[Android]笔记3-跟随手指的小球

为了实现一个跟随的小球，自定义UI组件，这个UI组件将会在指定位置绘制一个小球，这个位置可以动态改变。当用户通过手指在屏幕上拖动时，程序监听这个手指动作，把手指的位置传入自定义UI组件，并通知该组件重绘制即可。

DrawView UI组件

package com.dezai.customerviewactivity;

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

/**
* Created by A292 on 2017/5/8.
*/

public class DrawView  extends View {
public float currentX=40;
public float currentY=50;
Paint p=new Paint();
public DrawView(Context context) {
super(context);
}
public DrawView(Context context, AttributeSet set){
super(context,set);
}
@Override
public void onDraw(Canvas canvas){
super.onDraw(canvas);
//设置画笔颜色
p.setColor(Color.RED);
//绘制一个小圆(作为一个小球)
canvas.drawCircle(currentX,currentY,25,p);
}
@Override
public boolean onTouchEvent(MotionEvent event){
//修改currentX currentY 两个属性
currentX =event.getX();
currentY=event.getY();
//通知当前组件重绘制自己
invalidate();
//返回true表明该处理方法已处理该事件
return true;
}
}

XML:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.dezai.customerviewactivity.MainActivity">

<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:id="@+id/root">
<com.dezai.customerviewactivity.DrawView
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
</android.support.constraint.ConstraintLayout>

MainActivity

package com.dezai.customerviewactivity;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

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