你真的了解Context吗

发表于

引言

很多人应该知道Activity,Service中的Context和ApplicationContext的区别,而且也知道Context,ContextImpl,ContextWrapper,Activity,Service,Application构成的体系，在异步任务需要Context时,也知道为了防止内存泄露需要传递ApplicationContext而不是Activity的Context,但是这样的场景并不万能，因为并不是所有需要Activity的Context的地方都可以用ApplicationContext来代替。

1.Context继承体系

所以一个App进程中Context数量=Activity数量+Service数量+1.

2.Activity的startActivity()和Application的startActivity()的区别

Application.startActivity()本质上是调用其中的mBase(ContextImpl对象)的startActivity()方法，而ContextImpl.startActivity()如下: 

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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);
    }

可以看到这里对Intent的flag进行了检查，如果没有FLAG_ACTIVITY_NEW_TASK属性,就会抛出异常。
那为什么Activity中就不需要做这样的检查了?
根本原因在于Application中的Context并没有所谓的任务栈,所以待启动的Activity就找不到task了，这样的话要启动Activity就必须将它放到一个新的task中，即使用singleTask的方式启动。

3.Dialog的创建

如下示例: 

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AlertDialog imageDialog = new AlertDialog.Builder(context).setTitle("状态操作").setItems(items, listener).create();  
imageDialog.show();

如果其中的context是Application Context,那么会抛出以下异常： 

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android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application

这个异常是在ViewRoogImpl的setView()中抛出的。
而抛出这个异常的原因是与WMS进行Binder IPC(res=mWindowSession.addToDisplay())的结果,而这个结果是执行WMS中addWindow()的结果,该方法如下: 

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public int addWindow(Session session,IWindow client,int seq,WindowManager.LayoutParams attrs,int viewVisibility,int displayId,Rect outContentInsets, InputChannel outInputChannel){
	if(token==null){
		...
	} else if (type >= FIRST_APPLICATION_WINDOW && type <= LAST_APPLICATION_WINDOW) {
                AppWindowToken atoken = token.appWindowToken;
                if (atoken == null) {
                    Slog.w(TAG, "Attempted to add window with non-application token "
                          + token + ".  Aborting.");
                    return WindowManagerGlobal.ADD_NOT_APP_TOKEN;
                } 
                ...
    }        
}

显然,这是由于AppWindowToken==null导致的，这个AppWindowToken对应client端中Window的IBinder mAppToken这个成员。

由于AlertDialog中的super()会调用Dialog的构造方法，所以我们先看一下Dialog的构造方法: 

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Dialog(Context context, int theme, boolean createContextThemeWrapper) {
    if (createContextThemeWrapper) {
        if (theme == 0) {
            TypedValue outValue = new TypedValue();
            context.getTheme().resolveAttribute(com.android.internal.R.attr.dialogTheme,
                    outValue, true);
            theme = outValue.resourceId;
        }
        mContext = new ContextThemeWrapper(context, theme);
    } else {
        mContext = context;
    }
    mWindowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
    Window w = PolicyManager.makeNewWindow(mContext);
    mWindow = w;
    w.setCallback(this);
    w.setOnWindowDismissedCallback(this);
    w.setWindowManager(mWindowManager, null, null);
    w.setGravity(Gravity.CENTER);
    mListenersHandler = new ListenersHandler(this);
}

注意其中的w.setWindowManager(),显然，传递的appToken为null.这也是Dialog和Activity窗口的一个区别,Activity会将这个appToken设置为ActivityThread传过来的token.

