Android View.onDraw() 总是有一个干净的 Canvas [英] Android View.onDraw() always has a clean Canvas

问题描述

我正在尝试绘制动画.为此，我扩展了 View 并覆盖了 onDraw() 方法.我所期望的是，每次调用 onDraw() 时，画布都会处于我离开它的状态，我可以选择清除它或只绘制它的一部分(这就是我使用 SurfaceView 时的工作方式) 但每次画布回来时都已经清除了.有什么办法可以不清除吗?或者也许可以将之前的状态保存到位图中，这样我就可以绘制该位图，然后在其上方绘制?

I am trying to draw an animation. To do so I have extended View and overridden the onDraw() method. What I would expect is that each time onDraw() is called the canvas would be in the state that I left it in and I could choose to clear it or just draw over parts of it (This is how it worked when I used a SurfaceView) but each time the canvas comes back already cleared. Is there a way that I can not have it cleared? Or maybe save the previous state into a Bitmap so I can just draw that Bitmap and then draw over top of it?

推荐答案

我不确定是否有办法.但是对于我的自定义视图，我要么在每次调用 onDraw() 时重新绘制所有内容，要么绘制到位图，然后将位图绘制到画布(如您在问题中所建议的那样).

I'm not sure if there is a way or not. But for my custom views I either redraw everything each time onDraw() is called, or draw to a bitmap and then draw the bitmap to the canvas (like you suggested in your question).

这是我的做法

class A extends View {

    private Canvas canvas;
    private Bitmap bitmap;

    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        if (bitmap != null) {
            bitmap .recycle();
        }
        canvas= new Canvas();
        bitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        canvas.setBitmap(bitmap);
    }
    public void destroy() {
        if (bitmap != null) {
            bitmap.recycle();
        }
    }
    public void onDraw(Canvas c) {
      //draw onto the canvas if needed (maybe only the parts of animation that changed)
      canvas.drawRect(0,0,10,10,paint);

      //draw the bitmap to the real canvas c
      c.drawBitmap(bitmap, 
          new Rect(0,0,bitmap.getWidth(),bitmap.getHeight()), 
          new Rect(0,0,bitmap.getWidth(),bitmap.getHeight()), null);
    }
}

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