IOS内存泄漏检查方法及重写MLeakFinder

时间:2021-11-21 14:34:46

对于iOS开发来讲,内存泄漏的问题,已经是老生常谈的话题。在日常的面试中经常会提到这些问题。我们日常的开发过程中进行内存泄漏的检测,一般是使用instrument工具中的Leaks/Allocation来进行排查,网络上也有比较高效又好用的内存泄漏检测工具,MLeakFinder。

 

MLeakFinder-原理

 

首先看UIViewController,当一个UIViewController被pop或dismiss的时候,这个VC包括在这个VC上的View,或者子View都会很快的被释放。所以我们我们需要在UIViewController被POP或dismiss后一小段时间后,在这个VC上的view,subView等是否还存在。

在UIViewController+MemoryLeak.h的load方法中可以看到,早+load方法中通过runtime交换了viewWillAppear,viewDidAppear,dismissViewControllerAnimated:completion:这三个方法。

 

1,首先看viewWillAppear

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- (void)swizzled_viewWillAppear:(BOOL)animated {
    [self swizzled_viewWillAppear:animated];
    objc_setAssociatedObject(self, kHasBeenPoppedKey, @(NO), OBJC_ASSOCIATION_RETAIN);
}

当VC进来的时候,添加关联对象,并标记为NO

 

2,在看viewDidAppear

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- (void)swizzled_viewDidDisappear:(BOOL)animated {
    [self swizzled_viewDidDisappear:animated];
    if ([objc_getAssociatedObject(self, kHasBeenPoppedKey) boolValue]) {
        [self willDealloc];   
    }
}

通过代码可以看出,获取当前关联对象的标记,当标记为YES的时候,就会调用willDealloc。

 

3,我们看什么时候会被标记为YES呢?

在UINavigationController+MemoryLeak.h的popViewControllerAnimated:方法中我们可以看到

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- (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;
}

我们可以看出,在VC被pop或者左滑返回的时候,相当于视图销毁,就会被标记为YES。

 

4,我们重点看willDealloc

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- (BOOL)willDealloc {
   //第一步
    NSString *className = NSStringFromClass([self class]);
    if ([[NSObject classNamesWhitelist] containsObject:className])
        return NO;
 
   //第二步
    NSNumber *senderPtr = objc_getAssociatedObject([UIApplication sharedApplication], kLatestSenderKey)
    if ([senderPtr isEqualToNumber:@((uintptr_t)self)])
        return NO;
 
   //第三步
    __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;
}

1.第一步:我们可以看到,会先判断当前的class是否在白名单中,是的话就会return NO,即不是内存泄漏的。
同时我们查看构建白名单的源码:使用了一个单例实现,确保只有一个,是个私有方法

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+ (NSMutableSet *)classNamesWhitelist {
    static NSMutableSet *whitelist = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        whitelist = [NSMutableSet setWithObjects:
                     @"UIFieldEditor", // UIAlertControllerTextField
                     @"UINavigationBar",
                     @"_UIAlertControllerActionView",
                     @"_UIVisualEffectBackdropView",
                     nil];
        
        // System's bug since iOS 10 and not fixed yet up to this ci.
        NSString *systemVersion = [UIDevice currentDevice].systemVersion;
        if ([systemVersion compare:@"10.0" options:NSNumericSearch] != NSOrderedAscending) {
            [whitelist addObject:@"UISwitch"];
        }
    });
    return whitelist;
}

同时在还支持,自定义的添加白名单

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+ (void)addClassNamesToWhitelist:(NSArray *)classNames {
    [[self classNamesWhitelist] addObjectsFromArray:classNames];
}

2. 第二步:判断该对象是否是上一次发送action的对象,是的话,不进行内存检测

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//第二步
   
 NSNumber *senderPtr = objc_getAssociatedObject([UIApplication sharedApplication], kLatestSenderKey) 
  
if ([senderPtr isEqualToNumber:@((uintptr_t)self)])
        
return NO;

3,第三步:弱指针指向self,2s延迟,然后通过这个弱指针调用-assertNotDealloc,若被释放,给nil发消息直接返回,不触发-assertNotDealloc方法,认为已经释放;如果它没有被释放(泄漏了),-assertNotDealloc就会被调用

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__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];
});

 

5,现在我们回到:2的代码 [self willDealloc]

看一下他的源码

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- (BOOL)willDealloc {
    //第一步
    if (![super willDealloc]) {
        return NO;
    }
    //第二步
    [self willReleaseChildren:self.childViewControllers];
    [self willReleaseChild:self.presentedViewController];
    if (self.isViewLoaded) {
        [self willReleaseChild:self.view];
    }
    return YES;
}

1,第一步:会通过 super调用父类的willDealloc,即上面目录4

2,第二步:调用willReleaseChildren,willReleaseChild遍历该对象的子对象,看其是否释放

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- (void)willReleaseChild:(id)child {
    if (!child) {
        return;
    }
    
    [self willReleaseChildren:@[ child ]];
}
 
 
- (void)willReleaseChildren:(NSArray *)children {
    NSArray *viewStack = [self viewStack];
    NSSet *parentPtrs = [self parentPtrs];
    for (id child in children) {
        NSString *className = NSStringFromClass([child class]);
        [child setViewStack:[viewStack arrayByAddingObject:className]];
        [child setParentPtrs:[parentPtrs setByAddingObject:@((uintptr_t)child)]];
        [child willDealloc];
    }
}

