iOS 根据类名，获取已存在的controller，无论是模态出来还是push出来的controller，都可以找到

我们在写代码时，有时会遇到：需要知道某个controller是否已经存在，或着，要拿到已存在的某个controller对象，这里提供一种可以获取到你想要的controller对象，只需要传入类名即可，废话不多说直接上代码：

- (UIViewController *)getActivityViewController:(NSString *)controllerName {
    UIViewController *topVC = [UIApplication sharedApplication].keyWindow.rootViewController;
    while (topVC.presentedViewController) {
        topVC = topVC.presentedViewController;
        if ([[topVC.class description] isEqualToString:@"UINavigationController"]) {
            UINavigationController *navi = (UINavigationController *)topVC;
            if (navi && navi.viewControllers && navi.viewControllers.count > 0) {
                NSInteger count = navi.viewControllers.count;
                for (NSInteger i=count-1; i>=0; i--) {
                    UIViewController *controller = [navi.viewControllers objectAtIndex:i];
                    if ([[controller.class description] isEqualToString:controllerName]) {
                        return controller;
                    }
                }
            }
        }
    }
    return nil;
}

如果当前存在的同一类名的controller有多个，你要拿到最新的一个可以用下面这种方法：

- (UIViewController *)getActivityViewController:(NSString *)controllerName {
    UIViewController *topVC = [UIApplication sharedApplication].keyWindow.rootViewController;
    UIViewController *resaultVC = nil;
    while (topVC.presentedViewController) {
        topVC = topVC.presentedViewController;
        if ([[topVC.class description] isEqualToString:@"UINavigationController"]) {
            UINavigationController *navi = (UINavigationController *)topVC;
            if (navi && navi.viewControllers && navi.viewControllers.count > 0) {
                NSInteger count = navi.viewControllers.count;
                for (NSInteger i=count-1; i>=0; i--) {
                    UIViewController *controller = [navi.viewControllers objectAtIndex:i];
                    if ([[controller.class description] isEqualToString:controllerName]) {
                        resaultVC =  controller;
                    }
                }
            }
        }
    }
    return resaultVC;
}

当然这两个方法的前提是，你如果是 present 出来的congtroller，并且导航控制器是 UINavigationController 类型，如果导航控制器是自定义的，那么在判断 if ([[topVC.class description] isEqualToString:@"UINavigationController"]) 时@"UINavigationController" 要替换成你自定义的类名即可。