Android的：如何获得自定义视图重绘部分？ [英] Android: How to get a custom view to redraw partially?

问题描述

我有一个充满了我的整个屏幕自定义视图。 （钢琴键盘） 当用户触摸键时，它会导致无效（）被称为与全键盘被重新绘制以显示新的状态触摸键。

I have a custom view that fills my entire screen. (A piano keyboard) When a user touches the key, it causes invalidate() to be called and the whole keyboard gets redrawn to show the new state with a touched key.

目前的看法很简单，但我计划增加多一点漂亮的图形。由于整个键盘动态呈现，这将使得重画整个键盘更加昂贵。

Currently the view is very simple, but I plan to add a bit more nice graphics. Since the whole keyboard is dynamically rendered this would make redrawing the entire keyboard more expensive.

所以我想，让我们看看部分重绘。现在，我称之为无效（矩形脏）用正确的脏区。我把我的的OnDraw（帆布油画）法中之荤地区唯一绘制键，如果我确实想要部分重绘。这导致这些键被绘制，但键盘的其他部分完全是黑色/未拉伸的。

So I thought, let's look into partial redrawing. Now I call invalidate(Rect dirty) with the correct dirty region. I set my onDraw(Canvas canvas) method to only draw the keys in the dirty region if I do indeed want a partial redraw. This results in those keys being drawn, but the rest of the keyboard is totally black/not drawn at all.

我是错误的期待，调用无效（矩形脏）将缓存的电流画布，和只有允许图中的脏区？

Am I wrong in expecting that calling invalidate(Rect dirty) would "cache" the current canvas, and only "allows" drawing in the dirty region?

有没有什么办法可以达到我想要的吗？ （一办法缓存的画布，只有重绘脏区？

Is there any way I can achieve what I want? (A way to "cache" the canvas and only redraw the dirty area?"

推荐答案

目前不错的解决方法是手动缓存满画布为位图：

Current nice workaround is to manually cache the full canvas to a bitmap:

 private void onDraw(Canvas canvas)
 {
     if (!initialDrawingIsPerformed)
     {
          this.cachedBitmap = Bitmap.createBitmap(getWidth(), getHeight(),
            Config.ARGB_8888); //Change to lower bitmap config if possible.
          Canvas cacheCanvas = new Canvas(this.cachedBitmap);
          doInitialDrawing(cacheCanvas);
          canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
          initialDrawingIsPerformed = true;
     }
     else
     {
          canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
          doPartialRedraws(canvas);
     }
 }

Ofcourse，你需要存储哪些重绘自己和preferably不会使用新的油漆每次的信息，但都是一些细节。

Ofcourse, you need to store the info about what to redraw yourself and preferably not use a new Paint everytime, but that are details.

另外请注意：位图是在您的应用程序的内存使用量相当繁重。我有崩溃的时候我缓存这是一个滚动使用的视图，而且是像设备的5倍的高度，因为它使用> 10MB内存！

Also note: Bitmaps are quite heavy on the memory usage of your app. I had crashes when I cached a View that was used with a scroller and that was like 5 times the height of the device, since it used > 10MB memory!

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