我们Android平台是一个又一个的Activity组成的，每一个Activity有一个或者多个View构成。所以说，当我们想显示一个界面的时候，我们首先想到的是建立一个Activity，然后所有的操作在Activity里面实现，或者是一个Dialog或者Toast。这种方式固然简单，但是在有些情况下，我们要求的只是简单的显示，用Activity显然是多余，这个时候，我们如何处理呢？

      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。

一、一个简单WindowManager实例

 1 package com.example.qjm3662.simplewindowsmanager;
 2 
 3 import android.graphics.PixelFormat;
 4 import android.support.v7.app.AppCompatActivity;
 5 import android.os.Bundle;
 6 import android.view.WindowManager;
 7 import android.widget.TextView;
 8 
 9 public class MainActivity extends AppCompatActivity {
10 
11     WindowManager wManager;
12     @Override
13     protected void onCreate(Bundle savedInstanceState) {
14         super.onCreate(savedInstanceState);
15         setContentView(R.layout.activity_main);
16 
17         //Activity自带一个getWindowManager()方法，可以获得当前系统的WindowManager
18         wManager = getWindowManager();
19         TextView tv = new TextView(this);
20         tv.setText("sdgvsd");
21         //布局参数
22         WindowManager.LayoutParams params = new WindowManager.LayoutParams();
23         params.width = 200;
24         params.height = 200;
25         
26         //设置透明程度
27         params.format = PixelFormat.TRANSPARENT;
28         //params.format = PixelFormat.RGBA_8888;
29         //设置是否获得焦点，如果获得焦点，则底层的View无法获得焦点，也无法响应返回键，默认是获得
30         params.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE;
31         wManager.addView(tv,params);
32     }
33 }

二、WindowManager的属性（WindowManager.LayoutParms）

详细的属性表在：

http://www.cnblogs.com/shitianzeng/articles/2814050.html

三、如何让一个view显示在桌面（即Activity销毁后（或在后台）仍可在桌面显示指定view）—》eg:各种安全软件的悬浮球

—-》利用Service

 1 package com.example.qjm3662.windowmanager;
 2 
 3 import android.content.Intent;
 4 import android.os.Bundle;
 5 import android.support.v7.app.AppCompatActivity;
 6 import android.view.Menu;
 7 import android.view.MenuItem;
 8 import android.view.View;
 9 import android.widget.Button;
10 
11 public class MainActivity extends AppCompatActivity {
12 
13     Button button;
14     @Override
15     protected void onCreate(Bundle savedInstanceState) {
16         super.onCreate(savedInstanceState);
17         setContentView(R.layout.activity_main);
18 
19         button = (Button) findViewById(R.id.btn_start);
20         button.setOnClickListener(new View.OnClickListener() {
21             @Override
22             public void onClick(View v) {
23                 startService(new Intent(MainActivity.this,WindowManagerService.class));
24             }
25         });
26     }
27 
28     @Override
29     public boolean onCreateOptionsMenu(Menu menu) {
30         getMenuInflater().inflate(R.menu.menu_main,menu);
31         return true;
32     }
33 
34     @Override
35     public boolean onOptionsItemSelected(MenuItem item) {
36         int id = item.getItemId();
37 
38         if(id == R.id.action_settings){
39             return true;
40         }
41         return super.onOptionsItemSelected(item);
42     }
43 }

package com.example.qjm3662.windowmanager;

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.graphics.PixelFormat;
import android.os.Handler;
import android.os.IBinder;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.WindowManager;
import android.widget.TextView;

public class WindowManagerService extends Service {

    private Handler handler = new Handler();
    public WindowManagerService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        String msg = "This is a text message";
        final WindowManager wm = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
        final View view = LayoutInflater.from(getApplicationContext()).inflate(R.layout.view_system,null);

        TextView tv = (TextView) view.findViewById(R.id.tv);
        tv.setText(msg);
        final WindowManager.LayoutParams params = new WindowManager.LayoutParams();
        params.x = 0;
        params.y = 0;
        params.gravity = Gravity.RIGHT | Gravity.BOTTOM;
        params.width = 400;
        params.height = 400;
        params.type = WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY;
        params.format = PixelFormat.TRANSPARENT;
        params.flags |= WindowManager.LayoutParams.FLAG_FORCE_NOT_FULLSCREEN;
        wm.addView(view,params);

        new Thread(){
            @Override
            public void run() {
                super.run();
                try {
                    Thread.sleep(15000);
                    handler.post(new Runnable() {
                        @Override
                        public void run() {
                            wm.removeViewImmediate(view);
                        }
                    });
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }.start();
        return super.onStartCommand(intent, flags, startId);
    }
}