Android 自定义View控件，实现跟随手指触摸移动的小球

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Android UI组件是通过继承View类，然后绘制内容，比如ImageView，TextView等组件都是继承View类。

当Android系统提供的组件功能不能满足需求时，可以通过继承View类重写一个或多个方法，派生自定义的组件，View类常用重写方法：

1.构造器：View子类最基本的重写方法

protected voidonDraw(Canvas canvas)

public booleanonTouchEvent(MotionEvent event)

public boolean onTrackballEvent(MotionEvent event)

public boolean onKeyDown(int keyCode, KeyEvent event)
public boolean onKeyUp(int keyCode, KeyEvent event)
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)//检测View组件及子组件大小
protected void onSizeChanged(int w, int h, int oldw, int oldh)
protected boolean onSetAlpha(int alpha)
protected void onFocusChanged(boolean gainFocus, int direction, Rect previouslyFocusedRect)
public void onWindowFocusChanged(boolean hasWindowFocus)
protected void onFinishInflate()//回调方法，当应用从xml文件加载构建界面之后此方法会被回调
定义DrawView 实现跟随手指移动的小球 DrawView.java

package shortcut.song.com.myapplication;import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.View;/**
 * Created by Administrator on 2017/1/6 0006.
 */

public class DrawView extends View {public float currentX = 80;public float currentY = 60;public DrawView(Context context){super(context);}public DrawView(Context context, AttributeSet attributeSet){super(context, attributeSet);}@Override
    protected void onDraw(Canvas canvas) {Paint p = new Paint();p.setColor(Color.RED);canvas.drawCircle(currentX, currentY, 10, p);}@Override
    public boolean onTouchEvent(MotionEvent event) {this.currentX = event.getX(); //触摸座标X
        this.currentY = event.getY(); //触摸座标Y
        invalidate();//重绘组件
        return true;//返回true：事件已处理，此处必须返回true，否则小球移动不了。
    }
}

//Layout文件activity_draw_view.xml

//通过代码编程实现时没用到此文件

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_draw_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="shortcut.song.com.myapplication.DrawViewActivity">
    <!-- 自定义的DrawView组件 --><shortcut.song.com.myapplication.DrawView  
        android:id="@+id/drawview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" 
	android:background="#0ff0ff"/>
</RelativeLayout>

DrawViewActivity.java

package shortcut.song.com.myapplication;import android.graphics.Color;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;public class DrawViewActivity extends AppCompatActivity {@Override
    protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);//通过XML布局实现
        //setContentView(R.layout.activity_draw_view);
        //final DrawView mDrawView = (DrawView)findViewById(R.id.drawview);

        //通过代码编程实现
        LinearLayout main = new LinearLayout(this);main.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.MATCH_PARENT));main.setOrientation(LinearLayout.HORIZONTAL);main.setGravity(Gravity.CENTER);DrawView mDrawView = new DrawView(this);mDrawView.setBackgroundColor(Color.BLUE);main.addView(mDrawView);setContentView(main);//设置Aictivity 显示View

    }
}

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