如果表面不是在前台SurfaceHolder.lockCanvas返回null [英] SurfaceHolder.lockCanvas returns null if the surface is not in the foreground

问题描述

我目前做在Android上LiveWallpaper测试。我使用code，它看起来像这样在画布上绘制的内容：

I'm currently doing a test with a LiveWallpaper in Android. I am drawing something on the canvas using code that looks something like this:

final SurfaceHolder holder = getSurfaceHolder();
Canvas c = new Canvas();
c = holder.lockCanvas(); // c becomes null
c.save();
c.drawBitmap(currentBitmap);
c.restore();
holder.unlockCanvasAndPost(c);

的这部分是在正常情况下工作正常，的。不过，我有，每当设置在对应于该服务的设置被更改执行此code的监听器。似乎每当我从设置活动执行此code，我得到一个空指针在 c.save（）方法。

This part is working fine under normal circumstances. However, I have a listener that executes this code whenever a setting is changed in the Settings that correspond to this service. It seems that whenever I execute this code from the settings activity, I am getting a NullPointer on the c.save() method.

似乎只有当壁纸不是在前台，在holder.lockCanvas（）。难道是不可能得出这个表面时，它不是在前台？

It seems that only when the Wallpaper is not in the foreground, the holder.lockCanvas(). Is it impossible to draw to this surface when it's not in the foreground?

推荐答案

这是正确的。避免这种情况的常用方法是注销中的onPause或onVisibilityChanged（假）的监听器，并在onResume或onVisibilityChanged（真）重新注册，因为你要不要设置变化做出反应时，你的画布是不可见的。

That's right. A common way to avoid this is to unregister your listener in onPause or onVisibilityChanged(false), and reregister in onResume or onVisibilityChanged(true), since you shouldn't react to settings changes when your canvas isn't visible.

另一种解决方案是简单地围绕code的那款带有空检查，并忘掉它。我建议针对这一点，虽然，因为你真正想要做的是prevent您code甚至从试图绘制到表面时，它不是在视图中。

Another solution would be to simply surround that section of code with a null check, and forget about it. I'd recommend against this, though, since what you really want to do is prevent your code from even attempting to draw to the surface when it's not in view.

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