UIScreen mirroredScreen属性始终返回nil

Question

如果设备不支持镜像(原始iPad)，我希望在外部连接的显示器上显示独特的内容，但如果它是iPad 2，则希望使用屏幕镜像。现在，当我尝试按如下方式进行编码时：

if ([UIScreen instancesRespondToSelector:@selector(mirroredScreen)] && [[UIScreen mainScreen] mirroredScreen] == nil) {
    // Mirroring not supported. Present unique content on external display
}

[[UIScreen mainScreen] mirroredScreen]始终返回nil。

我做错了什么吗？

Answer 1

根据我对文档的理解，如果您访问实际上是镜像屏幕的辅助屏幕上的属性，mirroredScreen将引用主屏幕。如下所示：

if ([[UIScreen screens] count] > 1) {
    UIScreen *secondaryScreen = [[UIScreen screens] objectAtIndex:1];
    NSLog(@"%@", secondaryScreen.mirroredScreen); // will reference the mainScreen
}

然后，[[UIScreen mainScreen] mirroredScreen]将始终返回nil，因为mainScreen不会镜像自身。

Answer 2

苹果在这里有一个关于检测屏幕是否镜像的建议：http://developer.apple.com/library/ios/#qa/qa1738/_index.html

UIScreen *aScreen;

NSArray *screens = [UIScreen screens];
for (aScreen in screens) 
{
    if ([aScreen respondsToSelector:@selector(mirroredScreen)] 
              && [aScreen mirroredScreen] == [UIScreen mainScreen]) 
    {
        // The main screen is being mirrored.
    }
    else 
    {
        // The main screen is not being mirrored, or
        // you are not running on a compatible device.
    }
}