自定义view实现随手指移动的小球及画轨迹

先直接上代码：

step1.建立drawview.java

package com.example.joky27.customview;


import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

/**
 * Created by joky27 on 2017/4/17.
 */

public class DrawView extends View{
    public float currentx=40;
    public float currenty=50;
    public float prex;
    public float prey;
    //之前坐标
    private Path mpath= new Path();//定义路径
    public Paint p; //定义画笔
    public DrawView (Context context)
    {
        super( context);
    }
    public DrawView(Context context,AttributeSet set)
    {
        super(context,set);
        mycontext=context;
        p= new Paint();
        p.setColor(Color.RED);
        p.setStyle(Paint.Style.STROKE);//将画笔设置为空心，才会画成曲线
        p.setStrokeCap(Paint.Cap.ROUND);
        p.setStrokeWidth(5);//设置画笔宽度
        p.setAntiAlias(true);
    }

    @Override
    public void onDraw(Canvas canvas){
        super.onDraw(canvas);
        Paint m=new Paint();
        m.setColor(Color.BLUE);
        canvas.drawCircle(currentx,currenty,15,m);
        canvas.drawPath(mpath,p);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event)
    {   //不断触发触摸屏事件，将监听到的信息都传入DrawView 组件
        currentx=event.getX();
        currenty=event.getY();
        switch(event.getAction()){
            case MotionEvent.ACTION_DOWN:
                mpath.moveTo(currentx,currenty);
                prex = currentx;
                prey = currenty;
                break;
            case MotionEvent.ACTION_MOVE:
                mpath.quadTo(prex,prey,currentx,currenty);
                prex = currentx;
                prey = currenty;
                break;
            case MotionEvent.ACTION_UP:
                mpath.reset();
                break;
        }
        invalidate();
        return true;

    }
}

step2.建立activity_main.xml

step3.直接创建mainactivity.java,将activity_main.xml传入

package com.example.joky27.customview;
import android.app.Activity;
import android.os.Bundle;



public class MainActivity extends Activity {

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

主要思路及注意点：

1.设置paint()的style,这里用了两个画笔，画小球用的是蓝色实心，而画路径用的是红色空心（不设为空心画不了曲线）

2.重写onTouchevent中，invalidate()屏幕刷新，当动作为action_up时，重设路径，所以效果是当手指离开触摸屏，之前的轨迹被清屏。

void invalidate ()

Invalidate the whole view. If the view is visible, onDraw(android.graphics.Canvas) will be called at some point in the future.

This must be called from a UI thread. To call from a non-UI thread, call postInvalidate().

                                                                                                                              -------------https://developer.android.com/reference/android/view/View.html