通过代码可以看出,通过调用willReleaseChildren的方法,获取当前对象viewStack,parentPtrs,并且遍历children,为每个子对象设置viewStack,parentPtrs,然后调用willDealloc。

通过源码看一下viewStask,parentPtrs的实现

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- (NSArray *)viewStack {
    NSArray *viewStack = objc_getAssociatedObject(self, kViewStackKey);
    if (viewStack) {
        return viewStack;
    }
    
    NSString *className = NSStringFromClass([self class]);
    return @[ className ];
}
 
 
- (void)setViewStack:(NSArray *)viewStack {
    objc_setAssociatedObject(self, kViewStackKey, viewStack, OBJC_ASSOCIATION_RETAIN);
}
 
 
- (NSSet *)parentPtrs {
    NSSet *parentPtrs = objc_getAssociatedObject(self, kParentPtrsKey);
    if (!parentPtrs) {
        parentPtrs = [[NSSet alloc] initWithObjects:@((uintptr_t)self), nil];
    }
    return parentPtrs;
}
 
 
- (void)setParentPtrs:(NSSet *)parentPtrs {
    objc_setAssociatedObject(self, kParentPtrsKey, parentPtrs, OBJC_ASSOCIATION_RETAIN);
}

viewStack使用数组,parentPtrs使用的集合形式。都是通过运行时,用关联对象添加属性。

parentPtrs会在-assertNotDealloc中,会判断当前对象是否与父节点集合有交集。下面仔细看下-assertNotDealloc方法

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- (void)assertNotDealloc {    //第一步
    if ([MLeakedObjectProxy isAnyObjectLeakedAtPtrs:[self parentPtrs]]) {
        return;
    }    //第二步
    [MLeakedObjectProxy addLeakedObject:self];
    
    NSString *className = NSStringFromClass([self class]);
    NSLog(@"Possibly Memory Leak.\nIn case that %@ should not be dealloced, override -willDealloc in %@ by returning NO.\nView-ViewController stack: %@", className, className, [self viewStack]);
}

1,第一步我们看到,通过parentPtrs的判断是否有交集

产看其源码:

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+ (BOOL)isAnyObjectLeakedAtPtrs:(NSSet *)ptrs {
    NSAssert([NSThread isMainThread], @"Must be in main thread.");
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        leakedObjectPtrs = [[NSMutableSet alloc] init];
    });
    if (!ptrs.count) {
        return NO
    }
    if ([leakedObjectPtrs intersectsSet:ptrs]) {
        return YES;
    } else {
        return NO;
    }}

可以看到,创建了一个单例对象,通过集合的形式,判断是否有交集,是的话return。否则就进入第二步

2,第二步:addLeakedObject

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+ (void)addLeakedObject:(id)object {
    NSAssert([NSThread isMainThread], @"Must be in main thread.");
    MLeakedObjectProxy *proxy = [[MLeakedObjectProxy alloc] init];
    proxy.object = object;
    proxy.objectPtr = @((uintptr_t)object);
    proxy.viewStack = [object viewStack];
    static const void *const kLeakedObjectProxyKey = &kLeakedObjectProxyKey;
    objc_setAssociatedObject(object, kLeakedObjectProxyKey, proxy, OBJC_ASSOCIATION_RETAIN);
    [leakedObjectPtrs addObject:proxy.objectPtr];
#if _INTERNAL_MLF_RC_ENABLED    [MLeaksMessenger alertWithTitle:@"Memory Leak"                            message:[NSString stringWithFormat:@"%@", proxy.viewStack]
                           delegate:proxy
              additionalButtonTitle:@"Retain Cycle"];
#else
    [MLeaksMessenger alertWithTitle:@"Memory Leak"
                            message:[NSString stringWithFormat:@"%@", proxy.viewStack]];#endif
}

第一步:构造MLeakedObjectProxy对象,给传入的泄漏对象 object 关联一个代理即 proxy

第二步:通过objc_setAssociatedObject(object, kLeakedObjectProxyKey, proxy, OBJC_ASSOCIATION_RETAIN)方法,object强持有proxy, proxy若持有object,如果object释放,proxy也会释放

第三步:存储 proxy.objectPtr(实际是对象地址)到集合 leakedObjectPtrs 里边

第四步:弹框 AlertView若 _INTERNAL_MLF_RC_ENABLED == 1,则弹框会增加检测循环引用的选项;若 _INTERNAL_MLF_RC_ENABLED == 0,则仅展示堆栈信息。

对于MLeakedObjectProxy类而言,是检测到内存泄漏才产生的,作为泄漏对象的属性存在的,如果泄漏的对象被释放,那么MLeakedObjectProxy也会被释放,则调用-dealloc函数

集合leakedObjectPtrs中移除该对象地址,同时再次弹窗,提示该对象已经释放了

 

6,自己也在尝试重写该框架,欢迎大家一起交流

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原文链接:https://juejin.cn/post/6954726462544936973