在Log.d（TAG，“ downloadImgsUriArrayList：$ downloadImagesUriList”）之后，迭代器的.addOnCompleteListener上载代码已执行

Question

Log.d(TAG, "downloadImgsUriArrayList:$downloadImagesUriList ")上载代码在迭代器的.addOnCompleteListener导致var downloadImagesUriList: MutableList<String> = mutableListOf()为空之前执行。

如何解决此问题？

var downloadImagesUriList: MutableList<String>  = mutableListOf()

//Upload images button
upload_imgs_button.setOnClickListener {
    Log.d(TAG, "uploadImgsPath: $uploadImgsPath");
        //Upload images to firebase storage
    if (uploadImgsPath != null) {

        for (uriForUpload in imagesList) {
            val imgNameUUID = UUID.randomUUID().toString()
            val withAppendedPath =
                Uri.withAppendedPath(uriForUpload, imgNameUUID).toString()
            var imgFileRef = uploadImgsPath!!.child(withAppendedPath)
            imgFileRef.putFile(uriForUpload)
                .continueWithTask {
                    if (!it.isSuccessful) {
                        it.exception?.let {
                            throw it
                        }
                    }
                    imgFileRef.downloadUrl
                }.addOnCompleteListener {
                    if (it.isSuccessful) {
                        var uriForDownload = it.result.toString()
                        Log.d(TAG, "uriForDownload:$uriForDownload ");
                        downloadImagesUriList.add(uriForDownload)
                    }
                }

        }

//The process will run here before downloadImagesUriList.add(uriForDownload) was excuted.
//How to solve this problem?
        Log.d(TAG, "downloadImgsUriArrayList:$downloadImagesUriList ");
        var uploadedImages = UploadedImages(
            categoryDesignID, user!!.uid,
            downloadImagesUriList, Timestamp.now()
        )
        //Save uploaded images path info to firestore
        firestore.collection("uploadedImages").add(uploadedImages)
            .addOnSuccessListener {
                Toast.makeText(context, "Upload successful", Toast.LENGTH_LONG)
                    .show()
            }
    } else {
        Toast.makeText(context,"Please choose a category",Toast.LENGTH_LONG).show()
    }

Answer 1

Firebase API，与大多数现代云API一样，是异步的，因为它们与（可能很耗时的）服务器交互。客户端允许您的主代码继续运行，而不是在云交互发生时阻塞您的主线程，然后在云API完成工作后再调用。

在您的代码中，这意味着Log.d(TAG, "downloadImgsUriArrayList:$downloadImagesUriList ");运行之前 imgFileRef.downloadUrl。这解释了为什么add(uploadedImages)没有将任何内容写入数据库：尚未确定下载URL，因为仍在进行上载。

为了解决此问题，所有需要上载下载URL的代码都必须是该上载的完成侦听器[[inside。

最简单的方法是，每次文件上传完成时添加一个新URL：

for (uriForUpload in imagesList) {
    val imgNameUUID = UUID.randomUUID().toString()
    val withAppendedPath =
        Uri.withAppendedPath(uriForUpload, imgNameUUID).toString()
    var imgFileRef = uploadImgsPath!!.child(withAppendedPath)
    imgFileRef.putFile(uriForUpload)
        .continueWithTask {
            if (!it.isSuccessful) {
                it.exception?.let {
                    throw it
                }
            }
            imgFileRef.downloadUrl
        }.addOnCompleteListener {
            if (it.isSuccessful) {
                var uriForDownload = it.result.toString()
                Log.d(TAG, "uriForDownload:$uriForDownload ");
                downloadImagesUriList.add(uriForDownload)

                Log.d(TAG, "downloadImgsUriArrayList:$downloadImagesUriList ");
                var uploadedImages = UploadedImages(
                    categoryDesignID, user!!.uid,
                    downloadImagesUriList, Timestamp.now()
                )
                //Save uploaded images path info to firestore
                firestore.collection("uploadedImages").add(uploadedImages)
            }
        }
}

以上将在每次上传完成时创建一个新文档。每个文件完成后，您将
任一想要更新同一文档。或等到最后一个文件上传之后，再创建文档。可以通过多种方法进行操作，但是一种简单的方法是简单地对完成上传的文件数进行计数：
var uploadedCount = 0
for (uriForUpload in imagesList) {
    ...

        }.addOnCompleteListener {
            if (it.isSuccessful) {
                var uriForDownload = it.result.toString()
                Log.d(TAG, "uriForDownload:$uriForDownload ");
                downloadImagesUriList.add(uriForDownload)
                if (uploadedCount++ >= imagesList.size) {
                    Log.d(TAG, "downloadImgsUriArrayList:$downloadImagesUriList ");
                    var uploadedImages = UploadedImages(
                        categoryDesignID, user!!.uid,
                        downloadImagesUriList, Timestamp.now()
                    )
                    //Save uploaded images path info to firestore
                    firestore.collection("uploadedImages").add(uploadedImages)
                }
    ...
}