DocumentsProvider：什么是内容：//com.android.documentsui.recents/state为什么它不喜欢我？

我正在尝试将DocumentsProvider实现为我的书的演示.虽然提供程序显示在消费者示例应用程序中,但只要我在Storage Access Framework的UI中单击它,我就会得到以下堆栈跟踪：

09-15 18:40:46.290    1765-1829/com.android.documentsui E/AndroidRuntime﹕ FATAL EXCEPTION: ProviderExecutor: com.commonsware.android.documents.provider
    Process: com.android.documentsui,PID: 1765
    java.lang.RuntimeException: An error occured while execuTing doInBACkground()
            at android.os.AsyncTask$3.done(AsyncTask.java:300)
            at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
            at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
            at java.util.concurrent.FutureTask.run(FutureTask.java:242)
            at com.android.documentsui.ProviderExecutor.run(ProviderExecutor.java:107)
     Caused by: java.lang.UnsupportedoperationException: Unsupported Uri content://com.android.documentsui.recents/state/com.commonsware.android.documents.provider/thisIsMyRoot/
            at com.android.documentsui.RecentsProvider.query(RecentsProvider.java:192)
            at android.content.ContentProvider.query(ContentProvider.java:857)
            at android.content.ContentProvider$Transport.query(ContentProvider.java:200)
            at android.content.ContentResolver.query(ContentResolver.java:461)
            at android.content.ContentResolver.query(ContentResolver.java:404)
            at com.android.documentsui.DirectoryLoader.loadInBACkground(DirectoryLoader.java:124)
            at com.android.documentsui.DirectoryLoader.loadInBACkground(DirectoryLoader.java:65)
            at android.content.AsyncTaskLoader.onLoadInBACkground(AsyncTaskLoader.java:312)
            at android.content.AsyncTaskLoader$LoadTask.doInBACkground(AsyncTaskLoader.java:69)
            at android.content.AsyncTaskLoader$LoadTask.doInBACkground(AsyncTaskLoader.java:57)
            at android.os.AsyncTask$2.call(AsyncTask.java:288)
            at java.util.concurrent.FutureTask.run(FutureTask.java:237)
            at com.android.documentsui.ProviderExecutor.run(ProviderExecutor.java:107)

content：//com.commonsware.android.documents.provider/thisIsMyRoot/可能是基于queryRoots()实现为我的文档根生成的Uri.但我不知道是什么内容：//com.android.documentsui.recents/state/com.commonsware.android.documents.provider/thisIsMyRoot/或我应该做些什么来防止这个错误.

这是DocumentsProvider实现,旨在提供来自assets /的文件：

/***
  Copyright (C) 2014 CommonsWare,LLC
  Licensed under the Apache License,Version 2.0 (the "License"); you may not
  use this file except in compliance with the License. You may obtain a copy
  of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
  by applicable law or agreed to in wriTing,software diStributed under the
  License is diStributed on an "AS IS" BASIS,WITHOUT WARRANTIES OR CONDITIONS
  OF ANY KIND,either express or implied. See the License for the specific
  language governing permissions and limitations under the License.

  From _The Busy Coder's Guide to Android Development_
    http://commonsware.com/Android
 */

package com.commonsware.android.documents.provider;

import android.content.res.AssetManager;
import android.database.cursor;
import android.database.Matrixcursor;
import android.net.Uri;
import android.os.CancellationSignal;
import android.os.ParcelFileDescriptor;
import android.provider.DocumentsContract.Document;
import android.provider.DocumentsContract.Root;
import android.provider.DocumentsProvider;
import android.util.Log;
import android.webkit.MimeTypeMap;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.outputStream;
import java.util.ArrayList;

public class DemoDocumentProvider extends DocumentsProvider {
  private static final String[] SUPPORTED_ROOT_PROjeCTION=new String[] {
      Root.columN_ROOT_ID,Root.columN_FLAGS,Root.columN_titlE,Root.columN_DOCUMENT_ID,Root.COLUMN_ICON };
  private static final String[] SUPPORTED_DOCUMENT_PROjeCTION=
      new String[] { Document.columN_DOCUMENT_ID,Document.columN_MIME_TYPE,Document.columN_DISPLAY_NAME,Document.columN_FLAGS};
  private static final String ROOT_ID="thisIsMyRoot";
  private static final String ROOT_DOCUMENT_ID="thisCAnnotBeEmpty";
  private AssetManager assets;

