如何在JavaScript中使用Promise [英] How to use promises in javascript

问题描述

我正在使用Typescript在Cordova中为Android开发一个应用程序，并且将其与Webpack捆绑在一起.我不太了解如何在Javascript中执行异步任务.我看到了一些有关回调和Promise的教程，但是对我来说仍然很困惑，我无法设法使其适应我的代码.我不知道我的问题是来自代码本身，还是来自Typescript/Cordova/Webpack.

I am making an app for Android in Cordova, with Typescript, and I bundle with Webpack. I don't understand very well how to do asynchronous tasks in Javascript. I saw several tutorials on callbacks and promises, but it's still very confused for me and I don't manage to adapt it to my code. I don't know if my problem comes from my code itself, or from Typescript/Cordova/Webpack.

我创建了一个函数 getAllSMS ，该函数返回一个大对象，并且需要一些时间来执行.我知道该功能有效，因为如果我尝试在检查器的控制台中打印 allSMS 变量，则该变量首先为空，然后在1秒钟内填充.(我认为检查员的行为很奇怪，但确实如此).

I created a function getAllSMS that return a big object, and takes some time to execute. I know the function works because if I try to print the allSMS variable in the inspector's console, it's empty at first and then filled in like 1 second. (I think that behavior of the inspector is quite strange, but it really does that).

我看到我可以用promises来执行异步任务，但是我在理解它的工作方式时遇到了一些问题.我看过几本教程，但我仍然不明白如何将其应用到我的代码中.

I saw I can do asynchronous tasks with promises, but I have some problems to understand how it works. I have seen several tutorials but I still don't understand how to apply it to my code.

谢谢

:这是将我的getAllSMS函数转换为Promise:

: Here's my getAllSMS function converted to a promise :

public getAllSMS() {
    return new Promise(//return a promise
        (resolve,reject)=>{
            if (SMS) {
                SMS.listSMS(this.filters, function (data) {
                    resolve(data);//added this, resolve promise
                }, function (err) {
                    console.log('error list sms: ' + err);
                    reject(err);//added this reject promise
                });
            }else{
                resolve([]);//added this, resolving to empty array?
            }
        }
    ).then(
        data=>{
            let contacts = {};
                            for (let key in data) {
                                if ((data[key].address).length > 7 && (data[key].address).match("[0-9]+")) {
                                    let date = SMSManager.convertUnixDate(data[key].date); // on converti le format de date de listSMS
                                    if (contacts.hasOwnProperty(data[key].address)) {
                                        Object.defineProperty(contacts[data[key].address], data[key]._id, {
                                            value: {
                                                "body": data[key].body,
                                                "date": date
                                            }
                                        });
                                    } else {
                                        let myid = String(data[key]._id);
                                        Object.defineProperty(contacts, data[key].address, {
                                            value: {
                                                "000": {
                                                    "body": data[key].body,
                                                    "date": date
                                                }
                                            }
                                        });
                                    }
                                }
                            }
            return contacts;
        }
    );
}

我这样调用我的函数，但是我的for in循环仍未执行，尽管我的变量似乎已正确加载.

EDIT 2 : I call my function like that, but my for in loop is still not executed , although my variable seems correctly loaded.

    sms.getAllSMS().then( allSMS => {
        console.log('allSMS');
        console.log(allSMS);
        console.log('then is readed'); // this is readed
        for (let key in allSMS) {
            console.log(allSMS[key]); // this is not executed
        }
    }).catch(
        error => console.warn("something is wrong")
    );

这是控制台的屏幕截图(控制台中的对象"为allSMS).在对象"旁边有一个小"i"字样:下面的值现在才被求值".

Here's a screenshot of the console ('Object' in the console is allSMS). Next to "Object" there's a little "i" that says : "Value below was just evaluated right now".

推荐答案

我怀疑sms.getAllSMS返回了Promise，因此您可以尝试以下操作:

I suspect sms.getAllSMS returns a promise so you can try the following:

sms.getAllSMS()
.then(
  allSMS => {
    for (let key in allSMS) {
      console.log(allSMS[key]);
    }
  }
)
.catch(
  error=>console.warn("something went wrong, error is:",error)
)

您的getAllSMS不返回任何内容，请确保它返回了一个Promise:

Your getAllSMS does not return anything, make sure it returns a promise:

public getAllSMS(): object {
  return new Promise(//return a promise
    (resolve,reject)=>{
      if (SMS) {
        SMS.listSMS(this.filters, function (data) {
          resolve(data);//added this, resolve promise

        }, function (err) {
          console.log('error list sms: ' + err);
          reject(err);//added this reject promise
        });
      }else{
        resolve([]);//added this, resolving to empty array?
      }
    }
  ).then(
    data=>{
      let contacts = {};
      data.forEach(
        item=>{
          if ((item.address).length > 7 && (item.address).match("[0-9]+")) {
            let date = SMSManager.convertUnixDate(item.date); // on converti le format de date de listSMS
            if (contacts.hasOwnProperty(item.address)) {
              Object.defineProperty(contacts[item.address], item._id, {
                value: {
                  "body": item.body,
                  "date": date
                }
              });
            } else {
              Object.defineProperty(contacts, item.address, {
                value: {
                  "000": {
                    "body": item.body,
                    "date": date
                  }
                }
              });
            }
          };        
        }
      );
      return contacts;
    }
  );
}

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