Android 绘图缓存
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Android drawing cache
提问by Seva Alekseyev
Please explain how does the drawing cache work in Android. I'm implementing a custom View subclass. I want my drawing to be cached by the system. In the View constructor, I call
请解释绘图缓存在 Android 中是如何工作的。我正在实现一个自定义 View 子类。我希望我的绘图被系统缓存。在视图构造函数中，我调用
setDrawingCacheEnabled(true);
Then in the draw(Canvas c), I do:
然后在抽奖（Canvas c）中，我这样做：
Bitmap cac = getDrawingCache();
if(cac != null)
{
c.drawBitmap(cac, 0, 0, new Paint());
return;
}
Yet the getDrawingCache()returns null to me. My draw()is not called neither from setDrawingCacheEnabled(), nor from getDrawingCache(). Please, what am I doing wrong?
然而，getDrawingCache()返回 null 给我。Mydraw()既不是 fromsetDrawingCacheEnabled()也不是 from getDrawingCache()。请问，我做错了什么？
采纳答案by Seva Alekseyev
There's a hard limit on drawing cache size, available via the ViewConfiguration class.. My view is larger than allowed for caching.
绘图缓存大小有硬性限制，可通过 ViewConfiguration 类获得。我的视图大于缓存允许的大小。
FYI, the sources of the View class are available via the SDK Manager for some (not all) Android versions.
仅供参考，View 类的源可通过某些（并非所有）Android 版本的 SDK 管理器获得。
回答by Gavin Saunders
Hopefully this explains it.
希望这能解释它。
public class YourCustomView extends View {
private String mSomeProperty;
public YourCustomView(Context context) {
super(context);
}
public YourCustomView(Context context, AttributeSet attrs) {
super(context, attrs);
}
public YourCustomView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}
public void setSomeProperty(String value) {
mSomeProperty = value;
setDrawingCacheEnabled(false); // clear the cache here
invalidate();
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// specific draw logic here
setDrawingCacheEnabled(true); // cache
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
...
}
}
Example code explained.
示例代码解释。
- In the setSomeProperty() method call setDrawingCacheEnabled(false) to clear the cache and force a redraw by calling invalidate().
- Call setDrawingCacheEnabled(true) in the onDraw method after drawing to the canvas.
- Optionally place a log statement in the onDraw method to confirm it is only called once each time you call the setSomeProperty() method. Be sure to remove the log call once confirmed as this will become a performance issue.
- 在 setSomeProperty() 方法中调用 setDrawingCacheEnabled(false) 以清除缓存并通过调用 invalidate() 强制重绘。
- 绘制到画布后，在 onDraw 方法中调用 setDrawingCacheEnabled(true)。
- 可以选择在 onDraw 方法中放置一条日志语句，以确认每次调用 setSomeProperty() 方法时它只调用一次。请务必在确认后删除日志调用，因为这将成为性能问题。
