我們經常看到一些app中可以顯示圓角圖片，比如qq的聯絡人圖示等等，實現圓角圖片一種辦法是直接使用圓角圖片資源，當然如果沒有圓角圖片資源，我們也可以自己通過程式實現的，下面介紹一個自定義圓角ImageView的方法：

package com.yulongfei.imageview;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.widget.ImageView;

public class RoundAngleImageView extends ImageView {
	private int roundWidth = 13;
	private int roundHeight = 13;

	public RoundAngleImageView(Context context) {
		super(context);
		init(context, null);
	}

	public RoundAngleImageView(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);
		init(context, attrs);
	}

	public RoundAngleImageView(Context context, AttributeSet attrs) {
		super(context, attrs);
		init(context, attrs);
	}

	private void init(Context context, AttributeSet attrs) {
		if (attrs != null) {
			TypedArray a = context.obtainStyledAttributes(attrs,
					R.styleable.RoundAngleImageView);
			roundWidth = a.getDimensionPixelSize(
					R.styleable.RoundAngleImageView_roundWidth, roundWidth);
			roundHeight = a.getDimensionPixelSize(
					R.styleable.RoundAngleImageView_roundHeight, roundHeight);
			a.recycle();
		} else {
			float density = context.getResources().getDisplayMetrics().density;
			roundWidth = (int)(roundWidth * density);
			roundHeight = (int)(roundHeight * density);
		}
	}

	/** 重寫draw() */
	@Override
	public void draw(Canvas canvas) {
		
		//例項化一個和ImageView一樣大小的bitmap
		Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(),
				Config.ARGB_8888);
		
		//例項化一個canvas，這個canvas對應的記憶體為上面的bitmap
		Canvas canvas2 = new Canvas(bitmap);
		if (bitmap.isRecycled()) {
			bitmap = Bitmap.createBitmap(getWidth(), getHeight(),
					Config.ARGB_8888);
			canvas2 = new Canvas(bitmap);
		}
		
		//將imageView自己繪製到canvas2上，這個導致bitmap裡面存放了imageView
		super.draw(canvas2);
		
		//利用canvas畫一個圓角矩形，這個會修改bitmap的資料
		drawRoundAngle(canvas2);
		
		//將裁剪好的bitmap繪製到系統當前canvas上，這樣裁剪好的imageview就能顯示到螢幕上
		Paint paint = new Paint();
		paint.setXfermode(null);
		canvas.drawBitmap(bitmap, 0, 0, paint);
		bitmap.recycle();
	}

	public void setRoundWidth(int roundWidth, int roundHeight) {
		this.roundWidth = roundWidth;
		this.roundHeight = roundHeight;
	}

	private void drawRoundAngle(Canvas canvas)
	{
		Paint maskPaint = new Paint();
        maskPaint.setAntiAlias(true);
		maskPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        Path maskPath = new Path();  
        maskPath.addRoundRect(new RectF(0.0F, 0.0F, getWidth(), getHeight()), roundWidth, roundHeight, Path.Direction.CW);
        
        //這是設定了填充模式，非常關鍵
        maskPath.setFillType(Path.FillType.INVERSE_WINDING);
        canvas.drawPath(maskPath, maskPaint);
	}
}