angular源码分析之StaticInjector

摘要：上一篇说到了平台实例在初始化的时候会创建根注入器那现在就一起看看注入器是如何创建的又是如何工作的所有引用的代码都被简化了创建注入器程序初始化时调用的创建根注入器的静态方法调用此方法会返回一个类型的实例也就是注入器类注入器的构造函数在初始化过

上一篇说到了平台实例在初始化的时候会创建根注入器,那现在就一起看看注入器是如何创建的,又是如何工作的.(所有引用的代码都被简化了)

程序初始化时调用的创建根注入器的静态方法:

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abstract class Injector{
  static create(options: StaticProvider[]|{providers: StaticProvider[], parent?: Injector, name?: string},parent?: Injector): Injector {
      if (Array.isArray(options)) {
          return new StaticInjector(options, parent);
      } else {
      return new StaticInjector(options.providers, options.parent, options.name || null);
  }
}

调用此方法会返回一个StaticInjector类型的实例(也就是注入器).

StaticInjector类

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export class StaticInjector implements Injector {
  readonly parent: Injector;
  readonly source: string|null;
  private _records: Map;
  constructor(providers: StaticProvider[], parent: Injector = NULL_INJECTOR, source: string|null = null) {
    this.parent = parent;
    this.source = source;
    const records = this._records = new Map();
    records.set(Injector, {token: Injector, fn: IDENT, deps: EMPTY, value: this, useNew: false});
    records.set(INJECTOR, {token: INJECTOR, fn: IDENT, deps: EMPTY, value: this, useNew: false});
    recursivelyProcessProviders(records, providers);
  }
}

注入器的构造函数在初始化过程中的操作:

设置当前注入器的父注入器

设置注入器的源

新建注册表(_records属性,是一个Map类型)

将参数providers全部添加到注册表中

向注册表中添加服务调用了recursivelyProcessProviders函数

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const EMPTY = [];
const MULTI_PROVIDER_FN = function (): any[] { return Array.prototype.slice.call(arguments) };
function recursivelyProcessProviders(records: Map, provider: StaticProvider) {
    if (provider instanceof Array) {
      for (let i = 0; i < provider.length; i++) {
        recursivelyProcessProviders(records, provider[i]);
      }
    } else (provider && typeof provider === "object" && provider.provide) {
      let token = resolveForwardRef(provider.provide);// 方法`resolveForwardRef`的作用可能是向前兼容,可以忽略
      const resolvedProvider = resolveProvider(provider);
      if (provider.multi === true) {
        let multiProvider: Record | undefined = records.get(token);
        if (multiProvider) {
          if (multiProvider.fn !== MULTI_PROVIDER_FN) {
            throw multiProviderMixError(token);
          }
        } else {
          records.set(token, multiProvider = {
            token: provider.provide,
            deps: [],
            useNew: false, // 这个值在后面获取依赖实例的时候会用到,当做判断条件
            fn: MULTI_PROVIDER_FN,
            value: EMPTY // 这个值在后面获取依赖实例的时候会用到,当做判断条件
          });
        }
        token = provider;
        multiProvider.deps.push({ token, options: OptionFlags.Default });
      }
      records.set(token, resolvedProvider);
    }
}

recursivelyProcessProviders函数具体的执行过程:

如果provider是个数组,那就遍历后依次调用此方法.

如果provider是个对象:

1 获取token

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  let token = resolveForwardRef(provider.provide);

2 调用resolveProvider方法处理服务中可能出现的属性和依赖,返回一个Record对象,此对象会作为token的值

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function resolveProvider(provider: SupportedProvider): Record {
  const deps = computeDeps(provider);
  let fn: Function = function (value) { return value };
  let value: any = [];
  // useUew用来标识是否需要 new 
  let useNew: boolean = false;
  let provide = resolveForwardRef(provider.provide);
  if (USE_VALUE in provider) {
    value = provider.useValue;
  } else if (provider.useFactory) {
    fn = provider.useFactory;
  } else if (provider.useExisting) {
    //do nothing
  } else if (provider.useClass) {
    useNew = true;
    fn = resolveForwardRef(provider.useClass);
  } else if (typeof provide == "function") {
    useNew = true;
    fn = provide;
  } else {
    throw staticError("StaticProvider does not have [useValue|useFactory|useExisting|useClass] or [provide] is not newable", provider);
  }
  return { deps, fn, useNew, value }; // provider中不同的属性会返回包含不同值的对象
}

