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Context关键函数

    2018-10-04     修改 删除


转：//https://www.cnblogs.com/tsingke/p/9127758.html

public abstract class Context {

	// 获取应用程序包的AssetManager实例
	public abstract AssetManager getAssets(); // 获取应用程序包的Resources实例
	public abstract Resources getResources();

	// 获取PackageManager实例，以查看全局package信息
	public abstract PackageManager getPackageManager();

	// 获取应用程序包的ContentResolver实例
	public abstract ContentResolver getContentResolver();

	// 它返回当前进程的主线程的Looper，此线程分发调用给应用组件(activities, services等)
	public abstract Looper getMainLooper();

	// 返回当前进程的单实例全局Application对象的Context
	public abstract Context getApplicationContext();

	// 从string表中获取本地化的、格式化的字符序列
	public final CharSequence getText(int resId) {
		return getResources().getText(resId);
	}

	// 从string表中获取本地化的字符串
	public final String getString(int resId) {
		return getResources().getString(resId);
	}

	public final String getString(int resId, Object... formatArgs) {
		return getResources().getString(resId, formatArgs);
	}

	// 返回一个可用于获取包中类信息的class loader
	public abstract ClassLoader getClassLoader();

	// 返回应用程序包名
	public abstract String getPackageName();

	// 返回应用程序信息
	public abstract ApplicationInfo getApplicationInfo();

	// 根据文件名获取SharedPreferences
	public abstract SharedPreferences getSharedPreferences(String name, int mode);

	// 其根目录为: Environment.getExternalStorageDirectory()
	/*
	* @param type The type of files directory to return.  May be null for
	* the root of the files directory or one of
	* the following Environment constants for a subdirectory:
	* {@link android.os.Environment#DIRECTORY_MUSIC},
	* {@link android.os.Environment#DIRECTORY_PODCASTS},
	* {@link android.os.Environment#DIRECTORY_RINGTONES},
	* {@link android.os.Environment#DIRECTORY_ALARMS},
	* {@link android.os.Environment#DIRECTORY_NOTIFICATIONS},
	* {@link android.os.Environment#DIRECTORY_PICTURES}, or
	* {@link android.os.Environment#DIRECTORY_MOVIES}.
	*/
	public abstract File getExternalFilesDir(String type);

	// 返回应用程序obb文件路径
	public abstract File getObbDir();

	// 启动一个新的activity
	public abstract void startActivity(Intent intent);

	// 启动一个新的activity
	public void startActivityAsUser(Intent intent, UserHandle user) {
		throw new RuntimeException("Not implemented. Must override in a subclass.");
	}

	// 启动一个新的activity
	// intent: 将被启动的activity的描述信息
	// options: 描述activity将如何被启动
	public abstract void startActivity(Intent intent, Bundle options);

	// 启动多个新的activity
	public abstract void startActivities(Intent[] intents);

	// 启动多个新的activity
	public abstract void startActivities(Intent[] intents, Bundle options);

	// 广播一个intent给所有感兴趣的接收者，异步机制
	public abstract void sendBroadcast(Intent intent);

	// 广播一个intent给所有感兴趣的接收者，异步机制
	public abstract void sendBroadcast(Intent intent,String receiverPermission);

	public abstract void sendOrderedBroadcast(Intent intent,String receiverPermission);

	public abstract void sendOrderedBroadcast(Intent intent,
		String receiverPermission, BroadcastReceiver resultReceiver,
		Handler scheduler, int initialCode, String initialData,
		Bundle initialExtras);

	public abstract void sendBroadcastAsUser(Intent intent, UserHandle user);

	public abstract void sendBroadcastAsUser(Intent intent, UserHandle user,
		String receiverPermission);

	// 注册一个BroadcastReceiver，且它将在主activity线程中运行
	public abstract Intent registerReceiver(BroadcastReceiver receiver,
		IntentFilter filter);

	public abstract Intent registerReceiver(BroadcastReceiver receiver,
		IntentFilter filter, String broadcastPermission, Handler scheduler);

	public abstract void unregisterReceiver(BroadcastReceiver receiver);

	// 请求启动一个application service
	public abstract ComponentName startService(Intent service);

	// 请求停止一个application service
	public abstract boolean stopService(Intent service);

	// 连接一个应用服务，它定义了application和service间的依赖关系
	public abstract boolean bindService(Intent service, ServiceConnection conn,
		int flags);

