How to erase previous drawing on Canvas?

问题:

I have a background image (a map) on which I need to regularly draw the you-are-here icon. I use Canvas to draw the icon on top of the map. Assuming that the drawing process is triggered on button click (see code below), how can I erase the previous drawing?

private void displayUserPos(Point userPos) {
    Bitmap marker = BitmapFactory.decodeResource(getResources(), R.drawable.ic_yah);
    canvas.drawBitmap(marker, (float)userPos.getX(), (float)userPos.getY(), null);
    imgView.setImageBitmap(fmOverlay);
}

回答1:

canvas.drawColor(0, Mode.CLEAR);

More info http://developer.android.com/guide/topics/graphics/index.html

回答2:

canvas.drawColor(0, Mode.CLEAR);

回答3:

overlayBitmap.eraseColor(Color.TRANSPARENT);

This simply sets an existing Bitmap to all transparent.

I use this to "clear" a Bitmap object that I use to overlay on top of another to show a cropping window.

回答4:

Just fill in the canvas with a color or image:

canvas.drawColor(Color.BLACK);

If you want to keep certain elements and take certain elements away you can store these in an ArrayList. Then you can add and remove elements from this ArrayList when you want, and then iterate through them in onDraw().

for (Iterator<GraphicObject> it = _graphics.iterator(); it.hasNext();) {
    GraphicObject graphic = (GraphicObject)it.next();
    coords = graphic.getCoordinates();
    canvas.drawCircle(coords.getX(), coords.getY(), (float)coords.getRadius(), paint);
}

回答5:

Try as below,it can be used to clear the canvas totally.

Declaration should be like this,

ArrayList<Pair<Path, Paint>> paths = new ArrayList<Pair<Path, Paint>>();
ArrayList<Pair<Path, Paint>> undonePaths = new ArrayList<Pair<Path, Paint>>();

and while clearing use

    undonePaths.clear();
    paths.clear();
    invalidate();

回答6:

canvas.drawColor(0, PorterDuff.Mode.CLEAR);

For more info Click here

回答7:

I wrote a function to help with this. Usually I set up a boolean to check if the page has changed, if it has, I call this function, using the return boolean to set my newpage boolean.

    private boolean clearCanvas(Canvas canvas){
      canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
      return false;
    }

Color.TRANSPARENT isn't really needed in this, you could do with out.