这个方法会先调用computeDeps函数处理服务需要的依赖,它将useExisting类型的服务也转换成deps,最后返回[{ token, OptionFlags }]形式的数组(OptionFlags是枚举常量)

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  function computeDeps(provider: StaticProvider): DependencyRecord[] {
    let deps: DependencyRecord[] = EMPTY;
    const providerDeps: any[] = provider.deps;
    if (providerDeps && providerDeps.length) {
      deps = [];
      for (let i = 0; i < providerDeps.length; i++) {
        let options = OptionFlags.Default;
        let token = resolveForwardRef(providerDeps[i]);
        deps.push({ token, options });
      }
    } else if ((provider as ExistingProvider).useExisting) {
      const token = resolveForwardRef((provider as ExistingProvider).useExisting);
      deps = [{ token, options: OptionFlags.Default }];
    } 
    return deps;
  }

resolveProvider函数最终返回的Record对象有一个缺省值:

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{
  deps:[], // 包含依赖时 [{ token, options },{ token, options }]
  fn:function(value) { return value },
  useNew:false,
  value:[]
}

执行过程中会根据provider不同的属性修改Record对象的变量为不同的值:

useValue : 修改value为useValue的值

useFactory : 修改fn为对应的函数

useClass 或 typeof provide == "function"(令牌为一个函数时) : 修改fn为对应的函数,并设置useNew为true

useExisting : 不做修改,直接使用默认值

3 如果是多处理服务(multi:ture)且为首次注册,那么在注册表中额外注册一个占位的Record

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records.set(token, multiProvider = {
  token: provider.provide,
  deps: [],
  useNew: false,
  fn: MULTI_PROVIDER_FN,
  value: EMPTY
});

4 非多处理服务以token为键,多处理服务以provider对象为键,返回的Record对象为值,存入注册表records中

服务注册完,下一步就是怎么从注入器中获取服务的实例了,这会调用StaticInjector的get方法

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export class StaticInjector implements Injector {
  get(token: any, notFoundValue?: any, flags: InjectFlags = InjectFlags.Default): any {
    // 获取token对应的record
    const record = this._records.get(token);
    return resolveToken(token, record, this._records, this.parent, notFoundValue, flags);
}

get方法调用了resolveToken函数,这个函数会返回token对应的实例(就是被注入的对象)

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const EMPTY = [];
const CIRCULAR = IDENT;
const IDENT = function (value: T): T { return value };
function resolveToken(token: any, record: Record | undefined, records: Map, parent: Injector,
  notFoundValue: any, flags: InjectFlags): any {
  let value;
  if (record && !(flags & InjectFlags.SkipSelf)) {
    value = record.value;
    if (value == CIRCULAR) {
      throw Error(NO_NEW_LINE + "Circular dependency");
    } else if (value === EMPTY) {
      record.value = CIRCULAR;
      let obj = undefined;
      let useNew = record.useNew;
      let fn = record.fn;
      let depRecords = record.deps;
      let deps = EMPTY;
      if (depRecords.length) {
        deps = [];
        for (let i = 0; i < depRecords.length; i++) {
          const depRecord: DependencyRecord = depRecords[i];
          const options = depRecord.options;
          const childRecord = options & OptionFlags.CheckSelf ? records.get(depRecord.token) : undefined;
          deps.push(tryResolveToken(
            depRecord.token,
            childRecord,
            records,
            !childRecord && !(options & OptionFlags.CheckParent) ? NULL_INJECTOR : parent,
            options & OptionFlags.Optional ? null : Injector.THROW_IF_NOT_FOUND,
            InjectFlags.Default));
        }
      }
      record.value = value = useNew ? new (fn as any)(...deps) : fn.apply(obj, deps);
    }
  } else if (!(flags & InjectFlags.Self)) {
    value = parent.get(token, notFoundValue, InjectFlags.Default);
  }
  return value;
}

函数中会先判断当前请求的token是否存在,如果不存在则去当前注入器的父注入器中寻找,如果存在:

获取token对应的record

判断record.value是否为[](非useValue类型的服务/多处理服务的默认值是[]):

ture : 非useValue类型的服务/多处理服务或此服务没有被创建过

查看是否包含依赖,包含则优先创建依赖的实例,也是调用这个函数

根据record.fn创建当前token对应的实例并更新record.value(这里需要根据record.useNew来判断是否需要用new来实例化,比如useFactory类型就不需要new,而useExisting更是直接返回了deps)

返回这个值

false : useValue类型的服务或此服务已经被创建过

直接返回这个值