使用画布在位图中绘制在Android视图中不起作用

Question

I'm trying to use onDraw method and canvas to draw some items in a bitmap and cache it to draw it again and don't call onDraw again

This is part of my code :

@Override
    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        if (mCacheDrawing == null) {
            mCacheDrawing = Bitmap.createBitmap(mScrollWidth, mScrollHeight, Config.RGB_565);
            canvas.setBitmap(mCacheDrawing);

            for (int i = 0; i < mIcons.size(); i++) {

                prepareItem(canvas, paint, mIcons.get(i));
            }

            canvas.save();
        } else {
            canvas.setBitmap(mCacheDrawing);
        }
    }

The code is not working and show me an empty screen, can any one help me please?

EDIT : I have found the following post and it help me to solve the problem >> https://groups.google.com/forum/#!topic/android-beginners/6pO8SJN3CTY

and my working code now is following :

@Override
    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

        if (mCacheDrawing == null) {

            mCacheDrawing = Bitmap.createBitmap(mScrollWidth, mScrollHeight, Config.RGB_565);
            mCanvas = new Canvas(mCacheDrawing);
            for (int i = 0; i < mIcons.size(); i++) {

                prepareItem(mCanvas, paint, mIcons.get(i));
            }
        } 

        canvas.drawBitmap(mCacheDrawing,new Rect (0,0, mScrollWidth, mScrollHeight), new Rect (0,0, mScrollWidth, mScrollHeight), paint);
    }

Answer 1

Well i can't figure out functionality of your code as its a snippet from a section, But what I can figures out is the reason. quick check

1. If you trying to draw bitmaps from a view, make sure drawing cache is enabled/
2. canvas.drawBitmap , this is the API for drawing a Bitmap over a canvas and not setBitmap

you can use save and restore method of canvas accordingly.