	// 断开一个应用服务，当服务重新开始时，将不再接收到调用，
	// 且服务允许随时停止
	public abstract void unbindService(ServiceConnection conn);

	// 返回系统级service句柄
	/*
	* @see #WINDOW_SERVICE
	* @see android.view.WindowManager
	* @see #LAYOUT_INFLATER_SERVICE
	* @see android.view.LayoutInflater
	* @see #ACTIVITY_SERVICE
	* @see android.app.ActivityManager
	* @see #POWER_SERVICE
	* @see android.os.PowerManager
	* @see #ALARM_SERVICE
	* @see android.app.AlarmManager
	* @see #NOTIFICATION_SERVICE
	* @see android.app.NotificationManager
	* @see #KEYGUARD_SERVICE
	* @see android.app.KeyguardManager
	* @see #LOCATION_SERVICE
	* @see android.location.LocationManager
	* @see #SEARCH_SERVICE
	* @see android.app.SearchManager
	* @see #SENSOR_SERVICE
	* @see android.hardware.SensorManager
	* @see #STORAGE_SERVICE
	* @see android.os.storage.StorageManager
	* @see #VIBRATOR_SERVICE
	* @see android.os.Vibrator
	* @see #CONNECTIVITY_SERVICE
	* @see android.net.ConnectivityManager
	* @see #WIFI_SERVICE
	* @see android.net.wifi.WifiManager
	* @see #AUDIO_SERVICE
	* @see android.media.AudioManager
	* @see #MEDIA_ROUTER_SERVICE
	* @see android.media.MediaRouter
	* @see #TELEPHONY_SERVICE
	* @see android.telephony.TelephonyManager
	* @see #INPUT_METHOD_SERVICE
	* @see android.view.inputmethod.InputMethodManager
	* @see #UI_MODE_SERVICE
	* @see android.app.UiModeManager
	* @see #DOWNLOAD_SERVICE
	* @see android.app.DownloadManager
	*/
	public abstract Object getSystemService(String name);

	public abstract int checkPermission(String permission, int pid, int uid);

	// 返回一个新的与application name对应的Context对象
	public abstract Context createPackageContext(String packageName,
		int flags) throws PackageManager.NameNotFoundException;

	// 返回基于当前Context对象的新对象，其资源与display相匹配
	public abstract Context createDisplayContext(Display display);
}

ContextImpl关键成员和函数

    2018-10-04     修改 删除


/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
class ContextImpl extends Context {
	private final static String TAG = "ContextImpl";
	private final static boolean DEBUG = false;

	private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs =
		new HashMap<String, SharedPreferencesImpl>();

	/*package*/ LoadedApk mPackageInfo; // 关键数据成员
	private String mBasePackageName;
	private Resources mResources;
	/*package*/ ActivityThread mMainThread; // 主线程 @Override
	public AssetManager getAssets() {
		return getResources().getAssets();
	}

	@Override
	public Looper getMainLooper() {
		return mMainThread.getLooper();
	}

	@Override
	public Object getSystemService(String name) {
		ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
		return fetcher == null ? null : fetcher.getService(this);
	}

	@Override
	public void startActivity(Intent intent, Bundle options) {
		warnIfCallingFromSystemProcess();
		if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
			throw new AndroidRuntimeException(
			"Calling startActivity() from outside of an Activity "
			+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
			+ " Is this really what you want?");
		}
		mMainThread.getInstrumentation().execStartActivity(
			getOuterContext(), mMainThread.getApplicationThread(), null,
			(Activity)null, intent, -1, options);
	}
}

ContextWrapper

    2018-10-04     修改 删除


它只是对Context类的一种封装，它的构造函数包含了一个真正的Context引用，即ContextImpl对象。

/**
* Proxying implementation of Context that simply delegates all of its calls to
* another Context.  Can be subclassed to modify behavior without changing
* the original Context.
*/
public class ContextWrapper extends Context {
	Context mBase; //该属性指向一个ContextIml实例

	public ContextWrapper(Context base) {
		mBase = base;
	} /**
	* Set the base context for this ContextWrapper.  All calls will then be
	* delegated to the base context.  Throws
	* IllegalStateException if a base context has already been set.
	*
	* @param base The new base context for this wrapper.
	* 创建Application、Service、Activity，会调用该方法给mBase属性赋值
	*/
	protected void attachBaseContext(Context base) {
		if (mBase != null) { throw new IllegalStateException("Base context already set");
		}
		mBase = base;
	}

