圓形隨手指移動

package com.bwie.playbass;


import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.AttributeSet;
import android.view.Display;
import android.view.MotionEvent;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    private int screenW;//屏幕寬度
    private int screenH;//屏幕高度
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.activity_main);
        Display dis=this.getWindowManager().getDefaultDisplay();
        //設置全屏
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
        //獲取屏幕寬度
        screenW=dis.getWidth();
        //獲取屏幕高度
        screenH=dis.getHeight();
        setContentView(new BallView(this));
    }
    class BallView extends View{
        //定義畫筆
        private Paint paint;
        //圓點默認X座標
        private float cx=50;
        //圓點默認Y座標
        private float cy=50;
        //定義半徑
        private int radius=100;
        //定義畫筆顏色
        private int colorArray[] ={Color.BLACK,Color.GREEN,Color.YELLOW,Color.RED};
        private int paintColor = colorArray[0];
        public BallView(Context context) {
            super(context);
            //初始化畫筆
            initPaint();
        }

        private void initPaint() {
            paint=new Paint();
            paint.setAntiAlias(true);
            paint.setColor(paintColor);
        }
            //重寫onDraw方法
        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            //將屏幕設置爲白色
            canvas.drawColor(Color.WHITE);
            //修正圓點座標
            revise();
            //隨機設置畫筆顏色
            setPainRandomColor();
            //繪製小圓作爲小球
            canvas.drawCircle(cx,cy,radius,paint);
        }
            //爲畫筆設置隨機顏色
        private void setPainRandomColor() {
            Random rand=new Random();
            int randomIndex = rand.nextInt(colorArray.length);
            paint.setColor(colorArray[randomIndex]);
        }
            //修正圓點座標
        private void revise() {
            if(cx <= radius){
                cx = radius;
            }else if(cx >= (screenW-radius)){
                cx=screenW-radius;
            }
            if(cy <= radius){
                cy=radius;
            }else if(cy >= (screenH-radius)){
                cy=screenH-radius;
            }
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            switch(event.getAction()){
                //按下
                case MotionEvent.ACTION_DOWN:
                    cx = (int) event.getX();
                    cy = (int) event.getY();
                    // 通知重繪
                    postInvalidate();//該方法會調用onDraw方法，重新繪圖
                    break;
                //移動
                case MotionEvent.ACTION_MOVE:
                    cx=(int)event.getX();
                    cy=(int)event.getY();
                    postInvalidate();
                    break;
                //擡起
                case MotionEvent.ACTION_POINTER_UP:
                    cx=(int)event.getX();
                    cy=(int)event.getY();
                    postInvalidate();
                    break;
            }
            return true;
        }
    }
}