相对严谨的一种获取方式

- (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;
}