Clear/Reset entire ImageView canvas

Issue

I've got a simple layout with 2 imageViews:

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <ImageView
        android:id="@+id/takenPicture"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
     />

     <com.example.dochjavatestimplementation.pkgActivity.ExtendedImageView
        android:id="@+id/takenPicture2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
      />
    
</RelativeLayout>

The first ImageView displays a bitmap, the second ImageView (1) is a custom ImageView (ExtendedImageView) (2)which draws a canvas which gets displayed on top of the normal ImageView. This looks like this: displayimageviews

What I want is that after drawing the rect in my onDraw method, I want to clear the rect again (right now just for testing purposes). This is what my code looks like:

@Override
protected void onDraw(Canvas canvas) {

    Paint paint = new Paint();
    paint.setColor(Color.BLACK);
    paint.setStrokeWidth(3);
    canvas.drawRect(new Rect(212,0,-720,600),paint);
    
    //clear the rect/contents of canvas again
    //try 1
    canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.MULTIPLY);
    canvas.drawColor(0, PorterDuff.Mode.CLEAR);

    //try2
    Paint transparent = new Paint();
    transparent.setAlpha(0);
    canvas.drawPaint(transparent);

    //try3
    setImageResource(0);
}

I tried clearing the canvas/rect in three different ways as seen in code above, but it doesnt change the output as the black rect is still visible. My question now is what can the cause be? Is it cause I'm not "updating" the canvas or is it due to how I try clearing my canvas?

The endresult should basically be that I only see the first imageview

Solution

Drawing shapes on Canvas in View.onDraw() is low-level function that directly changes pixels on the display in the final stage. So, if you once draw something, it's impossible to erase it. It's not buffered. There remain only mixed RGB(w/o A) pixels on the screen. The former pixel colors (originated from the "takenPicture") are already lost.

If you'd like to make shapes erasable, prepare another canvas backed by an ARGB-bitmap and draw all on it. Then draw the bitmap on the canvas in the end.

@Override
protected void onDraw(Canvas canvas)
{

    // Prepare Bitmap and Canvas 
    Bitmap bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
    Canvas draw_canvas = new Canvas(bitmap);

    // Draw a rect.
    Paint paint = new Paint();
    paint.setColor(Color.BLACK);
    paint.setStrokeWidth(3);
    draw_canvas.drawRect(new Rect(212,0,-720,600),paint);

    // Clear
    draw_canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.SRC);

    // Draw the bitmap.
    canvas.drawBitmap(bitmap, 0, 0, null);
    bitmap.recycle();
}

Answered By - ardget