Android对话框之Context

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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