使用内容sheme时,保存文件的正确方法是什么? [英] Whats the correct way to save a file when using the Content sheme?
问题描述
我使用以下意图允许用户选择文件的保存位置:
I use the following intent to allow the user to choose where a file should be saved:
// https://developer.android.com/guide/topics/providers/document-provider
var intent = new Intent(Intent.ActionCreateDocument);
intent.AddCategory(Intent.CategoryOpenable);
intent.SetType("image/png");
intent.PutExtra(Intent.ExtraTitle, "myfile");
StartActivityForResult(Intent.CreateChooser(intent, "Select Save Location"), 43);
它将创建文件并返回文件的uri:
It creates the file and returns the file's uri:
content://com.android.providers.downloads.documents/document/436
但是现在我被绞死了,因为文档的该部分以
But now I am left hanging because that section of the documentation ends with
创建新文档后,可以在onActivityResult()中获取其URI,以便您可以继续对其进行写操作.
After you create a new document you can get its URI in onActivityResult() so that you can continue to write to it.
我不知道该怎么做.由于我的结果是使用content
方案,因此我不能只将其像普通的Java.IO.File
一样对待并将其写入磁盘.那么如何将文件保存到我拥有的内容uri中?
And I don't know how to do that. Since my result is using the content
scheme I can't just treat it like a regular Java.IO.File
and write it to the disk. So how do I save a file to the content uri that I have?
推荐答案
当您在OnActivityResult
中重新获得内容Uri时,您对该Uri具有临时权限(因此具有写访问权),因此您可以打开一个包裹-处于写入模式的基于文件的描述符,请从该包裹的文件描述符创建输出流,然后将其写入所需的任何内容.
When you get the content Uri back in the OnActivityResult
you have temporary permission (and thus write access) to that Uri, so you can open a parcel-based file descriptor in write-mode, create a output stream from that parcel's file descriptor and write whatever you need to it.
protected async override void OnActivityResult(int requestCode, [GeneratedEnum] Result resultCode, Intent data)
{
if (resultCode == Result.Ok && requestCode == 43)
{
var buffer = new byte[1024];
using (var pFD = ContentResolver.OpenFileDescriptor(data.Data, "w"))
using (var outputSteam = new FileOutputStream(pFD.FileDescriptor))
using (var inputStream = Assets.Open("somePicture.png"))
{
while (inputStream.CanRead && inputStream.IsDataAvailable())
{
var readCount = await inputStream.ReadAsync(buffer, 0, buffer.Length);
await outputSteam.WriteAsync(buffer, 0, readCount);
}
}
}
base.OnActivityResult(requestCode, resultCode, data);
}
更新(性能):
仅供参考,如果要保存/流式传输大文件,请避免在流上读取和写入异步版本,而只需旋转一个线程(或通过Task.Run从线程池使用一个线程).
Update (performance):
Just an FYI, if you are saving/streaming large files avoid the async versions of the Read and Write on the streams and just spin up a single thread (or use one from the threadpool via Task.Run).
注意:由于所有异步/等待开销,这将总是 更快,并且有点像我通常会做的那样(通常快2倍(+ ),具体取决于文件大小.
Note: This will always be faster due all the async/await overhead and is kind-of like normally how I would do it (typically faster by 2x(+) based upon file size).
if (resultCode == Result.Ok && requestCode == 43)
{
await Task.Run(() =>
{
// Buffer size can be "tuned" to enhance read/write performance
var buffer = new byte[1024];
using (var pFD = ContentResolver.OpenFileDescriptor(data.Data, "w"))
using (var outputSteam = new FileOutputStream(pFD.FileDescriptor))
using (var inputStream = Assets.Open("her.png"))
{
while (inputStream.CanRead && inputStream.IsDataAvailable())
{
var readCount = inputStream.Read(buffer, 0, buffer.Length);
outputSteam.Write(buffer, 0, readCount);
}
}
});
}
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