Question

我正在尝试使用Promise.all和map而不是forEach循环，因此任务可以是异步的。 Promise.all数组中的所有promise都会被执行并得到解决。代码如下所示：

loadDistances() {
    //return new Promise((resolve, reject) => {
      let rrr;
      let arr = [];
      this.geolocation.getCurrentPosition().then((resp) => {            
          // resp.coords.latitude
          rrr = resp;
          console.log(rrr + "              rrrrrrrrrrrrrrrrrrrrrrrrrr");

          setTimeout(() => {
            this.distancelist = this.af.list('/profiles/stylists');

            let x = 0;
            this.subscription6 = this.distancelist.subscribe(items => {

              let mapped = items.map((item) => {
                return new Promise(resolve => {
                  let rr;
                  //console.log(JSON.stringify(item) + "               *((*&*&*&*&^&*&*&*(&*(&*&*(&(&(&*(              :::" + x);
                  if(item.address == "") {
                    /*if(!item.picURL) {
                      item.picURL = 'assets/blankprof.png';
                    }*/
                    //arr.push({'pic':item.picURL, 'salon':item.username, 'distance':"No Address"});
                    //x++;
                  }
                  else {
                    console.log(item.address + " is the address empty??????");
                    this.nativeGeocoder.forwardGeocode(item.address)
                      .then((coordinates: NativeGeocoderForwardResult) => {
                        console.log("I AM IN THE GEOCODING ***&&*&*&*&*");
                          rr = this.round(this.distance(coordinates.latitude, coordinates.longitude, rrr.coords.latitude, rrr.coords.longitude, "M"), 1);
                          if(!item.picURL) {
                            item.picURL = 'assets/blankprof.png';
                          }
                          arr.push({'pic':item.picURL, 'salon':item.username, 'distance':rr});
                          console.log("push to the array of results");
                          //x++;
                          /*console.log(items.length + "         length   /    x:        " + x);
                          if(items.length - x == 1) {
                            console.log("getting resolved in geocoder ^&^&^&&^^&^&^&");
                            resolve(arr);
                          }*/
                          resolve();
                        }).catch(e => {
                          console.log(e.message + " caught this error");
                          /*x++;
                          if(items.length - x == 1) {
                            resolve(arr);
                          }*/
                          resolve();
                        })
                  }

                })
              });

              let results = Promise.all(mapped);
              results.then(() => {
                console.log(JSON.stringify(arr) + " :FOSIEJO:SFJ::EFIJSEFIJS:EFJS:IO THIS IODIOSJ:FDSIJ :DIS");
                arr.sort(function(a,b) {
                  return a.distance - b.distance;
                });

                this.distances = arr.slice();
              })

            });//);
          }, 1500)




      /*}).catch((error) => {
        this.diagnostic.switchToLocationSettings();
        console.log('Error getting location', error.message);
        resolve();
      });*/

    });


  }

控制台输出是：

[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:27]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:27]  console.log: push to the array of results 
[12:38:28]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:28]  console.log: push to the array of results 
[12:38:29]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:29]  console.log: push to the array of results 
[12:38:29]  console.log: I AM IN THE GEOCODING ***&&*&*&*&* 
[12:38:29]  console.log: push to the array of results

交替的消息使它看起来像是有效的，所有的承诺都得到了解决。但当我Promise.all时，这条线永远不会出现在控制台中：

console.log(JSON.stringify(arr) + " :FOSIEJO:SFJ::EFIJSEFIJS:EFJS:IO THIS IODIOSJ:FDSIJ :DIS");

所以它未达到then Promise.all的{{1}}。

Answer 1

您的代码路径无法解决，具体而言：

if(item.address == "") {
  /*if(!item.picURL) {
    item.picURL = 'assets/blankprof.png';
  }*/
  //arr.push({'pic':item.picURL, 'salon':item.username, 'distance':"No Address"});
  //x++;
}

Answer 2

在创建新的Promise时，很容易陷入像@ andy-gaskell所指出的僵局。为了避免这种情况，你最好做其中一个;所有这些都可能引发一个不可能的错误或返回1.

function promiseWithTryCatch() {
  return new Promise((resolve, reject) => {
    try {
      let result;
      // your code

      resolve(result);
    }
    catch(ex) {
      reject(ex);
    }
  })
}

function promiseWithResolve() {
  return Promise.resolve()
    .then(() => {
      let result;
      // your code

      return result; 
    })
}

async function promiseWithAsync() {
  let result;
  // your code

  return result;
}

用您的代码替换注释，并将结束结果放入变量结果中。如果您的代码包含异步代码，则最好使用相同的模式创建一个新函数并将其作为结果返回，例如：result = [promise method]

最可怕的是“新承诺”，如果任何代码无法解决或拒绝它将以“死锁”结束，这就是为什么重要的是尝试/捕获。
如果您正在使用Promise.resolve（），它将捕获内部的任何错误。但是不要在外部执行任何代码。然后，否则您必须捕获异常并返回Promise.reject（new Error（））< / LI>
最安全的是异步函数，因为任何“throw”都会返回一个Promise.reject，而“return”将返回一个Promise.resolve

使用此示例，但我必须注释掉async / await，因为此代码段工具无法处理ES2016。

function nestedPromise(num) {
  return Promise.resolve(' myNestedValue is ' + num);
}

function promiseWithTryCatch() {
  return new Promise((resolve, reject) => {
    try {
      let mynumber = 2 + 5;

      nestedPromise(mynumber)
        .then((answer) => {
          resolve(answer.trim());
        })
        .catch(ex => {
          // error handling for async-code
          reject(ex);
        })
    }
    catch(ex) {
      // error handling for sync-code
      reject(ex);
    }
  })
}

function promiseWithResolve() {
  return Promise.resolve()
    .then(() => {
      let mynumber = 2 + 5;

      return nestedPromise(mynumber);
    })
    .then((answer) => {
      // do something with the answer
      return answer.trim();
    })
}

/*
async function promiseWithAsync() {
  let mynumber = 2 + 5;
  let answer = await nestedPromise(mynumber);
  
  return answer.trim();
}
*/

promiseWithTryCatch()
  .then(answer => console.log('promiseWithTryCatch result is ' + answer))
   .catch(err => console.log('promiseWithAsync error is ' + err.message));
   
promiseWithResolve()
  .then(answer => console.log('promiseWithResolve result is ' + answer))
  .catch(err => console.log('promiseWithResolve error is ' + err.message));

/*
promiseWithAsync()
  .then(answer => console.log('promiseWithAsync result is' + answer))
  .catch(err => console.log('promiseWithAsync error is ' + err.message));
*/

然后，Promise.all结果永远不会执行

2 个答案: