从Angular中的诺言返回值 [英] Return value from a promise in Angular

问题描述

我对Promises有点困惑.我在Ionic + Angular中拥有以下提供商:

I am a little bit confused with Promises. I have the following provider in Ionic + Angular:

@Injectable()
export class StorageProvider { 

  constructor( public storage:Storage ) {}

  save(keyname: string, data: any): Promise<any> {
    return this.storage.set(keyname, data);
  }

  retrieve(keyname: string): Promise<any> {
    return this.storage.get(keyname);
  }

  clear(): Promise<void> {
    return this.storage.clear();
  }

  delete(keyname): Promise<any> {
    return this.storage.remove(keyname);
  }

  isLogged() {
      return this.retrieve('user').then( value => { value ? true : false });
  }

}

在我的组件之一中:

console.log(this.storage.isLogged()); 

问题在于它返回的是Promise对象，而不是true或false

The problem is that it returns the promise object and not true or false

如果我将组件更改为:

this.storage.isLogged().then(function(data) { console.log(data)});

然后我得到undefined

推荐答案

使用console.log登录诺言将仅返回诺言对象. StorageProvider的isLogged方法应具有返回true/false，以便基础调用方将在.then成功回调中接收值.

Logging promise using console.log will just return promise object. StorageProvider's isLogged method should have return true/false so that the underlying caller will receive value inside .then success callback.

isLogged() {
  return this.retrieve('user').then( value => { 
    //this return will `return` value in chained manner
     return value ? true : false; 
  });
}

来电者

this.storage.isLogged().then(
  (value) => console.log("isLogged", value)
)

---

为什么内部return语句解决了问题?

Promise是处理异步请求的方法，它基本上具有3个可以出现在.then中的功能.

---

Why inner return statement solved the problem?

Promise is way to deal async request, and it basically has 3 function that can appear in .then.

myPromiseCall().then(successFn, errorFn, notifyFn)

promise的甜美特性是它们可以轻松地链接起来.这样相互依赖的异步代码看起来更具可读性.因此，只要有可能将一个承诺的数据传递给另一个，就可以从promiseA成功返回数据，例如，在promiseB成功时将自动提供该数据.下方

Sweet properties of promise is they can easily chained up. So that interdependent async code looks more readable. So whenever there is case of passing a data of one promise to another, so that time you returns a data from promiseA success That will automatically available in promiseB success, for eg. below

promiseA(){return promise };
promiseB(){ return promise };

promiseA().then( 
   //success fn
   (data) => { 
      //do something
      return data;
   }
).then(
   //success fn
   (promiseAData) => {
      console.log("Promise A data", promiseAData);
      //do some awesome thing based on data retrieved from promiseA

      return promiseB();
   }
).then(successFn, errorFn)

从promiseA返回数据是一项重要的任务，这就是当您返回数据时，底层的链接保证成功回调函数如何获取该数据.如果假定promiseA函数未从其成功函数返回任何内容，则链接的promiseB将得到undefined.你也发生了类似的事情.

It was important task to return a data from promiseA, that is how when you returned a data, the underlying chained promise success callback function got that data. If suppose the promiseA function didn't returned anything from its success function, the chained promiseB would get undefined. The similar thing was happening with you.

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