如何使用buildChildDocumentsUriUsingTree过滤查询结果

Question

我想将文件保存在SD卡文件夹中.

我不能在我的项目上使用V4支持.

所以我打电话给:

Intent itent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
itent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
itent.addFlags(Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
startActivityForResult(itent, requestCodeTree);

然后在onActivityResult上,我有:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent intent) {
    super.onActivityResult(requestCode, resultCode, intent);

    if (resultCode == RESULT_OK) {
        switch(requestCode) {
            case requestCodeTree:
                saveFile(intent.getData());
                break;
        }
    }
}

而saveFile的代码是:

   private void saveFile(Uri data) {
        ContentResolver contentResolver = context.getContentResolver();

        InputStream in = null;
        OutputStream out = null;

        try {

            // Problems start here ************************
            Uri toUriFile= getUriBackupFile(context, data);
            // ********************************************

            if (toUriFile==null) {
                Uri toUriFolder = DocumentsContract.buildDocumentUriUsingTree(data, DocumentsContract.getTreeDocumentId(data));
                toUriFile = DocumentsContract.createDocument(contentResolver, toUriFolder, "", backupName);
            }

            out = contentResolver.openOutputStream(toUriFile);
            in = new FileInputStream(fromFile);

            byte[] buffer = new byte[1024];
            int read;
            while ((read = in.read(buffer)) != -1) {
                out.write(buffer, 0, read);
            }
            in.close();
            // write the output file (the file is now copied)
            out.flush();
            out.close();

        } catch (FileNotFoundException e) {
            Log.e(TAG, "Failed", e);
        } catch (Exception e) {
            Log.e(TAG, "Failed", e);
        }
    }

到现在为止还挺好.

当我调用getUriBackupFile来获取目标文件的uri 时,问题就开始了.

要做到这一点,我查询ContentResolver的与buildChildDocumentsUriUsingTree并试图筛选,其中结果DocumentsContract.Document.COLUMN_DISPLAY_NAME符合我的文件的显示名称,就像这样:

private static Uri getUriBackupFile(Context context, Uri treeUri) {
        final ContentResolver resolver = context.getContentResolver();

        final Uri childrenUri = DocumentsContract.buildChildDocumentsUriUsingTree(
                treeUri,
                DocumentsContract.getTreeDocumentId(treeUri));

        Cursor c = null;
        try {
            String[] projections = new String[] {
                    DocumentsContract.Document.COLUMN_DOCUMENT_ID,
                    DocumentsContract.Document.COLUMN_DISPLAY_NAME};

            // this line doesn't seem to have any effect !
        String selection = DocumentsContract.Document.COLUMN_DISPLAY_NAME + " = '" + backupName + "' ";
            // *************************************************************************

            c = resolver.query(childrenUri, projections, selection, null, null);

            if (c!=null && c.moveToFirst()) {
            // Here I expect to have c.getCount() == 1 or == 0
            // But actually c.getCount() == [Number of children in the treeUri] regardless of the selection
                String documentId = c.getString(0);
                Uri documentUri = DocumentsContract.buildDocumentUriUsingTree(treeUri,
                        documentId);
                return documentUri;
            }

        } catch (Exception e) {
            Log.w(TAG, "Failed query: " + e);
        } finally {
            if (c!=null) c.close();
        }

        return null;
    }

但是无论选择什么,查询总是返回treeUri的所有子节点.所以,似乎选择没有效果.

我总是可以循环遍历所有结果,但如果所选文件夹中包含大量文件,则对性能不利.

所以我的问题是:

1. 我如何过滤查询结果？
2. 这甚至是在SD卡路径上保存文件的正确方法吗？

Answer 1

1. 您需要在清单中添加读/写外部存储的权限。在应用程序标记之前添加此行。

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

2. 如果您的应用程序在 Android 6 或更高版本的设备上运行。您需要先请求许可，然后才能写入 SD 卡/外部存储。请遵循有关在运行时检查权限的文档。

3. 使用Environment.getExternalStorageDirectory()获取外部目录。这将返回外部目录。

另请参阅本文档（https://developer.android.com/training/data-storage/files）和这个问题。