4.4前后，读取系统图库的图片

** 为什么要分开不同版本呢?
其实在4.3或以下可以直接用ACTION_GET_CONTENT的,在4.4或以上,官方建议用ACTION_OPEN_DOCUMENT,但其实都不算太大区别,区别是他们返回的Uri。**

• 4.4之后的版本:

/*
根据返回的Uri得到文件的路径，从而得到需要获取的资源
*/
public static String getPath(final Context context, final Uri uri) {

    final boolean isKitKat = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT;

    // DocumentProvider
    if (isKitKat && DocumentsContract.isDocumentUri(context, uri)) {//判断是否是Document类型的Uri
        // ExternalStorageProvider
        if (isExternalStorageDocument(uri)) {//判断是否是手机SD卡根目录下的Document
            final String docId = DocumentsContract.getDocumentId(uri);//获取“类型:id”
            final String[] split = docId.split(":");//通过:来分隔类型和ID
            final String type = split[0];

            if ("primary".equalsIgnoreCase(type)) {//判断类型是否是primary(手机SD卡根目录path类型)
                return Environment.getExternalStorageDirectory() + "/" + split[1];//Environment提供对环境变量的访问。方法getExternalStorageDirectory()是返回手机根目录path
            }

            // TODO handle non-primary volumes
        }
        // DownloadsProvider
        else if (isDownloadsDocument(uri)) {//判断是否在系统下载path下的文件

            final String id = DocumentsContract.getDocumentId(uri);
            final Uri contentUri = ContentUris.withAppendedId(
                    Uri.parse("content://downloads/public_downloads"), Long.valueOf(id));//ContentUris对于Data Scheme采用content://的Uri的有效方法。其中withAppendedId（Uri uri，Long id）方法用于在给定的路径后面追加id

            return getDataColumn(context, contentUri, null, null);
        }
        // MediaProvider
        else if (isMediaDocument(uri)) {//判断是否是media类型的文件
            final String docId = DocumentsContract.getDocumentId(uri);
            final String[] split = docId.split(":");
            final String type = split[0];

            Uri contentUri = null;
            if ("image".equals(type)) {//判断是否是image
                contentUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;//返回content://样式的Uri
            } else if ("video".equals(type)) {//判断是否是video
                contentUri = MediaStore.Video.Media.EXTERNAL_CONTENT_URI;//返回content://样式的Uri
            } else if ("audio".equals(type)) {//判断是否是audio
                contentUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;//返回content://样式的Uri
            }

            final String selection = "_id=?";
            final String[] selectionArgs = new String[] {
                    split[1]
            };

            return getDataColumn(context, contentUri, selection, selectionArgs);
        }
    }
    // MediaStore (and general)
    else if ("content".equalsIgnoreCase(uri.getScheme())) {//属于普通的Uri，包含path

        // Return the remote address
        if (isGooglePhotosUri(uri))
            return uri.getLastPathSegment();

        return getDataColumn(context, uri, null, null);
    }
    // File
    else if ("file".equalsIgnoreCase(uri.getScheme())) {
        return uri.getPath();
    }

    return null;
}

/**
 * Get the value of the data column for this Uri. This is useful for
 * MediaStore Uris, and other file-based ContentProviders.
 *
 * @param context The context.
 * @param uri The Uri to query.
 * @param selection (Optional) Filter used in the query.
 * @param selectionArgs (Optional) Selection arguments used in the query.
 * @return The value of the _data column, which is typically a file path.
 */
public static String getDataColumn(Context context, Uri uri, String selection,
        String[] selectionArgs) {

    Cursor cursor = null;
    final String column = "_data";
    final String[] projection = {
            column
    };

    try {
        cursor = context.getContentResolver().query(uri, projection, selection, selectionArgs,
                null);
        if (cursor != null && cursor.moveToFirst()) {
            final int index = cursor.getColumnIndexOrThrow(column);
            return cursor.getString(index);
        }
    } finally {
        if (cursor != null)
            cursor.close();
    }
    return null;
}


/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is ExternalStorageProvider.
 */
public static boolean isExternalStorageDocument(Uri uri) {
    return "com.android.externalstorage.documents".equals(uri.getAuthority());
}

/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is DownloadsProvider.
 */
public static boolean isDownloadsDocument(Uri uri) {
    return "com.android.providers.downloads.documents".equals(uri.getAuthority());
}

/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is MediaProvider.
 */
public static boolean isMediaDocument(Uri uri) {
    return "com.android.providers.media.documents".equals(uri.getAuthority());
}

/**
 * @param uri The Uri to check.
 * @return Whether the Uri authority is Google Photos.
 */
public static boolean isGooglePhotosUri(Uri uri) {
    return "com.google.android.apps.photos.content".equals(uri.getAuthority());
}

• 4.4之前的版本

private void handleImageBeforeKitKia(Intent data) {
    Uri uri = data.getData();
    String imagePath = getImagePath(uri, null);
    displayImage(imagePath);
}
private String getImagePath(Uri uri, String selection) {
    String path = null;
    Cursor cursor = getContentResolver().query(uri, null, selection, null, null);
    if (cursor != null) {
        if (cursor.moveToFirst()) {
            path = cursor.getString(cursor.getColumnIndex(MediaStore.Images.Media.DATA));        }
        cursor.close();    }
    return path;
}
private void displayImage(String imagePath) {
    if (imagePath != null) {
        Bitmap bitmap = BitmapFactory.decodeFile(imagePath);
        picture.setImageBitmap(bitmap);    
} else {
        Toast.makeText(this, "failed to get image", Toast.LENGTH_SHORT).show();    
}
}