在开发中经常遇到一些需要弹框 UIAlertController 一直显示不消失的需求，如登录时等。但一般的，不管点击取消或确定按钮，弹框都会消失。今天就用 Runtime 这个“黑魔法”来实现这个需求。

首先在 UIAlertAction 的回调里打个断点，看下 UIAlertController 调用的方法：

Screenshot

可以看到 UIAlertController 调了 _invokeHandlersForAction: 和 _dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion: 两个方法，可以猜测前一个方法是用来回调 UIAlertAction 的，后一个方法可能与 UIAlertController 的消失与否有关，可以来验证一下。

UIAlertController+Dismiss.h

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//  UIAlertController+Dismiss.h
#import <UIKit/UIKit.h>

@interface UIAlertController (Dismiss)

@property (nonatomic, assign) BOOL wy_rejectDismiss;

@end

UIAlertController+Dismiss.m

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//  UIAlertController+Dismiss.m
#import "UIAlertController+Dismiss.h"
#import <objc/runtime.h>

@implementation UIAlertController (Dismiss)

@dynamic wy_rejectDismiss;

- (void)setWy_rejectDismiss:(BOOL)wy_rejectDismiss {
    objc_setAssociatedObject(self, @selector(wy_rejectDismiss), @(wy_rejectDismiss), OBJC_ASSOCIATION_ASSIGN);
}

- (BOOL)wy_rejectDismiss {
    return [(NSNumber *)objc_getAssociatedObject(self, _cmd) boolValue];
}

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = [self class];
        
        SEL originalSelector = NSSelectorFromString(@"_dismissAnimated:triggeringAction:triggeredByPopoverDimmingView:dismissCompletion:");
        SEL swizzledSelector = @selector(wy_dismissAnimated:
                                         triggeringAction:
                                         triggeredByPopoverDimmingView:
                                         dismissCompletion:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
//        动态添加方法，如果类中不存在这个方法的实现，则添加成功
//        这里 UIAlertController 类中存在 originalMethod，所以添加是失败的
        BOOL didAddMethod = class_addMethod(class,
                                            originalSelector,
                                            method_getImplementation(swizzledMethod),
                                            method_getTypeEncoding(swizzledMethod));
        if (didAddMethod) {
            // 如果添加成功，则用 originalMethod 替换添加的空方法 originalMethod
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originalMethod),
                                method_getTypeEncoding(originalMethod));
        } else {
            // 交换两个方法的实现
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

- (void)wy_dismissAnimated:(BOOL)animation
          triggeringAction:(UIAlertAction *)action
triggeredByPopoverDimmingView:(id)view
         dismissCompletion:(id)handler {
//    如果点击“取消”按钮或者允许弹框 dismiss，就调用原来的方法（originalMethod）
//      因为已经交换了两个方法的实现，所以其实是调用 swizzledMethod
//      所以这里并不会出现循环调用
//    否则就忽略原来的方法（originalMethod），直接下一步，掉用后面的方法
    if (action.style == UIAlertActionStyleCancel || self.wy_rejectDismiss == NO) {
        [self wy_dismissAnimated:animation
                triggeringAction:action
   triggeredByPopoverDimmingView:view
               dismissCompletion:handler];
    } else {
        SEL invokeHandler = NSSelectorFromString(@"_invokeHandlersForAction:");
//        这里如果使用 performSelector 来调 invokeHandler 这个方法
//          [self performSelector:invokeHandler withObject:action];
//        会报 "PerformSelector may cause a leak because its selector is unknown" 的警告
//        为消除警告，用下面的方法
        IMP imp = [self methodForSelector:invokeHandler];
        void (*func)(id, SEL, UIAlertAction *) = (void *)imp;
        func(self, invokeHandler, action);
    }
}

@end

ViewController.m

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//  ViewController.m
#import "ViewController.h"
#import "UIAlertController+Dismiss.h"

@interface ViewController ()

@property (nonatomic, strong) UIAlertController *alertController;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"提示" message:nil preferredStyle:UIAlertControllerStyleAlert];
    [alertController addTextFieldWithConfigurationHandler:^(UITextField * _Nonnull textField) {
        [textField addTarget:self action:@selector(handleTextFieldEditingChanged:) forControlEvents:UIControlEventEditingChanged];
    }];
    UIAlertAction *cancelAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
        // do something
    }];
    UIAlertAction *confirmActin = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
        // do something
    }];
    [alertController addAction:cancelAction];
    [alertController addAction:confirmActin];
    alertController.wy_rejectDismiss = YES;
    self.alertController = alertController;
    [self presentViewController:alertController animated:YES completion:nil];
}

- (void)handleTextFieldEditingChanged:(UITextField *)textField {
    if ([textField.text isEqualToString:@"123"]) {
        self.alertController.wy_rejectDismiss = NO;
    } else {
        self.alertController.wy_rejectDismiss = YES;
    }
}

@end

结果说明猜测是正确的。这样就实现了禁止弹框消失的需求，只需要设置 alertController 的 wy_rejectDismiss 这个属性为 YES，当需要弹框消失时只要把 wy_rejectDismiss 设置成 NO 即可。

相关链接

Method Swizzling
Runtime 隐藏 Status Bar 背景
performSelector may cause a leak because its selector is unknown