1:创建工程的目录结构

1.1工程的目录结构

工程目录结构.jpg

1.2 工程思维导图

工程类图结构

2:原理解释

MLFinder的原理就是在viewcontroller返回时候（pop）检测当前的viewcontroller是否还有能力调用指定的willDealloc的方法。

总结成一句话：利用runtime发送消息的时候，receiver可以为nil，如[nil msg]。不会奔溃。

3:运行流程

运行流程图

4:源码如下

4.1 NSObject+MLFinder.m

#import "NSObject+MLFinder.h"
#import <objc/runtime.h>
#import <UIKit/UIKit.h>
const void *const kLatestSenderKey = &kLatestSenderKey;
@implementation NSObject (MLFinder)
+ (void)swizzleSEL:(SEL)originalSEL withSEL:(SEL)swizzledSEL {
    Class class = [self class];
    
    Method originalMethod = class_getInstanceMethod(class, originalSEL);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSEL);
    
    BOOL didAddMethod =
    class_addMethod(class,
                    originalSEL,
                    method_getImplementation(swizzledMethod),
                    method_getTypeEncoding(swizzledMethod));
    
    if (didAddMethod) {
        class_replaceMethod(class,
                            swizzledSEL,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}
- (BOOL)willDealloc {
    
    __weak id weakSelf = self;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        __strong id strongSelf = weakSelf;
        [strongSelf assertNotDealloc];
    });
    
    return YES;
}
- (void)assertNotDealloc {
    NSLog(@"\n\n================assertNotDeallocassertNotDealloc================\n\n");
}
@end

4.2 UINavigationController+MLFinder.m

#import "UINavigationController+MLFinder.h"
#import "NSObject+MLFinder.h"
#import <objc/runtime.h>
static const void *const kPoppedDetailVCKey = &kPoppedDetailVCKey;
@implementation UINavigationController (MLFinder)
+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [self swizzleSEL:@selector(popViewControllerAnimated:) withSEL:@selector(swizzled_popViewControllerAnimated:)];
    });
}
- (UIViewController *)swizzled_popViewControllerAnimated:(BOOL)animated {
    UIViewController *poppedViewController = [self swizzled_popViewControllerAnimated:animated];
    
    if (!poppedViewController) {
        return nil;
    }
    
//    // Detail VC in UISplitViewController is not dealloced until another detail VC is shown
//    if (self.splitViewController &&
//        self.splitViewController.viewControllers.firstObject == self &&
//        self.splitViewController == poppedViewController.splitViewController) {
//        objc_setAssociatedObject(self, kPoppedDetailVCKey, poppedViewController, OBJC_ASSOCIATION_RETAIN);
//        return poppedViewController;
//    }
    
    // VC is not dealloced until disappear when popped using a left-edge swipe gesture
    extern const void *const kHasBeenPoppedKey;
    objc_setAssociatedObject(poppedViewController, kHasBeenPoppedKey, @(YES), OBJC_ASSOCIATION_RETAIN);
    
    return poppedViewController;
}
@end

4.3 UIViewController+MLFinder.m

#import "UIViewController+MLFinder.h"
#import "NSObject+MLFinder.h"
#import <objc/runtime.h>
const void *const kHasBeenPoppedKey = &kHasBeenPoppedKey;
@implementation UIViewController (MLFinder)
+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        [self swizzleSEL:@selector(viewDidDisappear:) withSEL:@selector(swizzled_viewDidDisappear:)];
        [self swizzleSEL:@selector(viewWillAppear:) withSEL:@selector(swizzled_viewWillAppear:)];
//        [self swizzleSEL:@selector(dismissViewControllerAnimated:completion:) withSEL:@selector(swizzled_dismissViewControllerAnimated:completion:)];
    });
}
- (void)swizzled_viewDidDisappear:(BOOL)animated {
    [self swizzled_viewDidDisappear:animated];
    BOOL hasBeenPop = [objc_getAssociatedObject(self, kHasBeenPoppedKey) boolValue];
    if (hasBeenPop) {
        [self willDealloc];
    }
}
- (void)swizzled_viewWillAppear:(BOOL)animated {
    [self swizzled_viewWillAppear:animated];
    BOOL hasBeenPop = [objc_getAssociatedObject(self, kHasBeenPoppedKey) boolValue];
//    NSLog(@"----%d",hasBeenPop);
    objc_setAssociatedObject(self, kHasBeenPoppedKey, @(NO), OBJC_ASSOCIATION_RETAIN);
}
@end

5:模拟代码

#import "TimerViewController.h"
@interface TimerViewController ()
@property (nonatomic, strong) NSTimer * timer;
@end
@implementation TimerViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.title = @"timer";
    
//    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
//        [self timerClick];
//    });
    
//    self.timer = [NSTimer timerWithTimeInterval:2 target:self selector:@selector(timerClick) userInfo:nil repeats:YES];
//    [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
    
    __weak typeof(self) ws = self;
    self.timer = [NSTimer scheduledTimerWithTimeInterval:2 repeats:YES block:^(NSTimer * _Nonnull timer) {
        [ws timerClick];
    }];
    [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
    
    
}
- (void)timerClick {
    NSLog(@"%s",__func__);
}
//- (void)viewWillDisappear:(BOOL)animated {
//    if (self.timer != nil) {
//          [_timer invalidate];
//            _timer = nil;
//    }
//}
-(void)dealloc {
    NSLog(@"%s",__func__);
    [_timer invalidate];
    _timer = nil;
}
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"---TimerViewController:viewWillAppear:%@",self);
}
- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    NSLog(@"---TimerViewController:viewDidDisappear:%@",self);
}
@end

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