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

package com.syntc.demo;

import android.annotation.TargetApi;
import android.content.Intent;
import android.os.Build;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

import com.example.lfk.myapplication.R;

public class MainActivity extends AppCompatActivity {

   
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Intent intent = new Intent();
        intent.setClass(this, MyService.class);
        startService(intent);
        this.finish();
    }

}

package com.syntc.demo;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;

import android.util.AttributeSet;
import android.view.View;

/**
 * Created by lfk on 2016/8/31.
 */
public class MyView extends View {
    private Paint mPaint;
    private Context mContext;
    private static final String mString = "Welcome to Mr Wei's blog";

    public MyView(Context context) {
        super(context);

    }
    public MyView(Context context,AttributeSet attr)
    {
        super(context,attr);

    }
    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        super.onDraw(canvas);

        mPaint = new Paint();

        //设置画笔颜色
        mPaint.setColor(Color.RED);
        //设置填充
        mPaint.setStyle(Paint.Style.FILL);

        //画一个矩形,前俩个是矩形左上角坐标，后面俩个是右下角坐标
        canvas.drawRect(new Rect(10, 10, 100, 100), mPaint);

        mPaint.setColor(Color.BLUE);
        //绘制文字
        canvas.drawText(mString, 10, 110, mPaint);
    }
}

package com.syntc.demo;

import android.app.Service;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.graphics.PixelFormat;
import android.os.IBinder;
import android.view.View;
import android.view.WindowManager;

public class MyService extends Service implements View.OnClickListener {
    private WindowManager mWindowManager;
    private WindowManager.LayoutParams mLayoutParams;
    private MyView mMyView;
    private boolean flag = true;
    public MyService() {
        
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mWindowManager = (WindowManager) getApplicationContext().getSystemService(
                Context.WINDOW_SERVICE);
        mLayoutParams = new WindowManager.LayoutParams();
        mLayoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ALERT;// 系统提示window
        mLayoutParams.format = PixelFormat.TRANSLUCENT;// 支持透明
        //mParams.format = PixelFormat.RGBA_8888;
        mLayoutParams.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE;// 焦点
        mLayoutParams.width = 490;//窗口的宽和高
        mLayoutParams.height = 160;
        mLayoutParams.x = 0;//窗口位置的偏移量
        mLayoutParams.y = 0;
        mMyView = new MyView(this);
        mMyView.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {

    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        if (flag) {
            flag = false;
            mWindowManager.addView(mMyView, mLayoutParams);//添加窗口
        }
        return super.onStartCommand(intent, flags, startId);
    }


    @Override
    public void onDestroy() {
        super.onDestroy();
        if (mMyView.getParent() != null)
            mWindowManager.removeView(mMyView);//移除窗口
        super.onDestroy();
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }
}