自定义view组件

参考《疯狂android讲义》第2版 2.1节P48，对应CustomViewDemo.zip。

若在开发过程中，发现现有的view均不能满足需要，可以自定义一个view。

自定义一个view 的关键在于重写view类的几个核心方法，如onDraw， onTouchEvent等。

自定义view类：

DrawBall.java

package com.ljh.customviewdemo.ui;

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;

public class DrawBall extends View {

	private Paint p = new Paint();
	private float currentX = 40;
	private float currentY = 50;

	public DrawBall(Context context) {
		super(context);
	}

	public DrawBall(Context context, AttributeSet set) {
		super(context, set);
	}

	@Override
	protected void onDraw(Canvas canvas) {
		super.onDraw(canvas);
		p.setColor(Color.RED);
		canvas.drawCircle(currentX, currentY, 15, p);
	}

	@Override
	public boolean onTouchEvent(MotionEvent event) {

		currentX = event.getX();
		currentY = event.getY();

		/*
		 * 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().
		 */
		invalidate();
		return true;
	}

}

Activity类：MainAtivity.java

package com.ljh.customviewdemo;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class MainActivity extends Activity {

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

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

布局文件：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    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=".MainActivity" >

	<!--  注意：此处使用类的全路径，标明ui类型。  -->
    <com.ljh.customviewdemo.ui.DrawBall
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</RelativeLayout>

注意，activity中包括多个view。一个activity对应一个布局文件，布局文件中包括多个view，这些view可以是android定义好的，或者自定义的。