相关疑难解决方法(0)

我尝试使用Porter-Duff Xfermodes在我的Android应用程序中擦除部分位图.

我有一个绿色背景,由蓝色位图覆盖.当我触摸屏幕时,应该创建覆盖位图中的"洞",使绿色背景可见.而不是一个洞,我当前的代码产生一个黑点.

以下是我的代码.任何想法,我在这里做错了什么？

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(new DrawView(this));
}

public class DrawView extends View implements OnTouchListener {

    private int x = 0;
    private int y = 0;

    Bitmap bitmap;
    Canvas bitmapCanvas;

    private final Paint paint = new Paint();
    private final Paint eraserPaint = new Paint();

    public DrawView(Context context) {
        super(context);
        setFocusable(true);
        setFocusableInTouchMode(true);

        this.setOnTouchListener(this);

        // Set background
        this.setBackgroundColor(Color.GREEN);

        // Set bitmap
        bitmap = Bitmap.createBitmap(320, 480, …

我试图在自定义视图的onDraw方法中绘制这样的形状.

不幸的是,我无法"剪切"画布上的透明圆圈(通过绘制带有Color.Transparent的圆圈).

我应该先在另一个位图中绘制形状,然后在onDraw提供的画布上绘制它吗？或者这是一种更好(更简单)的方法吗？

这是我尝试的代码(与Color.WHITE一起使用):

mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaint.setColor(Color.TRANSPARENT);
mPaint.setStrokeWidth(4);
mPaint.setStyle(Style.STROKE);

canvas.drawColor(getResources().getColor(R.color.black_overlay));
canvas.drawCircle(-3*(this.getBottom()-this.getTop())/4, (this.getTop()+this.getBottom())/2, this.getBottom()-this.getTop(), mPaint);

PS:使用Color.WHITE时,我得到了我想要的确切形状:

解

@Override
public void onDraw(Canvas canvas)
{
    mBitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
    mCanvas = new Canvas(mBitmap);
    mCanvas.drawColor(getResources().getColor(R.color.black_overlay));
    mCanvas.drawCircle(-3*(getHeight())/4, (getHeight())/2, getHeight(), mPaint);
    canvas.drawBitmap(mBitmap, 0, 0, null);
}
with
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
    mPaint.setStrokeWidth(4);
    mPaint.setStyle(Style.STROKE);
    mPaint.setXfermode(new PorterDuffXfermode(Mode.CLEAR));

注意:createBitamp和新画布应该移出onDraw方法.