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

问题描述

我想绘制动画。要做到这一点我已经扩展视图，并覆盖了的OnDraw（）方法。我会想到的是，每一次的OnDraw（）被调用画布会在我离开时的状态，我可以选择清除它，还是画在它的一部分（这是它是如何工作的时候我用了一个SurfaceView ），但每个画布回来时已经被清除。有没有办法，我不能有它清除？或者，也许节省previous状态转变成位图，所以我可以只画了位图，然后绘制在它的上面？

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

我不知道是否有一种方法还是不行。但是，对于我的自定义视图，我不是重绘一切每次的OnDraw（）被调用，或绘制为位图，然后绘制位图到画布上（就像你在你的问题的建议）。

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).

下面是我如何做到这一点

Here is how i do it

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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