Question

如何使下面的函数返回一个promise，以便我可以在调用此函数的Page中正确处理它？</ p>

getUploads() {
    const rootDef = this.db.database.ref();
    const uploadsRef = rootDef.child('userUploads').orderByChild('time');
    const userRef = rootDef.child("userProfile");
    var uploads = [];

    uploadsRef.once("value").then((uploadSnaps) => {

      uploadSnaps.forEach((uploadSnap) => {

        var upload = uploadSnap.val();

        userRef.child(uploadSnap.val().user).once("value").then((userSnap) => {
          upload.displayName = userSnap.val().displayName;
          upload.avatar = userSnap.val().avatar;
          uploads.push(upload);
        });

      });

    });

    return uploads;
}

我尝试了下面的内容，但它显示错误。我该如何修改？

return new Promise((resolve, reject) => {
  resolve(uploads);
});

我将调用此方法，如下所示。

this.db.getUploads().then((uploads) => {
  this.allUploads = uploads;
  console.log(this.allUploads);
});

Answer 1

我认为您可以使用

包围方法的内容

getUploads() {
    return new Promise((resolve, reject) => {
        // content of method
        resolve(uploads); // instead of "return uploads"
    });
}

Answer 2

您可以使用Promise.resolve：

Promise.resolve（value）方法返回一个Promise对象用给定的值解决。如果该值是可以的（即具有a ＆＃34;然后＆＃34;方法），返回的承诺将＆＃34;跟随＆＃34;那个，采用其最终状态;否则返回的承诺将是实现了价值。

如此简单：

return Promise.resolve(uploads);

但是代码的问题在于，在调用uploadsRef.once("value").then(...)之前返回值您应该只返回then：

的结果

return uploadsRef.once("value").then((uploadSnaps) => {
    ...
    return uploads
};

Answer 3

这应该正确处理异步调用并返回所有上传：

getUploads() {
    const rootDef = this.db.database.ref();
    const uploadsRef = rootDef.child('userUploads').orderByChild('time');
    const userRef = rootDef.child("userProfile");

    return uploadsRef.once("value").then((uploadSnaps) => {

      return Promise.all(uploadSnaps.map(uploadSnap => {
        var upload = uploadSnap.val();

        return userRef.child(uploadSnap.val().user).once("value").then((userSnap) => {
          upload.displayName = userSnap.val().displayName;
          upload.avatar = userSnap.val().avatar;
          return upload;
        });        
      }));
    });
}

将结果作为Typescript中的承诺返回

3 个答案: