iOS：获取最顶层window

方式一：（此方式不严谨，最好用方式二）

[[UIApplication sharedApplication].windows lastObject];

方式二：

+ (UIWindow *)lastWindow
{
    NSArray *windows = [UIApplication sharedApplication].windows;
    for(UIWindow *window in [windows reverseObjectEnumerator]) {
        if ([window isKindOfClass:[UIWindow class]] &&
            CGRectEqualToRect(window.bounds, [UIScreen mainScreen].bounds))
            return window;
    }
    return [UIApplication sharedApplication].keyWindow;
}