Drawing to a created Bitmap from onDraw()

问题

I'm trying to draw to a Bitmap so I can put my custom view inside an imageView.The code within the onDraw method is:

    public void onDraw(Canvas canvas) {     

    Bitmap drawGraph = Bitmap.createBitmap(canvas.getWidth(),canvas.getHeight(),Bitmap.Config.ARGB_8888);       
    canvas.setBitmap(drawGraph);    
    canvas.drawBitmap(drawGraph, 0, 0, bgPaint);

My problem is that if I try to use a Bitmap in this way, I just get a black screen. I know that the rest of my code works as it displays if I don't try to draw to a bitmap.

If I comment out the line

canvas.setBitmap(drawGraph);

Then everything works perfectly, so this is the problem but I dont know why.

where am I going wrong?

回答1:

Turns out I did have to create a second canvas. My working code is below just for anyone who might need it:

    public void onDraw(Canvas canvas) {

    Canvas singleUseCanvas = new Canvas();      

    drawGraph = Bitmap.createBitmap(canvas.getWidth(),canvas.getHeight(),Bitmap.Config.ARGB_8888);      
    singleUseCanvas.setBitmap(drawGraph);   

    canvas.drawBitmap(drawGraph, 100, 100, bgPaint);

回答2:

I think is the canvas and canvas2 dichotomy. Try to use only canvas2 (the parameter) to draw.

来源：https://stackoverflow.com/questions/6828595/drawing-to-a-created-bitmap-from-ondraw