我们知道，循环引用即:
当两个对象A和B， 分别强引用对方，那么就会产生循环引用。即A释放的时候必须先释放B，而B释放的时候必须释放A。导致谁也不能释放
而打破循环引用的方法就是其中一方弱引用另一方

1. Block是否一定会造成循环引用

根据上篇文我们知道，循环引用的关键是:相互强引用,如果没有达到这一必要条件，则block不会造成循环引用

两个简单的例子:

@interface BlockViewController ()

@property (nonatomic, copy) NSString *nickname;

@property (nonatomic, copy) void(^ testBlock1)(void);

@property (nonatomic, copy) void(^ testBlock2)(void);

@end

@implementation BlockViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor orangeColor];
    
    // 会造成循环引用，因为self->Block, block -> self
    self.testBlock1 = ^{
        NSLog(@"== self = %p",self);
    };

    //不会造成循环引用，因为只有self->block
    self.testBlock2 = ^{
    };
}

- (void)dealloc {
    NSLog(@"dealloc = %@", self);
}

@end

编译成cpp源码:

testBlock0

struct __BlockViewController__viewDidLoad_block_impl_0 {
  struct __block_impl impl;
  struct __BlockViewController__viewDidLoad_block_desc_0* Desc;
  BlockViewController *self;
  __BlockViewController__viewDidLoad_block_impl_0(void *fp, struct __BlockViewController__viewDidLoad_block_desc_0 *desc, BlockViewController *_self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __BlockViewController__viewDidLoad_block_func_0(struct __BlockViewController__viewDidLoad_block_impl_0 *__cself) {
  BlockViewController *self = __cself->self; // bound by copy

        NSLog((NSString *)&__NSConstantStringImpl__var_folders_44_1ht3l6g55dv59_5s62wsv_bm0000gn_T_BlockViewController_439f5f_mi_0,self);
    }
    
        ((void (*)(id, SEL, void (*)()))(void *)objc_msgSend)((id)self, sel_registerName("setTestBlock1:"), ((void (*)())&__BlockViewController__viewDidLoad_block_impl_0((void *)__BlockViewController__viewDidLoad_block_func_0, &__BlockViewController__viewDidLoad_block_desc_0_DATA, self, 570425344)));

testBlock1:

struct __BlockViewController__viewDidLoad_block_impl_1 {
  struct __block_impl impl;
  struct __BlockViewController__viewDidLoad_block_desc_1* Desc;
  __BlockViewController__viewDidLoad_block_impl_1(void *fp, struct __BlockViewController__viewDidLoad_block_desc_1 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __BlockViewController__viewDidLoad_block_func_1(struct __BlockViewController__viewDidLoad_block_impl_1 *__cself) {

    }

    ((void (*)(id, SEL, void (*)()))(void *)objc_msgSend)((id)self, sel_registerName("setTestBlock2:"), ((void (*)())&__BlockViewController__viewDidLoad_block_impl_1((void *)__BlockViewController__viewDidLoad_block_func_1, &__BlockViewController__viewDidLoad_block_desc_1_DATA)));

或者如下:也不会造成循环引用，虽然block持有self，但self没有持有block

    BlockViewController2 *vc = [[BlockViewController2 alloc] init];
    [self.navigationController pushViewController:vc animated:true];
    vc.testBlock10 = ^{
        NSLog(@"-----vc.testBlock10 = %p", self);
    };

2. UIView动画是否会造成循环引用

不会，+ (void)animateWithDuration:(NSTimeInterval)duration animations:是类方法，当前控制器无法强引用一个类，所以循环引用无法构成

3. Masonry是否会造成循环引用

不会 ， self持有了Masonry,但是Masonry源码中并没有持有View,,如果将源码的block(constrainMaker)改成self.block = block(constrainMaker)，那么此时view才会持有block，才会造成循环引用

    self.testButton = [UIButton buttonWithType:UIButtonTypeCustom];
    [self.view addSubview:self.testButton];
    [self.view mas_makeConstraints:^(MASConstraintMaker *make) {
        make.centerX.equalTo(self.view);
        make.centerY.equalTo(self.view);
        make.size.mas_equalTo(CGSizeMake(100, 50));
    }];
 
 Masonry源码:
- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
    self.translatesAutoresizingMaskIntoConstraints = NO;
    MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
    block(constraintMaker);
    return [constraintMaker install];
}

4. AFN 是否会造成循环引用

项目中用到AFN的代码

    AFHTTPSessionManager * manager = [AFHTTPSessionManager manager];
    [manager POST:completeURL parameters:params progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        if (success) {
            success(responseObject);
        }
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
        if (failure) {
            failure(error);
        }
    }];

AFN源码

- (NSURLSessionDataTask *)POST:(NSString *)URLString
                    parameters:(id)parameters
                      progress:(void (^)(NSProgress * _Nonnull))uploadProgress
                       success:(void (^)(NSURLSessionDataTask * _Nonnull, id _Nullable))success
                       failure:(void (^)(NSURLSessionDataTask * _Nullable, NSError * _Nonnull))failure
{
    NSURLSessionDataTask *dataTask = [self dataTaskWithHTTPMethod:@"POST" URLString:URLString parameters:parameters uploadProgress:uploadProgress downloadProgress:nil success:success failure:failure];

    [dataTask resume];

    return dataTask;
}

- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
                                       URLString:(NSString *)URLString
                                      parameters:(id)parameters
                                  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgress
                                downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgress
                                         success:(void (^)(NSURLSessionDataTask *, id))success
                                         failure:(void (^)(NSURLSessionDataTask *, NSError *))failure
{
    NSError *serializationError = nil;
    NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];
    if (serializationError) {
        if (failure) {
            dispatch_async(self.completionQueue ?: dispatch_get_main_queue(), ^{
                failure(nil, serializationError);
            });
        }

        return nil;
    }

    __block NSURLSessionDataTask *dataTask = nil;
    dataTask = [self dataTaskWithRequest:request
                          uploadProgress:uploadProgress
                        downloadProgress:downloadProgress
                       completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
        if (error) {
            if (failure) {
                failure(dataTask, error);
            }
        } else {
            if (success) {
                success(dataTask, responseObject);
            }
        }
    }];

    return dataTask;
}

可见： self并没有持有manager， dataTask也没有持有success和failure这两个block，即使block中持有了self。也不会构成循环引用