使用Android绘图技术绘制一个椭圆形，然后通过触摸事件让该椭圆形跟着手指移动

引言：在图形绘制中，控制一个图形（圆形，椭圆形，矩形，三角形）移动时，其实计算的都是该图形的中心点移动。在绘制过程中，首先计算出中心点，然后根据中心点的位置计算重图形的上下左右位置。我们假设图形左边为lx，图形右边为rx，图形上边为ty，图形下边为by，图形的中心点位置坐标为（cx,cy）.手指触摸位置坐标为(x,y)

计算公式:

lx = x - cx;

rx = x + cx;

ty = y - cy;

by = y + cy;

下面是控制图形移动的具体代码：

 

GameView.java

package com.yw.mygame.view;



import android.content.Context;

import android.graphics.Canvas;

import android.graphics.Color;

import android.graphics.Paint;

import android.graphics.RectF;

import android.view.KeyEvent;

import android.view.MotionEvent;

import android.view.View;



public class GameView extends View{

    private Paint paint = null;

    private int x = 0;    

    private int y = 0;

    

    //椭圆形的left，right,top,buttom初始位置

    private float lx = 0.0f;

    private float rx = 50.0f;

    private float ty = 0.0f;

    private float by = 100.0f;

    

    /**

     * 椭圆形的中心点坐标（要始终保证中心点的这两个值不变）

     */

    private float cx = 25.0f;

    private float cy = 50.0f;

    public GameView(Context context) {

        super(context);

        this.setFocusable(true);//允许获取上层焦点

        paint = new Paint();

        paint.setColor(Color.GREEN);

//        new GameViewThread().start();

    }

    @Override

    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        canvas.drawLine(x, 0, x, 480, paint);

        RectF rectf = new RectF(lx, ty, rx, by);

        canvas.drawOval(rectf, paint);

    }

    

    /*class GameViewThread extends Thread{

        @Override

        public void run() {

            while(x <= 480){

                x+=5;

                postInvalidate();

                try {

                    Thread.sleep(200);

                } catch (InterruptedException e) {

                    e.printStackTrace();

                }

            }

        }

    }*/

    @Override

    public boolean onTouchEvent(MotionEvent event) {

        x = (int)event.getX();

        //触摸位置

        float x = event.getX();

        float y = event.getY();

        /*lx = cx - (rx-lx)/2;

        rx = cx + (rx-lx)/2;

        ty = cy - (by-ty)/2;

        by = cy + (by-ty)/2;*/

        //改变椭圆形位置

        lx = x - cx;

        rx = x + cx;

        ty = y - cy;

        by = y + cy;

        //对View进行重新绘制

        postInvalidate();

        return true;

    }

    @Override

    public boolean onKeyDown(int keyCode, KeyEvent event) {

        return super.onKeyDown(keyCode, event);

    }

    @Override

    public boolean onKeyUp(int keyCode, KeyEvent event) {

        return super.onKeyUp(keyCode, event);

    }

}

GameViewActivity.java

package com.yw.mygame;



import com.yw.mygame.view.GameView;



import android.app.Activity;

import android.os.Bundle;



public class GameViewActivity extends Activity{

    private GameView gameView;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        gameView = new GameView(this);

        setContentView(gameView);

    }



}