CircleImageView实现方法有很多种，各有优缺点，因此需要按照不同的场景使用。我们今天使用修改图片像素的方法实现CircleImageView，主要知识点无非是勾股定理和点到圆形的距离。

素材图片：

效果如下：

1、clipPath裁剪画布

该方法支持的最小版本是Android 4.3(API Level 18)，方便快捷，但是不支持硬件加，此外也存在Path既有的缺点，不支持抗锯齿。

 @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        Paint paint = new Paint();

        mBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.pic);
        mPath = new Path();

        mPath.addCircle(mBitmap.getWidth() / 2, mBitmap.getHeight() / 2, mBitmap.getWidth() / 2, Path.Direction.CCW);
        canvas.clipPath(mPath);
        canvas.drawBitmap(mBitmap, 0, 0, paint);
    }

2、使用PorterDuffXfermode

PorterDuffXfermode是Android主流的图片合成工具，支持模式多，稳定性强，效果好，质量高，支持抗锯齿备受广大开发者喜爱，可以说是很多应用开发的首选。缺点是难度学习有些高，另外比较占内存。

 /**
     * 绘制圆形图片
     *
     */
    @Override  
    protected void onDraw(Canvas canvas) {  
  
        Drawable drawable = getDrawable();  
        if (null != drawable) {  
            Bitmap bitmap = ((BitmapDrawable) drawable).getBitmap();  
            Bitmap b = getCircleBitmap(bitmap);  
            final Rect rectSrc = new Rect(0, 0, b.getWidth(), b.getHeight());  
            final Rect rectDest = new Rect(0,0,getWidth(),getHeight());
            paint.reset();  
            canvas.drawBitmap(b, rectSrc, rectDest, paint);  
  
        } else {  
            super.onDraw(canvas);  
        }  
    }  
  
    /**
     * 获取圆形图片方法
     * @param bitmap
     * @param pixels
     * @return Bitmap
     */
    private Bitmap getCircleBitmap(Bitmap bitmap) {  
        Bitmap output = Bitmap.createBitmap(bitmap.getWidth(),  
                bitmap.getHeight(), Config.ARGB_8888);  
        Canvas canvas = new Canvas(output);  
          
        final int color = 0xff424242;
       
        final Rect rect = new Rect(0, 0, bitmap.getWidth(), bitmap.getHeight());  
        paint.setAntiAlias(true);  
        canvas.drawColor(Color.TRANSPARENT);  
        paint.setColor(color);  
        int x = bitmap.getWidth(); 
        
        canvas.drawCircle(x / 2, x / 2, x / 2, paint);  
        paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));  
        canvas.drawBitmap(bitmap, rect, rect, paint);  
        return output;  
        
        
    }  

4、设置画笔Paint的Shader，然后用该画笔绘制圆形图片

该方法是Glide和picasso使用的方法，用法简单便捷，占内占有率处于中等水平。

 
   
    @Override  
    protected void onDraw(Canvas canvas) {  
  
        Drawable drawable = getDrawable();  
        if (null != drawable) {  
            Bitmap bitmap = ((BitmapDrawable) drawable).getBitmap();  
            Bitmap b = transform(bitmap);  
            final Rect rectSrc = new Rect(0, 0, b.getWidth(), b.getHeight());  
            final Rect rectDest = new Rect(0,0,getWidth(),getHeight());
            paint.reset();  
            canvas.drawBitmap(b, rectSrc, rectDest, paint);  
  
        } else {  
            super.onDraw(canvas);  
        }  
    } 

public Bitmap transform(Bitmap source) {
            int size = Math.min(source.getWidth(), source.getHeight());

            int x = (source.getWidth() - size) / 2;
            int y = (source.getHeight() - size) / 2;

            Bitmap squaredBitmap = Bitmap.createBitmap(source, x, y, size, size);
            if (squaredBitmap != source) {
                source.recycle();
            }

            Bitmap bitmap = Bitmap.createBitmap(size, size, source.getConfig());

            Canvas canvas = new Canvas(bitmap);
            Paint paint = new Paint();
            BitmapShader shader = new BitmapShader(squaredBitmap,
                    BitmapShader.TileMode.CLAMP, BitmapShader.TileMode.CLAMP);

          float mScale = (mRadius * 2.0f) / Math.min(bitmap.getHeight(), bitmap.getWidth());

          Matrix matrix = new Matrix();
          matrix.setScale(mScale, mScale);
          bitmapShader.setLocalMatrix(matrix);


            paint.setShader(shader);
            paint.setAntiAlias(true);

            float r = size / 2f;
            canvas.drawCircle(r, r, r, paint);

            squaredBitmap.recycle();
            return bitmap;
        }

5、修改像素

该方法支持抗锯齿，用法简单，内存占有率同样处于中等水平。

public class CircleImageView extends AppCompatImageView {

    public CircleImageView(Context context) {
        this(context,null);
    }


    public CircleImageView(Context context, AttributeSet attrs) {
        this(context, attrs,0);
    }

    public CircleImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }


    @Override
    public void onDraw(Canvas canvas) {

        int width  = getWidth();
        int height = getHeight();


        int minSize = Math.min(width,height)/2;
        Drawable drawable = getDrawable();
        if(drawable!=null && minSize!=0) {

            if(Math.min(drawable.getIntrinsicWidth(),drawable.getIntrinsicHeight())==0) {
                super.onDraw(canvas);
                return;
            }

            int intrinsicWidth = drawable.getIntrinsicWidth();
            int intrinsicHeight = drawable.getIntrinsicHeight();

            float R = Math.min(intrinsicWidth,intrinsicHeight)/2;

            Bitmap bmp = Bitmap.createBitmap(intrinsicWidth, intrinsicHeight, Bitmap.Config.ARGB_8888);
            Canvas targetCanvas = new Canvas(bmp);

            drawable.draw(targetCanvas);

            for (int y=0;y<intrinsicHeight;y++){
                for (int x=0;x<intrinsicWidth;x++){
                    if((Math.pow(x-intrinsicWidth/2,2) + Math.pow(y-intrinsicHeight/2,2))<=Math.pow(R,2)){
                        continue;
                    }
                    bmp.setPixel(x,y, Color.TRANSPARENT);
                }
            }

            Matrix imageMatrix = getImageMatrix();
            if(imageMatrix!=null){
                canvas.concat(imageMatrix);
            }
            final int saveCount = canvas.getSaveCount();
            canvas.save();

            if (getCropToPadding()) {
                final int scrollX = getScrollX();
                final int scrollY = getScrollY();
                canvas.clipRect(scrollX + getPaddingLeft(), scrollY + getPaddingTop(),
                        scrollX + getRight() - getLeft() - getPaddingRight(),
                        scrollY + getBottom() - getTop() - getPaddingBottom());
            }

            canvas.translate(getPaddingLeft(),getPaddingTop());

            DrawFilter drawFilter = canvas.getDrawFilter();
            canvas.setDrawFilter(new PaintFlagsDrawFilter(0, Paint.ANTI_ALIAS_FLAG|Paint.FILTER_BITMAP_FLAG));
            canvas.drawBitmap(bmp,0,0,null);
            canvas.setDrawFilter(drawFilter);

            canvas.restoreToCount(saveCount);

            bmp.recycle();

        }else{
            super.onDraw(canvas);
        }
    }
}