你真的了解Context吗

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。

注:简书对于Markdown的支持还不是很好,希望有更好的代码阅读效果的童鞋,可以直接移步我的Blog: http://blog.imallen.wang/blog/2017/02/20/ni-zhen-de-liao-jie-contextma/

1.Context继承体系

context_hierarchy

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

```

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:

```

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：

```

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:

```

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:

```

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:

```

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:

```

@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:

```

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.