ = <ReflectiveInjector_>inj;
   const obj = inj_._getObjByKeyId(key.id);
   if (obj !== UNDEFINED) return obj;
   inj = inj_._parent;
  }
  if (inj !== null) {
   return inj.get(key.token, notFoundValue);
  } else {
   return this._throwOrNull(key, notFoundValue);
  }
}

// 获取keyId对应的对象，如依赖对象未创建，则调用_new()方法创建一个，然后保存到
// this.objs对象列表中
private _getObjByKeyId(keyId: number): any {
  for (let i = 0; i < this.keyIds.length; i++) {
   if (this.keyIds[i] === keyId) {
    // const UNDEFINED = new Object();
    if (this.objs[i] === UNDEFINED) {
     this.objs[i] = this._new(this._providers[i]);
    }
    return this.objs[i];
   }
  }
  return UNDEFINED;
}

用于创建对象

// 创建依赖对象
_new(provider: ResolvedReflectiveProvider): any {
  // 判断是否存在循环依赖
  if (this._constructionCounter++ > this._getMaxNumberOfObjects()) {
   throw cyclicDependencyError(this, provider.key);
  }
  return this._instantiateProvider(provider);
}

// 获取最大的对象个数
private _getMaxNumberOfObjects(): number { return this.objs.length; }

// 根据已解析的provider创建依赖对象。若是multi provider则，循环创建multi provider对象。
private _instantiateProvider(provider: ResolvedReflectiveProvider): any {
  if (provider.multiProvider) {
   const res = new Array(provider.resolvedFactories.length);
   for (let i = 0; i < provider.resolvedFactories.length; ++i) {
    res[i] = this._instantiate(provider, provider.resolvedFactories[i]);
   }
   return res;
  } else {
   return this._instantiate(provider, provider.resolvedFactories[0]);
  }
}

// 根据已解析的provider和已解析的工厂创建依赖对象
private _instantiate(
   provider: ResolvedReflectiveProvider,
   ResolvedReflectiveFactory: ResolvedReflectiveFactory): any {
  // 获取对象工厂函数
  const factory = ResolvedReflectiveFactory.factory;

  // 获取工厂函数所依赖的对象列表
  let deps: any[];
  try {
   deps = ResolvedReflectiveFactory.dependencies
       .map(dep => this._getByReflectiveDependency(dep));
  } catch (e) {
   if (e.addKey) {
    e.addKey(this, provider.key);
   }
   throw e;
  }

  // 调用对象工厂函数创建依赖对象
  let obj: any;
  try {
   obj = factory(...deps);
  } catch (e) {
   throw instantiationError(this, e, e.stack, provider.key);
  }
  return obj;
 }

用于获取工厂函数依赖对象

// 若通过@Optional装饰器定义该依赖对象，表示该依赖对象是可选的，当获取不到时返回null。
private _getByReflectiveDependency(dep: ReflectiveDependency): any {
  return this._getByKey(dep.key, dep.visibility, dep.optional ? null : THROW_IF_NOT_FOUND);
}

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