iOS block循环引用详解及常见误区

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
    self.translatesAutoresizingMaskIntoConstraints = NO;
    MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
    //这里并不是self.block （self并没有持有block 所以不会引起循环引用）
    block(constraintMaker);
    return [constraintMaker install];
}

#pragma mark - 添加代理
- (void)addDelegateForDataTask:(NSURLSessionDataTask *)dataTask
                uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
              downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
             completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler
{
    AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] init];
    delegate.manager = self;
     //block被代理引用
    delegate.completionHandler = completionHandler;
    dataTask.taskDescription = self.taskDescriptionForSessionTasks;
    //设置代理
    [self setDelegate:delegate forTask:dataTask];

    delegate.uploadProgressBlock = uploadProgressBlock;
    delegate.downloadProgressBlock = downloadProgressBlock;
}
#pragma mark - 设置代理
- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
    NSParameterAssert(task);
    NSParameterAssert(delegate);

    [self.lock lock];
    //代理被mutableTaskDelegatesKeyedByTaskIdentifier字典引用
    self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
    [delegate setupProgressForTask:task];
    [self addNotificationObserverForTask:task];
    [self.lock unlock];
}
#pragma mark - 任务完成
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didBecomeDownloadTask:(NSURLSessionDownloadTask *)downloadTask
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:dataTask];
    if (delegate) {
        //任务完成，移除
        [self removeDelegateForTask:dataTask];
        [self setDelegate:delegate forTask:downloadTask];
    }

    if (self.dataTaskDidBecomeDownloadTask) {
        self.dataTaskDidBecomeDownloadTask(session, dataTask, downloadTask);
    }
}
#pragma mark - 移除任务代理
- (void)removeDelegateForTask:(NSURLSessionTask *)task {
    NSParameterAssert(task);

    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:task];
    [self.lock lock];
    [delegate cleanUpProgressForTask:task];
    [self removeNotificationObserverForTask:task];
    //移除
    [self.mutableTaskDelegatesKeyedByTaskIdentifier removeObjectForKey:@(task.taskIdentifier)];
    [self.lock unlock];
}

//会造成循环引用
_person1 = [[Person alloc] init];
_person2 = [[Person alloc] init];
_person2.name = @"张三";
    
[_person1 Block:^{
    NSLog(@"%@",_person2.name)
}];

//在延迟执行期间，控制器被释放了，打印出来的会是**(null)**
_person1 = [[Person alloc] init];
_person2 = [[Person alloc] init];
_person2.name = @"张三";
__weak __typeof(self) weakSelf = self;
[_person1 Block:^{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"%@",weakSelf.person2.name);
    });
}];

//不会引起循环引用
[self dismissViewControllerAnimated:YES completion:^{
    NSLog(@"%@",self.string);
}];

_person1 = [[Person alloc] init];
_person2 = [[Person alloc] init];
_person2.name = @"张三";
__weak __typeof(self) weakSelf = self;
[_person1 Block:^{
    __typeof(&*weakSelf) strongSelf = weakSelf;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"%@",strongSelf.person2.name);
    });
}];

@interface CLViewController ()
//弱引用指针
@property (nonatomic,weak) Person *person1;
@property (nonatomic,strong) Person *person2;

@end

@implementation CLViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor redColor];
    //局部强引用对象
    Person *person1 = [[Person alloc] init];
    _person1 = person1;
    
    
    _person2 = [[Person alloc] init];
    _person2.name = @"张三";
    [_person1 Block:^{
            NSLog(@"%@",self.person2.name);
    }];

}

_person1 = [[Person alloc] init];
_person2 = [[Person alloc] init];
_person2.name = @"张三";
[_person1 Block:^{
    NSLog(@"%@",self.person2.name);
    //置空，避免循环引用
    _person1 = nil;
}];
//由于上面已经将对象置空，所以这里block里边的代码不会执行
[_person1 Block:^{
    NSLog(@"%@",self.person2.name);
}];

- (void)back
{
    if (self.BackBlock)
    {
        self.BackBlock(button);
    }
//使用完，马上置空当前block
    self.BackBlock = nil;
}