小球移动轨迹渐变 android,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