代码就这么一段：
new AlertDialog.Builder(getApplicationContext(),R.style.MyAlertDialogStyle)
.setTitle("温柔")
.setMessage("不知道 不明了 不想要\n" +
"为什么 我的心")
.setPositiveButton("确定",null)
.setNegativeButton("取消",null)
.show();
这个时候报错了： Unable to add window -- token null is not for an application
android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
at android.view.ViewRootImpl.setView(ViewRootImpl.java:576)
at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:269)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:69)
at android.app.Dialog.show(Dialog.java:289)
at android.app.AlertDialog$Builder.show(AlertDialog.java:951)
at cn.bvin.app.androidtest_asgit.MainActivity$1.onClick(MainActivity.java:24)
at android.view.View.performClick(View.java:4446)
at android.view.View$PerformClick.run(View.java:18480)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5294)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:864)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:680)
at dalvik.system.NativeStart.main(Native Method)
为什么会这样呢，看一下源码：
/**
* Create a Dialog window that uses the default dialog frame style.
*
* @param context The Context the Dialog is to run it. In particular, it
* uses the window manager and theme in this context to
* present its UI.
*/
public Dialog(Context context) {
this(context, 0, true);
} /**
* Create a Dialog window that uses a custom dialog style.
*
* @param context The Context in which the Dialog should run. In particular, it
* uses the window manager and theme from this context to
* present its UI.
* @param theme A style resource describing the theme to use for the
* window. See <a href="{@docRoot}guide/topics/resources/available-resources.html#stylesandthemes">Style
* and Theme Resources</a> for more information about defining and using
* styles. This theme is applied on top of the current theme in
* <var>context</var>. If 0, the default dialog theme will be used.
*/
public Dialog(Context context, int theme) {
this(context, theme, true);
} 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);
}
看到上面public的构造方法都是重载下面那个缺省构造方法，三个形参，但是我们看到public的构造方法里最后的createContextThemeWrapper参数都为true
Context context, int theme, boolean createContextThemeWrapper
所以这时mContext的是创建了一个ContextThemeWrapper对象，这下就明白了。 Service,Application,Broadcast都是继承ContextWraper,只有Activity是继承ContextThemeWrapper。
而ContextThemeWrapper又是继承ContextWraper，ContextWraper继承Context。 所以传Activity.this拿到的Context是可以创建ContextThemeWrapper对象的。
再看一下ContextThemeWrapper的源码：
public ContextThemeWrapper() {
super(null);
}
public ContextThemeWrapper(Context base, int themeres) {
super(base);
mThemeResource = themeres;
}
@Override protected void attachBaseContext(Context newBase) {
super.attachBaseContext(newBase);
}
...
@
Override public void setTheme(int resid) {
mThemeResource = resid;
initializeTheme();
}
ContextThemeWrapper有两个构造函数，一个是不带参的，一个是带一个Context和themeRes的，上面dialog的构造方法中是通过后者来创建的Context。
当然用无构造方法也可以创建ContextThemeWrapper对象，就如上图，attachBaseContext方法可以把Context传递进去，
setTheme则可以把themeRes设置进去。。。
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