angular源碼分析之StaticInjector

上一篇說到了平臺實例在初始化的時候會創建根注入器,那現在就一起看看注入器是如何創建的,又是如何工作的.(所有引用的代碼都被簡化了)

創建注入器

程序初始化時調用的創建根注入器的靜態方法:

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);
  }
}

調用此方法會返回一個StaticInjector類型的實例(也就是注入器).

StaticInjector類

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

注入器的構造函數在初始化過程中的操作:

• 設置當前注入器的父注入器
• 設置注入器的源
• 新建註冊表(_records屬性,是一個Map類型)
• 將參數providers全部添加到註冊表中

向註冊表中添加服務調用了recursivelyProcessProviders函數

const EMPTY = <any[]>[];
const MULTI_PROVIDER_FN = function (): any[] { return Array.prototype.slice.call(arguments) };
function recursivelyProcessProviders(records: Map<any, Record>, 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 = <Record>{
            token: provider.provide,
            deps: [],
            useNew: false, // 這個值在後面獲取依賴實例的時候會用到,當做判斷條件
            fn: MULTI_PROVIDER_FN,
            value: EMPTY // 這個值在後面獲取依賴實例的時候會用到,當做判斷條件
          });
        }
        token = provider;
        multiProvider.deps.push({ token, options: OptionFlags.Default });
      }
      records.set(token, resolvedProvider);
    }
}

recursivelyProcessProviders函數具體的執行過程:

如果provider是個數組,那就遍歷後依次調用此方法.

如果provider是個對象:

1 獲取token

  let token = resolveForwardRef(provider.provide);

2 調用resolveProvider方法處理服務中可能出現的屬性和依賴,返回一個Record對象,此對象會作爲token的值<!-- (useValue,useClass,deps,useExisting,useFactory) -->

function resolveProvider(provider: SupportedProvider): Record {
  const deps = computeDeps(provider);
  let fn: Function = function (value) { return value };
  let value: any = [];
  // useUew用來標識是否需要 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中不同的屬性會返回包含不同值的對象
}

這個方法會先調用computeDeps函數處理服務需要的依賴,它將useExisting類型的服務也轉換成deps,最後返回[{ token, OptionFlags }]形式的數組(OptionFlags是枚舉常量)

  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函數最終返回的Record對象有一個缺省值:

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

執行過程中會根據provider不同的屬性修改Record對象的變量爲不同的值:

• useValue : 修改value爲useValue的值
• useFactory : 修改fn爲對應的函數
• useClass 或 typeof provide == 'function'(令牌爲一個函數時) : 修改fn爲對應的函數,並設置useNew爲true
• useExisting : 不做修改,直接使用默認值

3 如果是多處理服務(multi:ture)且爲首次註冊,那麼在註冊表中額外註冊一個佔位的Record

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

4 非多處理服務以token爲鍵,多處理服務以provider對象爲鍵,返回的Record對象爲值,存入註冊表records中

---

從注入器中獲取實例

服務註冊完,下一步就是怎麼從注入器中獲取服務的實例了,這會調用StaticInjector的get方法

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

get方法調用了resolveToken函數,這個函數會返回token對應的實例(就是被注入的對象)

const EMPTY = <any[]>[];
const CIRCULAR = IDENT;
const IDENT = function <T>(value: T): T { return value };
function resolveToken(token: any, record: Record | undefined, records: Map<any, Record>, 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;
}

函數中會先判斷當前請求的token是否存在,如果不存在則去當前注入器的父注入器中尋找,如果存在:

獲取token對應的record

判斷record.value是否爲[](非useValue類型的服務/多處理服務的默認值是[]):

ture : 非useValue類型的服務/多處理服務或此服務沒有被創建過

• 查看是否包含依賴,包含則優先創建依賴的實例,也是調用這個函數
• 根據record.fn創建當前token對應的實例並更新record.value(這裏需要根據record.useNew來判斷是否需要用new來實例化,比如useFactory類型就不需要new,而useExisting更是直接返回了deps)
• 返回這個值

false : useValue類型的服務或此服務已經被創建過

• 直接返回這個值