Android4.2_Launcher_LauncherApplication

LauncherApplication 作为Launcher唯一的应用程序实例，一般会用来保存一些全局的数据，做一些全局的初始化工作。

1. 加入节点：

<application
        android:name="com.android.launcher2.LauncherApplication"
        android:label="@string/application_name"
        android:icon="@mipmap/ic_launcher_home"
        android:hardwareAccelerated="true"  
        android:largeHeap="@bool/config_largeHeap"
        android:supportsRtl="true">

android:hardwareAccelerated="true"  整个程序使用硬件加速，3.0才有。

android:largeHeap                   是否使用大内存。可以通过ActivityManager.getMemoryClass() 

android:supportsRtl="true"     是否支持从右到左的布局，支持的话，Activity中使用：layoutDirection="rtl"

2.主要工作：

<1.创建全局数据并初始化：

public LauncherModel mModel;
public IconCache mIconCache;

其中 

mModel 是一个 BroadcastReceiver。

mIconCache 是图片的缓存。mModel 持有对图标缓存的引用。

mModel关心的广播Activon有：

（1）ACTION_PACKAGE_REMOVED  ACTION_PACKAGE_CHANGED 应用程序包卸载及包信息改变的广播。

（2）ACTION_EXTERNAL_APPLICATIONS_AVAILABLE SD卡上程序可用。 ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE SD卡上程序不可用，         ACTION_LOCALE_CHANGED 语言环境改变时的广播。ACTION_CONFIGURATION_CHANGED 也是语言改变时会发出？
（3）INTENT_GLOBAL_SEARCH_ACTIVITY_CHANGED 通知全局搜索提高者已经改变了。

<2.设置一个监听器，来监听数据库中 favorites表的改变。  注册一个ContentResolver，用来响应 favorite 表的改变。这个表在 launcher.db数据库中。workspace 正是根据里面的数据来加载的。

注：
与包相关的Broadcast Action还有：

ACTION_PACKAGE_ADDED 一个新应用包已经安装在设备上，数据包括包名（最新安装的包程序不能接收到这个广播）
ACTION_PACKAGE_REPLACED 一个新版本的应用安装到设备，替换之前已经存在的版本
ACTION_PACKAGE_CHANGED 一个已存在的应用程序包已经改变，包括包名
ACTION_PACKAGE_REMOVED 一个已存在的应用程序包已经从设备上移除，包括包名（正在被安装的包程序不能接收到这个广播）
ACTION_PACKAGE_RESTARTED 用户重新开始一个包，包的所有进程将被杀死，所有与其联系的运行时间状态应该被移除，包括包名（重新开始包程序不能接收到这个广播）
ACTION_PACKAGE_DATA_CLEARED 用户已经清楚一个包的数据，包括包名（清除包程序不能接收到这个广播）

需要注意对一个 Receiver注册多个 IntentFilter的方法。

源码：

/*
 * Copyright (C) 2008 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.android.launcher2;

import android.app.Application;
import android.app.SearchManager;
import android.content.ContentResolver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.res.Configuration;
import android.database.ContentObserver;
import android.os.Handler;

import com.android.launcher.R;

import java.lang.ref.WeakReference;

public class LauncherApplication extends Application {
    public LauncherModel mModel;
    public IconCache mIconCache;
    private static boolean sIsScreenLarge;
    private static float sScreenDensity;
    private static int sLongPressTimeout = 300;
    private static final String sSharedPreferencesKey = "com.android.launcher2.prefs";
    WeakReference<LauncherProvider> mLauncherProvider;

    @Override
    public void onCreate() {
        super.onCreate();

        // set sIsScreenXLarge and sScreenDensity *before* creating icon cache
        sIsScreenLarge = getResources().getBoolean(R.bool.is_large_screen);
        sScreenDensity = getResources().getDisplayMetrics().density;

        mIconCache = new IconCache(this);
        mModel = new LauncherModel(this, mIconCache);

        // Register intent receivers
        IntentFilter filter = new IntentFilter(Intent.ACTION_PACKAGE_ADDED);
        filter.addAction(Intent.ACTION_PACKAGE_REMOVED);
        filter.addAction(Intent.ACTION_PACKAGE_CHANGED);
        filter.addDataScheme("package");
        registerReceiver(mModel, filter);
        filter = new IntentFilter();
        filter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE);
        filter.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE);
        filter.addAction(Intent.ACTION_LOCALE_CHANGED);
        filter.addAction(Intent.ACTION_CONFIGURATION_CHANGED);
        registerReceiver(mModel, filter);
        filter = new IntentFilter();
        filter.addAction(SearchManager.INTENT_GLOBAL_SEARCH_ACTIVITY_CHANGED);
        registerReceiver(mModel, filter);
        filter = new IntentFilter();
        filter.addAction(SearchManager.INTENT_ACTION_SEARCHABLES_CHANGED);
        registerReceiver(mModel, filter);

        // Register for changes to the favorites
        ContentResolver resolver = getContentResolver();
        resolver.registerContentObserver(LauncherSettings.Favorites.CONTENT_URI, true,
                mFavoritesObserver);
    }

    /**
     * There's no guarantee that this function is ever called.
     */
    @Override
    public void onTerminate() {
        super.onTerminate();

        unregisterReceiver(mModel);

        ContentResolver resolver = getContentResolver();
        resolver.unregisterContentObserver(mFavoritesObserver);
    }

    /**
     * Receives notifications whenever the user favorites have changed.
     */
    private final ContentObserver mFavoritesObserver = new ContentObserver(new Handler()) {
        @Override
        public void onChange(boolean selfChange) {
            // If the database has ever changed, then we really need to force a reload of the
            // workspace on the next load
            mModel.resetLoadedState(false, true);
            mModel.startLoaderFromBackground();
        }
    };

    LauncherModel setLauncher(Launcher launcher) {
        mModel.initialize(launcher);
        return mModel;
    }

    IconCache getIconCache() {
        return mIconCache;
    }

    LauncherModel getModel() {
        return mModel;
    }

    void setLauncherProvider(LauncherProvider provider) {
        mLauncherProvider = new WeakReference<LauncherProvider>(provider);
    }

    LauncherProvider getLauncherProvider() {
        return mLauncherProvider.get();
    }

    public static String getSharedPreferencesKey() {
        return sSharedPreferencesKey;
    }

    public static boolean isScreenLarge() {
        return sIsScreenLarge;
    }

    public static boolean isScreenLandscape(Context context) {
        return context.getResources().getConfiguration().orientation ==
            Configuration.ORIENTATION_LANDSCAPE;
    }

    public static float getScreenDensity() {
        return sScreenDensity;
    }

    public static int getLongPressTimeout() {
        return sLongPressTimeout;
    }
}