android 自定义View

哈哈，兄弟我终于自己写了一个view，不是网上那种简单的哦，还是有一定技术含量的，

我是通过学习ApiDemo（android自带的sample）里面LabelView实现的，

先谈谈学习过程，觉得一开始不应当盲目的动手做，应对想把原理搞明白，哪怕一个很小的View，也应当将各个细节弄清楚，

等这些搞定了，接下来的工作就是水道渠成了！

自定义一个View那必须继承View，

首先说说我的View是啥，恩，很简单，就是一个椭圆，其中可以设置4个参数，分别是top、left、right、bottom，应该很清楚吧，因为canvas.drawOval时用到了4个值，

首先得定义4个属性值，是我的View专有的，

建立attr.xml放到values下面

然后写个RocketView

package com.myos;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.View.MeasureSpec;

public class RocketView extends View{
	private Paint mOvalPaint;
	private int mStrokeWidth = 2;
	private int padding = 3;
	
	//椭圆参数
	private int mOval_l;
	private int mOval_t;
	private int mOval_r;
	private int mOval_b;
	
	
	
	//构造
	public RocketView(Context context, AttributeSet attrs) {
		super(context, attrs);
		initRocketView();
		
		TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.RocketView);

        mOval_l = a.getDimensionPixelOffset(R.styleable.RocketView_ovalLeft, padding);  
        mOval_t = a.getDimensionPixelOffset(R.styleable.RocketView_ovalTop, padding);  
        mOval_r = a.getDimensionPixelOffset(R.styleable.RocketView_ovalRight, 100);  
        mOval_b = a.getDimensionPixelOffset(R.styleable.RocketView_ovalBottom, 100);  

        a.recycle();
	}

    private void initRocketView() {
    	mOvalPaint = new Paint();
    	mOvalPaint.setAntiAlias(true);
    	mOvalPaint.setColor(Color.BLUE);
    	mOvalPaint.setStyle(Paint.Style.STROKE);
    	mOvalPaint.setStrokeWidth(mStrokeWidth);
    	setPadding(padding,padding,padding,padding);
    }
    
    public void setOvalRect(int l, int t, int r, int b){
    	mOval_l = l + padding;
    	mOval_t = t + padding;
    	mOval_r = r;
    	mOval_b = b;
    	requestLayout();
        invalidate();
    }
	@Override
	protected void onDraw(Canvas canvas) {
		// TODO Auto-generated method stub
		super.onDraw(canvas);
		canvas.drawColor(Color.WHITE);
		// 绘制椭圆
		RectF re11 = new RectF(mOval_l, mOval_t, mOval_r, mOval_b);
		canvas.drawOval(re11, mOvalPaint);	
		
//		// 绘制圆形
//		canvas.drawCircle(mCircle_x, mCircle_y, mCircle_r, mPaint);
	}
	
	 /**
	  * @see android.view.View#measure(int, int)
	  */
	 @Override
	 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	     setMeasuredDimension(measureWidth(widthMeasureSpec),
	             measureHeight(heightMeasureSpec));
	 }

	 /**
	  * Determines the width of this view
	  * @param measureSpec A measureSpec packed into an int
	  * @return The width of the view, honoring constraints from measureSpec
	  */
	 private int measureWidth(int measureSpec) {
	     int result = 0;
	     int specMode = MeasureSpec.getMode(measureSpec);
	     int specSize = MeasureSpec.getSize(measureSpec);

	     if (specMode == MeasureSpec.EXACTLY) {
	         // We were told how big to be
	         result = specSize;
	     } else {
	         // Measure the text
	         result = mOval_r + getPaddingLeft()
	                 + getPaddingRight();
	         if (specMode == MeasureSpec.AT_MOST) {
	             // Respect AT_MOST value if that was what is called for by measureSpec
	             result = Math.min(result, specSize);
	         }
	     }
		 
	     return result;
	 }

	 /**
	  * Determines the height of this view
	  * @param measureSpec A measureSpec packed into an int
	  * @return The height of the view, honoring constraints from measureSpec
	  */
	 private int measureHeight(int measureSpec) {
	     int result = 0;
	     int specMode = MeasureSpec.getMode(measureSpec);
	     int specSize = MeasureSpec.getSize(measureSpec);

	     if (specMode == MeasureSpec.EXACTLY) {
	         // We were told how big to be
	         result = specSize;
	     } else {
	         // Measure the text (beware: ascent is a negative number)
	         result = mOval_b + getPaddingTop()
	                 + getPaddingBottom();
	         if (specMode == MeasureSpec.AT_MOST) {
	             // Respect AT_MOST value if that was what is called for by measureSpec
	             result = Math.min(result, specSize);
	         }
	     }
	     
	     return result;
	 }
}

在构造中getDimensionPixelOffset检索出一个属性值，没有的话就使用第2个参数做默认值，

可以在布局xml中初始化这个属性值，下面是我main.xml

我这里没啥用处，仅仅试一下，待扩展属性时再说，说明一下xmlns:app中app可以随便命名的，这个android的规矩还真多

另外构造函数中onMeasure很重要的，onDraw很明显就是画椭圆了，android在画的时候呢，会去先测量，需要我们来提供View的宽和高，

大家都知道wrap_content、fill_parent，测量时会有3种模式，分别是UNSPECIFIED、EXACTLY、AT_MOST，

当使用

wrap_content时就是AT_MOST模式，fill_parent就是EXACTLY模式，可以看看官方文档：

http://developer.android.com/guide/topics/ui/how-android-draws.html

相信研读一下代码就明白了

RocketView定义好了，接下来就是使用了

package com.myos;

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

public class MainActivity extends Activity {
	RocketView mRocketView;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        mRocketView = (RocketView)findViewById(R.id.rv);
        mRocketView.setOvalRect(0, 0, 500, 100);
    }
}

可以通过setOvalRect函数来动态调整椭圆的大小！

下面是运行效果，ko啦！