AlertDialog传递application context报错原因分析
前言：在Activity中某个地方需要弹对话框时，通常会构造一个AlertDialog然后show一下搞定。其中在构造AlertDialog时，也很自然的传递了当前Activity作为上下文context，但这里如果我们故意传递Application context，那么会发生应用崩溃。本着知其然知其所以然的精神，来初探一下原因。
1.根据堆栈信息，ViewRootImpl如下地方跑出异常：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(),
mAttachInfo.mContentInsets, mInputChannel);
...
if (DEBUG_LAYOUT) Log.v(TAG, "Added window " + mWindow);
if (res < WindowManagerGlobal.ADD_OKAY) {
mAttachInfo.mRootView = null;
mAdded = false;
mFallbackEventHandler.setView(null);
unscheduleTraversals();
setAccessibilityFocus(null, null);
switch (res) {
case WindowManagerGlobal.ADD_NOT_APP_TOKEN:
throw new WindowManager.BadTokenException(
"Unable to add window -- token " + attrs.token
+ " is not for an application");上面，我们看到，应用在向WMS申请添加窗口失败。
2.再来看看WMS中发生了什么：
public int addWindow(Session session, IWindow client, int seq,
WindowManager.LayoutParams attrs, int viewVisibility, int displayId,
Rect outContentInsets, InputChannel outInputChannel) {
...
boolean addToken = false;
WindowToken token = mTokenMap.get(attrs.token);
...
} 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;3.最后我们再回到应用层，来分析具体原因。
AlertDialog对话框显示类似Toast、悬浮窗，（Acitivity实质上也是这个流程，由ActivityThread在onResume后触发）：即通过WindowManagerImpl.addView(View view, ViewGroup.LayoutParams params) -> WindowManagerGlobal.addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) ->
ViewRootImpl.setView(View view, WindowManager.LayoutParams attrs, View panelParentView)
从前面的分析发现，对比正常情况，attrs.token值在这里为null。
AlertDialog继承自Dialog，添加窗口的地方如下：
public void show() {
...
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 {
}继续来看下WindowManager.addView -> WindowManagerGlobal.addView (Android5.0):
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);void adjustLayoutParamsForSubWindow(WindowManager.LayoutParams wp) {
CharSequence curTitle = wp.getTitle();
if (wp.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wp.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
if (wp.token == null) {
View decor = peekDecorView();
if (decor != null) {
wp.token = decor.getWindowToken();
}
}
if (curTitle == null || curTitle.length() == 0) {
String title;
if (wp.type == WindowManager.LayoutParams.TYPE_APPLICATION_MEDIA) {
title="Media";
} else if (wp.type == WindowManager.LayoutParams.TYPE_APPLICATION_MEDIA_OVERLAY) {
title="MediaOvr";
} else if (wp.type == WindowManager.LayoutParams.TYPE_APPLICATION_PANEL) {
title="Panel";
} else if (wp.type == WindowManager.LayoutParams.TYPE_APPLICATION_SUB_PANEL) {
title="SubPanel";
} else if (wp.type == WindowManager.LayoutParams.TYPE_APPLICATION_ATTACHED_DIALOG) {
title="AtchDlg";
} else {
title=Integer.toString(wp.type);
}
if (mAppName != null) {
title += ":" + mAppName;
}
wp.setTitle(title);
}
} else {
if (wp.token == null) {
wp.token = mContainer == null ? mAppToken : mContainer.mAppToken;
}
if ((curTitle == null || curTitle.length() == 0)
&& mAppName != null) {
wp.setTitle(mAppName);
}
}
if (wp.packageName == null) {
wp.packageName = mContext.getPackageName();
}
if (mHardwareAccelerated) {
wp.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
原来WindowManager.LayoutParam.token在这里赋值的，而需要的条件是WindowManagerImpl.mParentWindow不为null。
4.最后，我们来看看Dialog中的mWindowManager
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);
}答案就在这里，如果我们的context是activity，那么将调用的是Activity.getSystemService()，之后才是contextImpl.getSystemService()
首先来看看Activity.getSystemService()：
@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);
}
@Activity.java
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;
@Window.java
/**
* Set the window manager for use by this Window to, for example,
* display panels. This is not used for displaying the
* Window itself -- that must be done by the client.
*
* @param wm The window manager for adding new windows.
*/
public void setWindowManager(WindowManager wm, IBinder appToken, String appName) {
setWindowManager(wm, appToken, appName, false);
}
/**
* Set the window manager for use by this Window to, for example,
* display panels. This is not used for displaying the
* Window itself -- that must be done by the client.
*
* @param wm The window manager for adding new windows.
*/
public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
boolean hardwareAccelerated) {
mAppToken = appToken;
mAppName = appName;
mHardwareAccelerated = hardwareAccelerated
|| SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
if (wm == null) {
wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
}
mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
}@WindowManagerImpl.java
private WindowManagerImpl(Display display, Window parentWindow) {
mDisplay = display;
mParentWindow = parentWindow;
}
public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
return new WindowManagerImpl(mDisplay, parentWindow);
}5.总结：
1.当构造AlertDialog时，正常情况下传递Activity context时，我们将复用Activity的WindowManager，其具有parentWindow，AlertDialog会继承parentWindow属性，这样在像WMS申请添加窗口时，窗口属性中会具备添加TYPE_APPLICATION窗口必要的WindowManager.LayoutParams.token属性。
2.当构造AlertDialog时，正常情况下传递Application context时,我们获取的WindowManager是ContextImpl中注册的一个WindowManagerImpl实例，其parentWindow为null，在向WMS申请添加窗口时，会缺少WindowManager.LayoutParams.token属性而报错。
3.可以尝试修改AlertDialog显示前，修改窗口默认type类型，规避问题。待验证