	@Override
	public Looper getMainLooper() {
		return mBase.getMainLooper();
	}

	@Override
	public Object getSystemService(String name) {
		return mBase.getSystemService(name);
	}

	@Override
	public void startActivity(Intent intent) {
		mBase.startActivity(intent);
	}
}

ContextThemeWrapper

    2018-10-04     修改 删除


该类内部包含了主题(Theme)相关的接口，即android:theme属性指定的。只有Activity需要主题，Service不需要主题，所以Service直接继承于ContextWrapper类。

/**
* A ContextWrapper that allows you to modify the theme from what is in the
* wrapped context.
*/
public class ContextThemeWrapper extends ContextWrapper {
	private Context mBase;
	private int mThemeResource;
	private Resources.Theme mTheme;
	private LayoutInflater mInflater;
	private Configuration mOverrideConfiguration;
	private Resources mResources;

	public ContextThemeWrapper() {
		super(null);
	}

	public ContextThemeWrapper(Context base, int themeres) {
		super(base);
		mBase = base;
		mThemeResource = themeres;
	}  @Override 
	protected void attachBaseContext(Context newBase) {
		super.attachBaseContext(newBase);
		mBase = newBase; }

	@Override 
	public void setTheme(int resid) {
		mThemeResource = resid;
		initializeTheme();
	}

	@Override 
	public Resources.Theme getTheme() {
		if (mTheme != null) {
		return mTheme;
	}

	mThemeResource = Resources.selectDefaultTheme(mThemeResource,
		getApplicationInfo().targetSdkVersion);
		initializeTheme();

		return mTheme;
	}
}

Context内存泄露问题

    2018-10-04     修改 删除


转：//KaelQ-https://www.jianshu.com/p/f24707874b04

静态资源导致的内存泄漏

有时候旋转屏幕时候，会先销毁原来的Activity，重新建立一个Activity，这时候图片我们并不想重新加载，所以将图片设置为静态对象。
但是静态对象的view.setBackgroundDrawable();方法很容易造成内存泄露。
public class MyCustomResource {
    //静态变量drawable
    private static Drawable drawable;
    private View view；
    public MyCustomResource(Context context) {
        Resources resources = context.getResources();
        drawable = resources.getDrawable(R.drawable.ic_launcher);
        view = new View(context);
        view.setBackgroundDrawable(drawable);
    }
}
public void setBackgroundDrawable(Drawable background) {
         ..........
         /**此处的this就是当前View对象，而View对象又是有Context对象获得
         因此，变量background持有View对象的引用，View持有Context的引用，
         所有background间接持有Context对象的引用了*/
         background.setCallback(this);
         .......
    }
这时候想要销毁原来的Activity时，发现静态对象Drawable间接持有Context对象的引用，导致Activity的内存无法完全释放内存，这时候就造成了内存泄露。
因此，Android系统在在3.0版本之后修改了setBackgroundDrawable内部方法中的 background.setCallback(this);方法，里面的实现使用了弱引用来持有View对象的引用，从而避免了内存泄漏隐患。所以，以后代码中避免使用静态资源，或者使用弱引用来解决相应的问题也是可以的。



单例模式导致内存泄漏

public class CustomManager {
    private static CustomManager sInstance;
    public static CustomManager getInstance(Context context) {
        if (sInstance == null) {
            sInstance = new CustomManager(context);
        }
        return sInstance;
    }
    private Context mContext;
    private CustomManager(Context context) {
        mContext = context;
    }
}
这样单例，有内存泄露的隐患，如果是在Activity中创建这个单例的话，传入的context为Activity的context，如果想要销毁Activity，但是单例的生命周期是整个APP，导致Activity的内存释放不完全。
所以需要修改成如下：

public class CustomManager {
    private static CustomManager sInstance;
    public static CustomManager getInstance(Context context) {
        if (sInstance == null) {
            sInstance = new CustomManager(context.getApplicationContext());
        }
        return sInstance;
    }
    private Context mContext;
    private CustomManager(Context context) {
        mContext = context;
    }
}
将context改为整个Application的Context，这时候单例与Activity就无关了，Activity释放的时候就不会出现内存泄露的问题了。

总结


以后在使用Context对象获取静态资源，创建单例对象或者静态方法的时候，请多考虑Context的生命周期，一定要记得不要使用Activity的Context，切记要使用生命周期长的Application的Context对象。但是并不是所有情况使用Application的Context对象，在创建Dialog，View控件的时候都必须使用Activity的Context对象。

根据生命周期选择适合的Context类型。