需求
在使用UIImagePickerController进行拍照时，如果没有相机权限，则需要弹出一个提示框提醒用户。

实现方式
1、每个弹出UIImagePickerController的地方写上弹框代码
2、写个UIImagePickerController的子类，重写其父类方法
3、使用Method Swizzling

使用Method Swizzling来实现需求
步骤
1、给UIViewController添加一个Category，

//
//  UIViewController+UIImagePickerCamera.m
//
//  Created by 黄瑞 on 2016/11/24.
//  Copyright © 2016年 黄瑞. All rights reserved.
//

#import "UIViewController+UIImagePickerCamera.h"

@implementation UIViewController (UIImagePickerCamera)

+ (void)load {
    SEL orig_present = @selector(presentViewController:animated:completion:);
    SEL swiz_present = @selector(swiz_presentViewController:animated:completion:);
    [UIViewController swizzleMethods:[self class] originalSelector:orig_present swizzledSelector:swiz_present];
}

//exchange implementation of two methods

+ (void)swizzleMethods:(Class)class originalSelector:(SEL)origSel swizzledSelector:(SEL)swizSel {
    
    Method origMethod = class_getInstanceMethod(class, origSel);
    Method swizMethod = class_getInstanceMethod(class, swizSel);
    
    //class_addMethod will fail if original method already exists
    
    BOOL didAddMethod = class_addMethod(class, origSel, method_getImplementation(swizMethod), method_getTypeEncoding(swizMethod));
    
    if (didAddMethod) {
        class_replaceMethod(class, swizSel, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    } else {
        //origMethod and swizMethod already exist
        method_exchangeImplementations(origMethod, swizMethod);
    }
}

- (void)swiz_presentViewController:(UIViewController *)vc animated:(BOOL)animated completion:(void(^)(void))completion {
    if ([vc isKindOfClass:[UIImagePickerController class]]) {
        UIImagePickerController *imgPicker = (UIImagePickerController *)vc;
        if (imgPicker.sourceType == UIImagePickerControllerSourceTypeCamera) {
            completion = ^{
                completion();
                AVAuthorizationStatus status = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
                if (status == AVAuthorizationStatusDenied || status == AVAuthorizationStatusRestricted) {
                    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"请在iPhone的“设置-隐私-相机”选项中，允许XX访问您的相机。" delegate:nil cancelButtonTitle:@"好" otherButtonTitles:nil];
                    [alert show];
                }
            };
        }
    }
    [self swiz_presentViewController:vc animated:animated completion:completion];
}

@end

需要在.h文件中引入AVFoundation框架

//
//  UIViewController+UIImagePickerCamera.h
//
//  Created by 黄瑞 on 2016/11/24.
//  Copyright © 2016年 黄瑞. All rights reserved.
//

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <objc/runtime.h>

@interface UIViewController (UIImagePickerCamera)

@end

完成