iphone - UIScreen mirroredScreen 属性总是返回 nil

Question

如果设备（原始 iPad）不支持镜像，我想在外部连接的显示器上呈现独特的内容，但如果它是 iPad 2，我想使用屏幕镜像。现在，当我尝试如下编码时：

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

[[UIScreen mainScreen] mirroredScreen]总是返回nil。

难道我做错了什么？

Answer 1

据我了解文档，mirroredScreen如果您在实际上是镜像屏幕的辅助屏幕上访问该属性，将参考主屏幕。如：

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

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

Answer 2

Apple have a recommendation about the way to detect if the screen is mirrored or not here : 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.
    }
}