文件分享（使用Content URI）

一、在manifest文件中声明FileProvider，如：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapp">
    <application
        ...>
        <provider
            android:name="android.support.v4.content.FileProvider"
            android:authorities="com.example.myapp.fileprovider"
            android:grantUriPermissions="true"
            android:exported="false">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/filepaths" />
        </provider>
        ...
    </application>
</manifest>

authorities属性指明FileProvider为content URI产生的URI域名部分，meta-data下的resource则指明你要分享的文件夹。

二、指明可分享的文件夹
在res文件夹下新建xml文件夹，在xml文件夹下新建resource属性指明的filepaths.xml文件，在其中声明可分享的文件夹，如：

<paths>
    <files-path path="images/" name="myimages" />
</paths>

<paths>标签可以有多个子节点，每个子节点指明了一个分享的文件夹；
<file-path>标签指示要分享的文件夹位于你的app的内存的files目录下，path属性表明一个子文件夹，name属性添加对应的路径到content URI；
<external-path>标签指示要分享的文件位于外部存储的文件夹；
<cache-path>标签指示要分享的文件位于app的缓存文件夹下。


三.设计一个文件选择的界面
在manifest文件中声明该界面，action为ACTION_PICK，category为CATEGORY_DEFAULT和CATEGORY_OPENABLE,也可以声明你的app可以分享的文件类型，如：

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
    ...
        <application>
        ...
            <activity
                android:name=".FileSelectActivity"
                android:label="@File Selector" >
                <intent-filter>
                    <action
                        android:name="android.intent.action.PICK"/>
                    <category
                        android:name="android.intent.category.DEFAULT"/>
                    <category
                        android:name="android.intent.category.OPENABLE"/>
                    <data android:mimeType="text/plain"/>
                    <data android:mimeType="image/*"/>
                </intent-filter>
            </activity>

四、用具体代码定义你的文件分享界面，如用一个listview显示指定文件夹下所有的文件的路径，如：

public class FileSelectActivity extends Activity {
    // The path to the root of this app's internal storage
    private File mPrivateRootDir;
    // The path to the "images" subdirectory
    private File mImagesDir;
    // Array of files in the images subdirectory
    File[] mImageFiles;
    // Array of filenames corresponding to mImageFiles
    String[] mImageFilenames;
    // Initialize the Activity
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...
        // Set up an Intent to send back to apps that request a file
        mResultIntent =
                new Intent("com.example.myapp.ACTION_RETURN_FILE");
        // Get the files/ subdirectory of internal storage
        mPrivateRootDir = getFilesDir();
        // Get the files/images subdirectory;
        mImagesDir = new File(mPrivateRootDir, "images");
        // Get the files in the images subdirectory
        mImageFiles = mImagesDir.listFiles();
	/*Display the file names in the ListView */
	...
	}
}

五、当用户点击listview中的一条数据时，生成该文件的content URI：

 protected void onCreate(Bundle savedInstanceState) {
        ...
        // Define a listener that responds to clicks on a file in the ListView
        mFileListView.setOnItemClickListener(
                new AdapterView.OnItemClickListener() {
            @Override
            /*
             * When a filename in the ListView is clicked, get its
             * content URI and send it to the requesting app
             */
            public void onItemClick(AdapterView<?> adapterView,
                    View view,
                    int position,
                    long rowId) {
                /*
                 * Get a File for the selected file name.
                 * Assume that the file names are in the
                 * mImageFilename array.
                 */
                File requestFile = new File(mImageFilename[position]);
                /*
                 * Most file-related method calls need to be in
                 * try-catch blocks.
                 */
                // Use the FileProvider to get a content URI
                try {
                    fileUri = FileProvider.getUriForFile(
                            FileSelectActivity.this,
                            "com.example.myapp.fileprovider",
                            requestFile);
                } catch (IllegalArgumentException e) {
                    Log.e("File Selector",
                          "The selected file can't be shared: " +
                          clickedFilename);
                }
                ...
            }
        });
        ...
    }

六、为你的content URI保证用户使用权限，通过调用addFlags()方法：

if (fileUri != null) {
                    // Grant temporary read permission to the content URI
                    mResultIntent.addFlags(
                        Intent.FLAG_GRANT_READ_URI_PERMISSION);
                }

七、设置界面返回数据：

 if (fileUri != null) {
                    ...
                    // Put the Uri and MIME type in the result Intent
                    mResultIntent.setDataAndType(
                            fileUri,
                            getContentResolver().getType(fileUri));
                    // Set the result
                    MainActivity.this.setResult(Activity.RESULT_OK,
                            mResultIntent);
                    } else {
                        mResultIntent.setDataAndType(null, "");
                        MainActivity.this.setResult(RESULT_CANCELED,
                                mResultIntent);
                    }
                }

八、finish该界面，可以在该界面显示一个Done按钮，当用户点击时关闭当前页面，如：

 public void onDoneClick(View v) {
        // Associate a method with the Done button
        finish();
    }

这样我们就响应了其他应用获取文件的请求，并开启了相应的界面，并将content URI作为结果返回。

九、访问获取到的文件：
使用FileDescriptor查看返回的content URI，如：

@Override
    public void onActivityResult(int requestCode, int resultCode,
            Intent returnIntent) {
        // If the selection didn't work
        if (resultCode != RESULT_OK) {
            // Exit without doing anything else
            return;
        } else {
            // Get the file's content URI from the incoming Intent
            Uri returnUri = returnIntent.getData();
            /*
             * Try to open the file for "read" access using the
             * returned URI. If the file isn't found, write to the
             * error log and return.
             */
            try {
                /*
                 * Get the content resolver instance for this context, and use it
                 * to get a ParcelFileDescriptor for the file.
                 */
                mInputPFD = getContentResolver().openFileDescriptor(returnUri, "r");
            } catch (FileNotFoundException e) {
                e.printStackTrace();
                Log.e("MainActivity", "File not found.");
                return;
            }
            // Get a regular file descriptor for the file
            FileDescriptor fd = mInputPFD.getFileDescriptor();
            ...
        }
    }