Android Assert工具类

/*
* Copyright (C) 2013 The Android Open Source Project
*
* 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.
*/

package com.example.android.justforus;

import android.content.ContentProvider;
import android.content.ContentValues;
import android.content.Context;
import android.content.res.AssetFileDescriptor;
import android.content.res.AssetManager;
import android.database.Cursor;
import android.database.MatrixCursor;
import android.net.Uri;
import android.provider.OpenableColumns;
import android.util.Log;

import java.io.FileNotFoundException;
import java.io.IOException;

import static java.net.URLConnection.guessContentTypeFromName;

/**
* Generic content provider, which makes any files available in this app's "assets" directory
* available publicly.
*
* <p>To use, add the following to your AndroidManifest.xml:
*
* <code><pre>
* <provider
*   android:name=".AssetProvider"
*   android:authorities="[YOUR CONTENT PROVIDER DOMAIN HERE]"
*   android:grantUriPermissions="true"
*   android:exported="true"/>
* </pre></code>
*/
public class AssetProvider extends ContentProvider {

/**
* Content provider authority that identifies data that is offered by this
* {@link AssetProvider}.
*/
public static String CONTENT_URI = "com.example.android.justforus";

private static final String TAG = "AssetProvider";

AssetManager mAssets;

@Override
public boolean onCreate() {
Context ctx = getContext();
if (ctx == null) {
// Context not available. Give up.
return false;
}
mAssets = ctx.getAssets();
return true;
}

@Override
public String getType(Uri uri){
// Returns the MIME type for the selected URI, in conformance with the ContentProvider
// interface. Looks up the file indicated by /res/assets/{uri.path}, and returns the MIME
// type for that file as guessed by the URLConnection class.

// Setup
String path = uri.getLastPathSegment();

// Check if file exists
if (!fileExists(path)) {
return null;
}

// Determine MIME-type based on filename
return guessContentTypeFromName(uri.toString());
}

@Override
public AssetFileDescriptor openAssetFile (Uri uri, String mode)
throws FileNotFoundException, SecurityException {
// ContentProvider interface for opening a file descriptor by URI. This content provider
// maps all URIs to the contents of the APK's assets folder, so a file handle to
// /res/assets/{uri.path} will be returned.

// Security check. This content provider only supports read-only access. (Also, the contents
// of an APKs assets folder are immutable, so read-write access doesn't make sense here.)
if (!"r".equals(mode)) {
throw new SecurityException("Only read-only access is supported, mode must be [r]");
}

// Open asset from within APK and return file descriptor
String path = uri.getLastPathSegment();
try {
return mAssets.openFd(path);
} catch (IOException e) {
throw new FileNotFoundException();
}
}

/**
* Check if file exists inside APK assets.
*
* @param path Fully qualified path to file.
* @return true if exists, false otherwise.
*/
private boolean fileExists(String path) {
try {
// Check to see if file can be opened. If so, file exists.
mAssets.openFd(path).close();
return true;
} catch (IOException e) {
// Unable to open file descriptor for specified path; file doesn't exist.
return false;
}
}

@Override
public Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder) {
String path = uri.getLastPathSegment();
if (!fileExists(path)) {
Log.e(TAG, "Requested file doesn't exist at " + path);
return null;
}

// Create matrix cursor
if (projection == null) {
projection = new String[]{
OpenableColumns.DISPLAY_NAME,
OpenableColumns.SIZE,
};
}

MatrixCursor matrixCursor = new MatrixCursor(projection, 1);
Object[] row = new Object[projection.length];
for (int col = 0; col < projection.length; col++) {
if (OpenableColumns.DISPLAY_NAME.equals(projection[col])) {
row[col] = path;
} else if (OpenableColumns.SIZE.equals(projection[col])) {
try {
AssetFileDescriptor afd = openAssetFile(uri, "r");
if (afd != null) {
row[col] = Long.valueOf(afd.getLength());
}
afd.close();
} catch (IOException e) {
Log.e(TAG, e.toString());
}
}
}
matrixCursor.addRow(row);
return matrixCursor;
}

// Required/unused ContentProvider methods below.
@Override
public Uri insert(Uri uri, ContentValues contentValues) {
throw new RuntimeException("Operation not supported");
}

@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
throw new RuntimeException("Operation not supported");
}

@Override
public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
throw new RuntimeException("Operation not supported");
}
}