  @Override
  public Boolean onCreate() {
    assets=getContext().getAssets();

    return(true);
  }

  @Override
  public cursor queryRoots(String[] projection)
      throws FileNotFoundException {
    String[] netProjection=
        netProjection(projection,SUPPORTED_ROOT_projection);
    Matrixcursor result=new Matrixcursor(netprojection);
    Matrixcursor.RowBuilder row=result.newRow();

    row.add(Root.columN_ROOT_ID,ROOT_ID);
    row.add(Root.COLUMN_ICON,R.drawable.ic_launcher);
    row.add(Root.columN_FLAGS,Root.FLAG_LOCAL_ONLY);
    row.add(Root.columN_titlE,getContext().getString(R.String.root));
    row.add(Root.columN_DOCUMENT_ID,ROOT_DOCUMENT_ID);

    return(result);
  }

  @Override
  public cursor queryChildDocuments(String parentDocumentId,String[] projection,String sortOrder)
      throws FileNotFoundException {
    String[] netProjection=
        netProjection(projection,SUPPORTED_DOCUMENT_projection);
    Matrixcursor result=new Matrixcursor(netprojection);

    parentDocumentId=fixUpDocumentId(parentDocumentId);

    try {
      String[] children=assets.list(parentDocumentId);

      for (String child : children) {
        addDocumentRow(result,child,parentDocumentId+child);
      }
    }
    catch (IOException E) {
      Log.e(getClass().getSimplename(),"Exception reading asset dir",E);
    }

    return(result);
  }

  @Override
  public cursor queryDocument(String documentId,String[] projection)
      throws FileNotFoundException {
    String[] netProjection=
        netProjection(projection,SUPPORTED_DOCUMENT_projection);
    Matrixcursor result=new Matrixcursor(netprojection);

    documentId=fixUpDocumentId(documentId);

    try {
      addDocumentRow(result,Uri.parse(documentId).getLastPathSegment(),documentId);
    }
    catch (IOException E) {
      Log.e(getClass().getSimplename(),E);
    }

    return(result);
  }

  @Override
  public ParcelFileDescriptor openDocument(String documentId,String mode,CancellationSignal signal)
      throws FileNotFoundException {
    ParcelFileDescriptor[] pipe=null;

    try {
      pipe=ParcelFileDescriptor.createPipe();
      AssetManager assets=getContext().getresources().getAssets();

      new TransferThread(assets.open(documentId),new ParcelFileDescriptor.AutoCloSEOutputStream(pipe[1])).start();
    }
    catch (IOException E) {
      Log.e(getClass().getSimplename(),"Exception opening pipe",E);
      throw new FileNotFoundException("Could not open pipe for: "
          + documentId);
    }

    return(pipe[0]);
  }

  private void addDocumentRow(Matrixcursor result,String child,String assetPath) throws IOException {
    Matrixcursor.RowBuilder row=result.newRow();

    row.add(Document.columN_DOCUMENT_ID,assetPath);

    if (isDirectory(assetPath)) {
      row.add(Document.columN_MIME_TYPE,Document.MIME_TYPE_DIR);
    }
    else {
      row.add(Document.columN_MIME_TYPE,MimeTypeMap.getFileExtensionFromUrl(assetPath));
      row.add(Document.columN_SIZE,lastModified(assetPath));
    }

    row.add(Document.columN_DISPLAY_NAME,child);
    row.add(Document.columN_FLAGS,0);
  }

  private Boolean isDirectory(String assetPath) throws IOException {
    return(assets.list(assetPath).length>1);
  }

  private long lastModified(String assetPath) throws IOException {
    return(assets.openFd(assetPath).getLength());
  }

  private String fixUpDocumentId(String documentId) {
    if (ROOT_DOCUMENT_ID.equals(documentId)) {
      return("");
    }

    return(documentId);
  }

  private static String[] netProjection(String[] requested,String[] supported) {
    if (requested==null) {
      return(supported);
    }

    ArrayList

所以,我的问题是：我哪里错了？