效果圖

UIAlertController 點擊確認時候會消失，無法校驗是否輸入，所以在他dismiss前要先做校驗

通過小技巧（點擊確認在block中添加一個數組，使其崩潰）找到以下兩個方法（系統版本不同）

_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion:
_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:

hook掉以上兩個方法

#import <JRSwizzle/JRSwizzle.h>

#pragma mark - Private

+ (void)load {
    
    SEL originalSelector = nil;
    SEL swizzledSelector = nil;
    
    // _dismissAnimated
    originalSelector = NSSelectorFromString(@"_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion:");
    swizzledSelector = @selector(tm_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion:);
    [UIAlertController jr_swizzleMethod:originalSelector withMethod:swizzledSelector error:nil];
    
    originalSelector = NSSelectorFromString(@"_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:");
    swizzledSelector = @selector(tm_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:);
    [UIAlertController jr_swizzleMethod:originalSelector withMethod:swizzledSelector error:nil];
}

- (void)tm_dismissAnimated:(BOOL)flag triggeringAction:(UIAlertAction *)action triggeredByPopoverDimmingView:(id)view dismissCompletion:(void (^)(void))completion {
    // 保證點擊取消依舊可以消失
    if (action.style != UIAlertActionStyleCancel) {
        for (UITextField *textField in self.textFields) {
            if ([textField.text isEqualToString:@""]) {
                [textField placeholderShake]; // placeholder 抖動動畫
                return;
            }
        }
    }
    
    [self tm_dismissAnimated:flag triggeringAction:action triggeredByPopoverDimmingView:view dismissCompletion:completion];
}

- (void)tm_dismissAnimated:(BOOL)flag triggeringAction:(UIAlertAction *)action triggeredByPopoverDimmingView:(id)view {
    // 保證點擊取消依舊可以消失
    if (action.style != UIAlertActionStyleCancel) {
        for (UITextField *textField in self.textFields) {
            if ([textField.text isEqualToString:@""]) {
                [textField placeholderShake]; // placeholder 抖動動畫
                return;
            }
        }
    }
    
    [self tm_dismissAnimated:flag triggeringAction:action triggeredByPopoverDimmingView:view];
}