如何在android canvas上绘制而不丢失先前绘制的对象？

Question

In android, is it possible to draw something on canvas, Then draw something else to the same canvas without losing the previously drawn objects?

Details :

• Display a drawable (based on image url).
• Upon clicking a node, display 5 more nodes (which drawables to display is determined by a remote api call).
• Now clicking on any of the 5 nodes will follow same steps to display 5 more nodes & so on.

I have gotten to a point where the first drawable is clicked & 5 more drawables are fetched. But when I translate the canvas & render the 5 nodes (invoking OnDraw), the first drawable is gone (expected). I dont want to draw all nodes again.

1) So how to append to the same canvas?

2) Is canvas.concat the way to go? If so any pointers on how it could be used?

My OnDraw method:

public void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.save();
    canvas.translate(mPosX, mPosY);
    canvas.scale(mScaleFactor, mScaleFactor);
    this.canvas = canvas;
    canvas.translate(50,50);
    drawable.setBounds(0, 0, drawable.getIntrinsicWidth(), drawable.getIntrinsicHeight());
    drawable.draw(canvas);
    canvas.restore();
}

UPDATE1 : I found that this can be achieved only with a workaround. Basically, I need to create a bitmap/Canvas outside onDraw, draw my drawables on this new canvas & then, when done, draw this pre-drawn bitmap on the canvas supplied by onDraw. But the problem with this approach is that when I create a bitmap, I need to provide a dimension. But the nodes can grow in size & there is no guarantee that I can accommodate them grown-nodes within 1000x1000. Assigning bigger numbers can result in out of memory. So unless there is a way to make the bitmap dimension match the canvas (impractical right?) I need to change my approach (SurfaceView?) here... suggestions welcome.

    mBitmap = Bitmap.createBitmap(1000, 1000, Bitmap.Config.ARGB_8888);
    mCanvas = new Canvas(mBitmap);

Answer 1

Did you call "canvas.drawColor( your_background_color )" before you call onDraw(Canvas canvas) function? If you do that just remove that line, which will clean your canvas as the color you assigned.