如何从 Firebase 存储中获取 DownloadURL

Question

我正在处理存储火力库，我正在尝试获取 URL 并将其设置为变量，不幸的是我无法做到这一点，我的代码有问题。有人可以帮帮我吗？

这是错误：

firebase.js:1 Uncaught (in promise) 错误：Reference.push 失败：第一个参数在属性“PostUsers.ImageURL.i”中包含未定义

这是代码：

console.log('Nice, The file has been successfully uploaded to the Firebase storage!');
var DownloadURL = uploadTask.snapshot.downloadURL;
var Url_File = uploadTask.snapshot.ref.getDownloadURL().then(function (URL) {
           return URL;
           });
           alert(Url_File);
                                    uploadTask.snapshot.ref.getDownloadURL().then(function (downloadURL) {
                                        console.log('File available at', downloadURL);
                                        //Url_File = downloadURL;
                                    });
                                    //getting the publication time
                                    var dayObj = new Date();
                                    var day = dayObj.getDate();
                                    var monthNames = ["January", "February", "March", "April", "May", "June",
                                        "July", "August", "September", "October", "November", "December"
                                    ];
                                    var month = monthNames[dayObj.getMonth()]
                                    var year = dayObj.getFullYear();
                                    var hour = dayObj.getHours();
                                    var minutes = dayObj.getMinutes();
                                    var seconds = dayObj.getSeconds();
                                    var GlobalUserName = <%=Session["UserId"] %>;
                                        firebase.database().ref('PostUsers/').push({
                                            ImageURL: Url_File,
                                            userAuthor: GlobalUserName,
                                            day: day,
                                            month: month,
                                            year: year,
                                            hour: hour,
                                            minutes: minutes,
                                            seconds: seconds
                                        });
                                        //console.log('the download Url of your file is: '+ downloadURL);
                                        console.log('User post successfully added to realtime data bases');
                                    });

我尝试了很多不同的方法，但是没有一个能正常工作。

提前致谢。

Answer 1

下载 URL 仅在您从 getDownloadURL() 获得的 then 回调中可用。您无法在此之外访问它。

所以解决方案是将所有需要下载 URL 的代码移到回调中：

uploadTask.snapshot.ref.getDownloadURL().then(function (URL) {
   //getting the publication time
    var dayObj = new Date();
    var day = dayObj.getDate();
    var monthNames = ["January", "February", "March", "April", "May", "June",
        "July", "August", "September", "October", "November", "December"
    ];
    var month = monthNames[dayObj.getMonth()]
    var year = dayObj.getFullYear();
    var hour = dayObj.getHours();
    var minutes = dayObj.getMinutes();
    var seconds = dayObj.getSeconds();
    var GlobalUserName = <%=Session["UserId"] %>;
    firebase.database().ref('PostUsers/').push({
        ImageURL: URL,
        userAuthor: GlobalUserName,
        day: day,
        month: month,
        year: year,
        hour: hour,
        minutes: minutes,
        seconds: seconds
    });
    console.log('User post successfully added to realtime database');
});

在处理现代 Web API 时，这是一种非常常见的模式：它们都是异步的，您的代码将不得不处理它。

有关更多信息，请参阅：

• Doug's blog posts
• Firebase use query result outside function（对于 Firebase 数据库，这是更常见的混淆原因）

Answer 2

在这段代码中：

var Url_File = uploadTask.snapshot.ref.getDownloadURL().then(function (URL) {
    return URL;
});

下载 URL 在一个名为 URL 的变量中的承诺回调中。它不在Url_File 中。 Url_File 将包含由 then() 返回的承诺。

您需要将使用下载 URL 的代码放在第一次可用的 Promise 回调中。按照您现在的方式，使用下载 URL 的代码在 URL 可用之前执行。