android的window基础介绍。

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原来，整个 Android的窗口机制是基于一个叫做 WindowManager，这个接口可以添加view到屏幕，也可以从屏幕删除view。它面向的对象一端是屏幕，另一端就是View，直接忽略我们以前的Activity或者Dialog之类的东东。其实我们的Activity 或者Diolog底层的实现也是通过WindowManager，这个 WindowManager是全局的，整个系统就是这个唯一的东东。它是显示 View的最底层了。但是要注意，显示出来就要销毁掉，这个是必须的，销毁其实就是一个remove

Android中悬浮窗口的实现原理和示例代码
http://www.xsmile.net/?p=404

调用WindowManager，并设置WindowManager.LayoutParams的相关属性，通过WindowManager的addView方法创建View，这样产生出来的View根据WindowManager.LayoutParams属性不同，效果也就不同了。比如创建系统顶级窗口，实现悬浮窗口效果！

WindowManager的方法很简单，基本用到的就三个addView，removeView，updateViewLayout。

而WindowManager.LayoutParams的属性就多了，非常丰富，具体请查看SDK文档。这里给出Android中的WindowManager.java源码，可以具体看一下。
WindowManager 的源码地址：
http://www.netmite.com/android/mydroid/frameworks/base/core/java/android/view/WindowManager.java

下面是简单示例代码：
public class myFloatView extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Button bb=new Button(getApplicationContext());
        WindowManager wm=(WindowManager)getApplicationContext().getSystemService("window");
        WindowManager.LayoutParams wmParams = new WindowManager.LayoutParams();
        wmParams.type=2002; //type是关键，这里的2002表示系统级窗口，你也可以试试2003。
        wmParams.format=1;
        /**
         *这里的flags也很关键
         *代码实际是wmParams.flags |= FLAG_NOT_FOCUSABLE;
         *40的由来是wmParams的默认属性（32）+ FLAG_NOT_FOCUSABLE（8）
         */
        wmParams.flags=40;
        wmParams.width=40;
        wmParams.height=40;
        wm.addView(bb, wmParams);//创建View
    }
}

PS：本代码在Android2.3下测试无错（从API Level来看，实际上android1.5都可行）！另外别忘了在AndroidManifest.xml文件中加入如下权限：

<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />

关于代码中wmParams.type的值的问题，下面给出一些数值参考：
/**
         * Window type: the status bar. There can be only one status bar
         * window; it is placed at the top of the screen, and all other
         * windows are shifted down so they are below it.
         */
        public static final int TYPE_STATUS_BAR         = FIRST_SYSTEM_WINDOW;

        /**
         * Window type: the search bar. There can be only one search bar
         * window; it is placed at the top of the screen.
         */
        public static final int TYPE_SEARCH_BAR         = FIRST_SYSTEM_WINDOW+1;

        /**
         * Window type: phone. These are non-application windows providing
         * user interaction with the phone (in particular incoming calls).
         * These windows are normally placed above all applications, but behind
         * the status bar.
         */
        public static final int TYPE_PHONE              = FIRST_SYSTEM_WINDOW+2;

        /**
         * Window type: system window, such as low power alert. These windows
         * are always on top of application windows.
         */
        public static final int TYPE_SYSTEM_ALERT       = FIRST_SYSTEM_WINDOW+3;

        /**
         * Window type: keyguard window.
         */
        public static final int TYPE_KEYGUARD           = FIRST_SYSTEM_WINDOW+4;

        /**
         * Window type: transient notifications.
         */
        public static final int TYPE_TOAST              = FIRST_SYSTEM_WINDOW+5;

        /**
         * Window type: system overlay windows, which need to be displayed
         * on top of everything else. These windows must not take input
         * focus, or they will interfere with the keyguard.
         */
        public static final int TYPE_SYSTEM_OVERLAY     = FIRST_SYSTEM_WINDOW+6;

        /**
         * Window type: priority phone UI, which needs to be displayed even if
         * the keyguard is active. These windows must not take input
         * focus, or they will interfere with the keyguard.
         */
        public static final int TYPE_PRIORITY_PHONE     = FIRST_SYSTEM_WINDOW+7;

        /**
         * Window type: panel that slides out from the status bar
         */
        public static final int TYPE_STATUS_BAR_PANEL   = FIRST_SYSTEM_WINDOW+8;

        /**
         * Window type: panel that slides out from the status bar
         */
        public static final int TYPE_SYSTEM_DIALOG      = FIRST_SYSTEM_WINDOW+8;

        /**
         * Window type: dialogs that the keyguard shows
         */
        public static final int TYPE_KEYGUARD_DIALOG    = FIRST_SYSTEM_WINDOW+9;

        /**
         * Window type: internal system error windows, appear on top of
         * everything they can.
         */
        public static final int TYPE_SYSTEM_ERROR       = FIRST_SYSTEM_WINDOW+10;

        /**
         * End of types of system windows.
         */
        public static final int LAST_SYSTEM_WINDOW      = 2999;

---------------------------------
这个FIRST_SYSTEM_WINDOW的值就是2000。2003和2002的区别就在于2003类型的View比2002类型的还要top，能显示在系统下拉状态栏之上！

    可以看出来，2002的值的含义其实就是2000+2。数值2000的含义就是系统级窗口，2002和2003的区别就是 2003 比 2002还要更上一层！比如，type为2003的view，能显示在手机下拉状态栏之上！

而关于flags等其他的属性请参考SDK文档

http://www.cnblogs.com/GnagWang/archive/2011/02/06/1949569.html

http://gundumw100.javaeye.com/blog/899977