Android自定义控件

下面通过自定义的View创建了一个同心圆的视图：

com.mxd.view.BullsEyeView.java

public class BullsEyeView extends View
{
	private Paint mPaint;

	private Point mCenter;
	private float mRadius;

	/**
	*  Java构造函数
	*/
	public BullsEyeView(Context context)
	{
		this(context, null);
	}

	/**
	* XML构造函数
	*/
	public BullsEyeView(Context context, AttributeSet attrs)
	{
		this(context, attrs, 0);
	}

	/**
	* 带有样式的XML构造函数
	*/
	public BullsEyeView(Context context, AttributeSet attrs, int defStyle)
	{
		super(context, attrs, defStyle);
		// 创建用于绘制视图的画笔
		mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
		// 画笔的样式
		mPaint.setStyle(Style.FILL);
		// 圆心
		mCenter = new Point();
	}

	@Override
	protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
	{
		int width, height;
		// 内容的大小
		int contentWidth = 100;
		int contentHeight = 100;

		width = getMeasurement(widthMeasureSpec, contentWidth);
		height = getMeasurement(heightMeasureSpec, contentHeight);

		setMeasuredDimension(width, height);
	}

	/**
	* 用于测量宽度和高度
	*/
	private int getMeasurement(int measureSpec, int contentSize)
	{
		int specSize = MeasureSpec.getSize(measureSpec);
		switch(MeasureSpec.getMode(measureSpec))
		{
			case MeasureSpec.AT_MOST:
				return Math.min(specSize, contentSize);
			case MeasureSpec.UNSPECIFIED:
				return contentSize;
			case MeasureSpec.EXACTLY:
				return specSize;
			default:
				return 0;
		}
	}

	@Override
	protected void onSizeChanged(int w, int h, int lodw, int oldh)
	{
		if(w != oldh || h != lodh)
		{ // 有参数变化，则复位参数
			mCenter.x = w / 2;
			mCenter.y = h / 2;
			mRadius = Math.min(mCenter.x, mCenter.y);
		}
	}

	@Override
	protected void onDraw(Canvas canvas)
	{
		// 绘制一系列从大到小的同心圆
		mPaint.setColor(Color.RED);
		canvas.drawCircle(mCenter.x, mCenter.y mRadius, mPaint);

		mPaint.setColor(Color.WHITE);
		canvas.drawCircle(mCenter.x, mCenter.y, mRadius * 0.7, mPaint);

		mPaint.setColor(Color.GREEN);
		canvas.drawCircle(mCenter.x, mCenter.y, mRadius*0.5, mPaint);

		mPaint.setColor(Color.WHITE);
		canvas.drawCircle(mCenter.x, mCenter.y, mRadius*0.3, mPaint);
	}

}

三个构造函数：

View(Context context) ：通过java代码来创建视图

View(Context context, AttributeSet attrs)：从XML文件中创建视图，AttributeSet包括xml中加载到View上的所有属性

View(Context context, AttributeSet attrs, int delStyle)

在xml文件中使用自定义控件：

<com.mxd.view.BullsEyeView
	android:layout_width="match_parent"
	android:layout_height="wrap_content" />