目录：
1、预备：阅读本篇文章，需具备View的绘制基本知识。
2、讲解：PorterDuff.Mode https://developer.android.com/reference/android/graphics/PorterDuff.Mode
3、实战：使用PorterDuff.Mode实现圆形头像。

核心代码：

    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_ATOP));

PorterDuff.Mode

1、SRC

PorterDuff.Mode.SRC

composite_SRC.png

PorterDuff.Mode.SRC_OVER

composite_SRC_OVER.png

PorterDuff.Mode.SRC_IN

composite_SRC_IN.png

PorterDuff.Mode.SRC_OUT

composite_SRC_OUT.png

PorterDuff.Mode.SRC_ATOP

composite_SRC_ATOP.png

2、Destination

PorterDuff.Mode.DST

composite_DST.png

PorterDuff.Mode.DST_OVER

composite_DST_OVER.png

PorterDuff.Mode.DST_IN

composite_DST_IN.png

PorterDuff.Mode.DST_OUT

composite_DST_OUT.png

PorterDuff.Mode.DST_ATOP

composite_DST_ATOP.png

3、Clear

PorterDuff.Mode.CLEAR

composite_CLEAR.png

3、Exclusive or

PorterDuff.Mode.XOR

composite_XOR.png

PorterDuff.Mode,实现圆形头像 （支持Padding、wrap_content属性）

1、核心代码

public class AirXfermodeCircleView extends ImageView {

private static final String TAG = "AirXfermodeCircleView";

private static final int MIN_SIZE_DEFAULT = 100;

private Paint mDrawablePaint;

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

public AirXfermodeCircleView(Context context, @Nullable AttributeSet attrs) {
    this(context, attrs, 0);
}

public AirXfermodeCircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    this(context, attrs, defStyleAttr, 0);
}

public AirXfermodeCircleView(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super(context, attrs, defStyleAttr, defStyleRes);


    initializePaints();
}

private void initializePaints() {

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

}

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

    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);

    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);


    switch (widthMode) {
        case MeasureSpec.AT_MOST:
            widthSize = dpToPx(MIN_SIZE_DEFAULT);
            break;
    }

    switch (heightMode) {
        case MeasureSpec.AT_MOST:
            heightSize = dpToPx(MIN_SIZE_DEFAULT);
            break;
    }

    int minSize = Math.min(widthSize, heightSize);

    //Update measured dimension.
    setMeasuredDimension(minSize, minSize);
}

@Override
protected void onDraw(Canvas canvas) {
    
    //Canvas circle image.
    drawCircleImage(canvas);

}

/**
 * Draw circle image with paint PorterDuff.
 *
 * @param canvas
 *         canvas.
 */
private void drawCircleImage(Canvas canvas) {
   
    //Save layer with parameter.
    canvas.saveLayerAlpha(getLeft(), getTop(), getRight(), getBottom(), 200);

    //Get drawable source.
    Drawable drawable = getDrawable();
    BitmapDrawable bitmapDrawable = (BitmapDrawable) drawable;
    Bitmap bitmap = bitmapDrawable.getBitmap();

    //Computer circle image radius.
    float radiusX = (getWidth() - getPaddingLeft() - getPaddingRight()) / 2f;
    float radiusY = (getHeight() - getPaddingTop() - getPaddingBottom()) / 2f;
    float radius = Math.min(radiusX, radiusY);

    //Computer
    Rect rectF = new Rect();
    rectF.left = getLeft() + getPaddingLeft();
    rectF.top = getTop() + getPaddingTop();
    rectF.right = getRight() - getPaddingRight();
    rectF.bottom = getBottom() - getPaddingBottom();

    int targetBitmapWidth = rectF.right - rectF.left;
    int targetBitmapHeight = rectF.bottom - rectF.top;

    int diameter = (int) (radius * 2 + 0.5);

    float circleX = getPaddingLeft() + radius;
    float circleY = getPaddingTop() + radius;

    Bitmap scaledBitmap = Bitmap.createScaledBitmap(bitmap, targetBitmapWidth, targetBitmapHeight, false);

    /*
    Draw destination bitmap.
     */
    Bitmap bitmapDestination = Bitmap.createBitmap(targetBitmapWidth, targetBitmapHeight, Bitmap.Config.ARGB_8888);
    Canvas canvasDestination = new Canvas(bitmapDestination);
    //Draw circle.
    canvasDestination.drawCircle(circleX, circleY, radius, mDrawablePaint);
    //Draw destination bitmap.
    canvas.drawBitmap(bitmapDestination, 0, 0, mDrawablePaint);

    /*
    Set  paint's porterDuffMode
     */
    mDrawablePaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));

    /*
    Draw source bitmap.
     */
    canvas.drawBitmap(scaledBitmap, rectF, rectF, mDrawablePaint);

    //Reset mode value.
    mDrawablePaint.setXfermode(null);
    //Restore.
    canvas.restore();
}


private int dpToPx(int dp) {
    return (int) (dp * getResources().getDisplayMetrics().density + 0.5);
}

}

圆形头像效果图

如果，您认为到这里就万事大吉了，那么很遗憾：请看下图

收尾点

所以，在绘制完成后，切记要加上：

Paint.setXfermode(null);

欲知后事如何，请看下篇分解

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自此，结束了本篇的学习😊😊😊
如果，您还意犹未尽，请查阅作者更多文集😊😊😊