返回结果作为Typescript中的承诺

Question

如何使下面的函数返回一个承诺，以便我可以在调用此函数的页面中正确处理它？

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; 
} 

我试过以下，但它显示错误。我应该如何修改？

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

我想你可以围绕你的方法的内容与

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

Answer 2

您可以使用Promise.resolve：

的Promise.resolve（值）方法返回一个对象承诺即用给定值解决了 。如果该值是可靠的（即，具有 “然后”方法），则返回的承诺将“跟随”可接受的， 采用其最终状态;否则返还的承诺将为 ，并以此值完成。

所以干脆：

return Promise.resolve(uploads); 

但随着你的代码的问题是，你返回值已调用uploadsRef.once("value").then(...)之前。
你应该简单地从then返回结果：

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

Answer 3

这应该正确处理异步调用和返回所有上传：

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; 
     });   
     })); 
    }); 
}