在Dialog的show()方法中: 

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public void show() {
    if (mShowing) {
        if (mDecor != null) {
            if (mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
                mWindow.invalidatePanelMenu(Window.FEATURE_ACTION_BAR);
            }
            mDecor.setVisibility(View.VISIBLE);
        }
        return;
    }
    mCanceled = false;
    
    if (!mCreated) {
        dispatchOnCreate(null);
    }
    onStart();
    mDecor = mWindow.getDecorView();
    if (mActionBar == null && mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
        final ApplicationInfo info = mContext.getApplicationInfo();
        mWindow.setDefaultIcon(info.icon);
        mWindow.setDefaultLogo(info.logo);
        mActionBar = new WindowDecorActionBar(this);
    }
    WindowManager.LayoutParams l = mWindow.getAttributes();
    if ((l.softInputMode
            & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) == 0) {
        WindowManager.LayoutParams nl = new WindowManager.LayoutParams();
        nl.copyFrom(l);
        nl.softInputMode |=
                WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
        l = nl;
    }
    try {
        mWindowManager.addView(mDecor, l);
        mShowing = true;
        sendShowMessage();
    } finally {
    }
}

其中mWindow是PhoneWindow类型，mWindow.getAttributes()获取到的Type为TYPE_APPLICATION.

Dialog也是通过WindowManager把自己的Window添加到WMS上，但是这里在addView()之前,mWindow的token为null(前面已经分析了,w.setWindowManager的第二个参数为null).而WMS要求TYPE_APPLICATION的窗口的token不能为null.

而如果用Application或者Service的Context区获取这个WindowManager服务的话,会得到一个WindowManagerImpl对象,这个实例中token也是空的。

那为什么Activity就可以呢?
原来是Activity重写了getSystemService()方法: 

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@Override
public Object getSystemService(@ServiceName @NonNull String name) {
	if (getBaseContext() == null) {
		throw new IllegalStateException("System services not available to Activities before onCreate()");
	}
	if (WINDOW_SERVICE.equals(name)) {
		return mWindowManager;
	} else if (SEARCH_SERVICE.equals(name)) {
		ensureSearchManager();
		return mSearchManager;
	}
	return super.getSystemService(name);
}

显然，对于WINDOW_SERVICE,返回的是mWindowManager对象，而这个对象的创建是在Activity的attach()方法中: 

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final void attach(Context context, ActivityThread aThread, Instrumentation instr, IBinder token, int ident,
			Application application, Intent intent, ActivityInfo info, CharSequence title, Activity parent, String id,
			NonConfigurationInstances lastNonConfigurationInstances, Configuration config,
			IVoiceInteractor voiceInteractor) {
		attachBaseContext(context);
		mFragments.attachActivity(this, mContainer, null);
		mWindow = PolicyManager.makeNewWindow(this);
		mWindow.setCallback(this);
		mWindow.setOnWindowDismissedCallback(this);
		mWindow.getLayoutInflater().setPrivateFactory(this);
		if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
			mWindow.setSoftInputMode(info.softInputMode);
		}
		if (info.uiOptions != 0) {
			mWindow.setUiOptions(info.uiOptions);
		}
		mUiThread = Thread.currentThread();
		mMainThread = aThread;
		mInstrumentation = instr;
		mToken = token;
		mIdent = ident;
		mApplication = application;
		mIntent = intent;
		mComponent = intent.getComponent();
		mActivityInfo = info;
		mTitle = title;
		mParent = parent;
		mEmbeddedID = id;
		mLastNonConfigurationInstances = lastNonConfigurationInstances;
		if (voiceInteractor != null) {
			if (lastNonConfigurationInstances != null) {
				mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
			} else {
				mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this, Looper.myLooper());
			}
		}
		mWindow.setWindowManager((WindowManager) context.getSystemService(Context.WINDOW_SERVICE), mToken,
				mComponent.flattenToString(), (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
		if (mParent != null) {
			mWindow.setContainer(mParent.getWindow());
		}
		mWindowManager = mWindow.getWindowManager();
		mCurrentConfig = config;
	}

注意其中的mWindow.setWindowManager(),在这里将Activity的mToken给了mWindow,所以这就是Activity中的mWindow和Dialog中的mWindow的区别。

所以不能通过Application和Service的Context来创建Dialog,只能通过Activity的Context来创建Dialog.