在安卓开发中，很多都要实现图片圆角的功能，大致有两种实现方式：

1：Xfermode http://blog.csdn.net/lmj623565791/article/details/42094215

2：BitmapShader http://blog.csdn.net/lmj623565791/article/details/41967509

以下是Xfermode实现的方式，相对比较简单：

public class RoundImageViewByXfermode extends ImageView
{       
        private Paint mPaint;
    private Xfermode mXfermode = new PorterDuffXfermode(Mode.DST_IN);
    private Bitmap mMaskBitmap;
    private WeakReference<Bitmap> mWeakBitmap;

    /**
     * 图片的类型，圆形or圆角
     */
    private int type;
    public static final int TYPE_CIRCLE = 0;
    public static final int TYPE_ROUND = 1;
    /**
     * 圆角大小的默认值
     */
    private static final int BODER_RADIUS_DEFAULT = 10;
    /**
     * 圆角的大小
     */
    private int mBorderRadius;

    public RoundImageViewByXfermode(Context context)
    {
        this(context,null);

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
    }

    public RoundImageViewByXfermode(Context context, AttributeSet attrs)
    {
        super(context, attrs);

        mPaint = new Paint();
        mPaint.setAntiAlias(true);

        TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.RoundImageViewByXfermode);

        mBorderRadius = a.getDimensionPixelSize(
                R.styleable.RoundImageViewByXfermode_borderRadius, (int) TypedValue
                        .applyDimension(TypedValue.COMPLEX_UNIT_DIP,
                                BODER_RADIUS_DEFAULT, getResources()
                                        .getDisplayMetrics()));// 默认为10dp
        Log.e("TAG", mBorderRadius+"");
        type = a.getInt(R.styleable.RoundImageViewByXfermode_type, TYPE_CIRCLE);// 默认为Circle

        a.recycle();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        /**
         * 如果类型是圆形，则强制改变view的宽高一致，以小值为准
         */
        if (type == TYPE_CIRCLE)
        {
            int width = Math.min(getMeasuredWidth(), getMeasuredHeight());
            setMeasuredDimension(width, width);
        }

    }

    @Override
    public void invalidate()
    {
        mWeakBitmap = null;
        if (mMaskBitmap != null)
        {
            mMaskBitmap.recycle();
            mMaskBitmap = null;
        }
        super.invalidate();
    }

    @SuppressLint("DrawAllocation")
    @Override
    protected void onDraw(Canvas canvas)
    {
        //在缓存中取出bitmap
        Bitmap bitmap = mWeakBitmap == null ? null : mWeakBitmap.get();

        if (null == bitmap || bitmap.isRecycled())
        {
            //拿到Drawable
            Drawable drawable = getDrawable();
            //获取drawable的宽和高
            int dWidth = drawable.getIntrinsicWidth();
            int dHeight = drawable.getIntrinsicHeight();

            if (drawable != null)
            {
                //创建bitmap
                bitmap = Bitmap.createBitmap(getWidth(), getHeight(),
                        Config.ARGB_8888);
                float scale = 1.0f;
                //创建画布
                Canvas drawCanvas = new Canvas(bitmap);
                //按照bitmap的宽高，以及view的宽高，计算缩放比例；因为设置的src宽高比例可能和imageview的宽高比例不同，这里我们不希望图片失真；
                if (type == TYPE_ROUND)
                {
                    // 如果图片的宽或者高与view的宽高不匹配，计算出需要缩放的比例；缩放后的图片的宽高，一定要大于我们view的宽高；所以我们这里取大值；
                    scale = Math.max(getWidth() * 1.0f / dWidth, getHeight()
                            * 1.0f / dHeight);
                } else
                {
                    scale = getWidth() * 1.0F / Math.min(dWidth, dHeight);
                }
                //根据缩放比例，设置bounds，相当于缩放图片了
                drawable.setBounds(0, 0, (int) (scale * dWidth),
                        (int) (scale * dHeight));
                drawable.draw(drawCanvas);
                if (mMaskBitmap == null || mMaskBitmap.isRecycled())
                {
                    mMaskBitmap = getBitmap();
                }
                // Draw Bitmap.
                mPaint.reset();
                mPaint.setFilterBitmap(false);
                mPaint.setXfermode(mXfermode);
                //绘制形状
                drawCanvas.drawBitmap(mMaskBitmap, 0, 0, mPaint);
                mPaint.setXfermode(null);
                //将准备好的bitmap绘制出来
                canvas.drawBitmap(bitmap, 0, 0, null);
                //bitmap缓存起来，避免每次调用onDraw，分配内存
                mWeakBitmap = new WeakReference<Bitmap>(bitmap);
            }
        }
        //如果bitmap还存在，则直接绘制即可
        if (bitmap != null)
        {
            mPaint.setXfermode(null);
            canvas.drawBitmap(bitmap, 0.0f, 0.0f, mPaint);
            return;
        }

    }
/**
     * 绘制形状
     * @return
     */
    public Bitmap getBitmap()
    {
        Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(),
                Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.BLACK);

        if (type == TYPE_ROUND)
        {
            canvas.drawRoundRect(new RectF(0, 0, getWidth(), getHeight()),
                    mBorderRadius, mBorderRadius, paint);
        } else
        {
            canvas.drawCircle(getWidth() / 2, getWidth() / 2, getWidth() / 2,
                    paint);
        }

        return bitmap;
    }
}