安卓基础知识：Context解析

作用

Activity本身继承了Context抽象类，可以通过Context获取一些相关信息
获取应用相关信息

 // 作用1：获取应用相关信息
    public abstract ApplicationInfo getApplicationInfo();
    public abstract String getPackageName();
    public abstract Looper getMainLooper();
    public abstract int checkPermission(@NonNull String permission, int pid, int uid);
1
2
3
4
5

获取系统/应用资源

// 如 AssetManager、PackageManager、Resources、System Service 以及 color、string、drawable 等
    public abstract AssetManager getAssets();
    public abstract Resources getResources();
    public abstract PackageManager getPackageManager();
    public abstract Context getApplicationContext();
    public abstract ClassLoader getClassLoader();
    public final @Nullable <T> T getSystemService(@NonNull Class<T> serviceClass) { ... }
    public final String getString(@StringRes int resId) { ... }
    public final int getColor(@ColorRes int id) { ... }
    public final Drawable getDrawable(@DrawableRes int id) { ... }
    public abstract Resources.Theme getTheme();
    public abstract void setTheme(@StyleRes int resid);
    public final TypedArray obtainStyledAttributes(@StyleableRes int[] attrs) { ... }
1
2
3
4
5
6
7
8
9
10
11
12
13

四大组件交互

// 如启动 Activity、Broadcast、Service，获取 ContentResolver 等
    public abstract void startActivity(@RequiresPermission Intent intent);
    public abstract void sendBroadcast(@RequiresPermission Intent intent);
    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,
                                            IntentFilter filter);
    public abstract void unregisterReceiver(BroadcastReceiver receiver);
    public abstract ComponentName startService(Intent service);
    public abstract boolean stopService(Intent service);
    public abstract boolean bindService(@RequiresPermission Intent service,
            @NonNull ServiceConnection conn, @BindServiceFlags int flags);
    public abstract void unbindService(@NonNull ServiceConnection conn);
    public abstract ContentResolver getContentResolver();
1
2
3
4
5
6
7
8
9
10
11
12

文件相关

// 如：获取缓存文件夹、删除文件、SharedPreference 相关等
    public abstract File getSharedPreferencesPath(String name);
    public abstract File getDataDir();
    public abstract boolean deleteFile(String name);
    public abstract File getExternalFilesDir(@Nullable String type);
    public abstract File getCacheDir();
    ...
    public abstract SharedPreferences getSharedPreferences(String name, @PreferencesMode int mode);
    public abstract boolean deleteSharedPreferences(String name);
1
2
3
4
5
6
7
8
9

数据库相关

// 如打开数据库、删除数据库、获取数据库路径等
    public abstract SQLiteDatabase openOrCreateDatabase(...);
    public abstract boolean deleteDatabase(String name);
    public abstract File getDatabasePath(String name);
1
2
3
4

不同类型

ActivityContext：与UI相关，对UI资源进行操作
ApplicationContext：生命周期较长（ApplicaitonContext生命周期与应用保持一致，注意内存泄漏）

额外问题

Activity继承ContextThemeWrapper来间接继承ContextWrapper，而Service与Applicaiton直接继承ContextWrapper，为什么需要间接继承？
因为只有Activity需要在ContextThemeWrapper内包含的Theme接口

getApplication()与getApplicationContext()有什么区别？
返回的都是同一个Applicaiton对象，但作用域不同：
getApplication()：作用在Activity、Service，不能用在BroadcastReceiver
getApplicationContext()：作用域更广，可在BroadcastReceiver中使用

getApplicationContext()、getBaseContext()和Activity.this区别？
getApplicationContext()：返回Applicaiton类型的Context
getBaseContext()：返回在构造函数/setBaseContext()里执行的Context
Activity.this：返回当前Activity的Context