iOS - GCD 和 __strong 引用

Question

我有这段代码，我想做的是让 self 在将在主线程上执行的 block 中保持事件状态。结果有点随机，有时会打印 null。

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.proArray = [[NSMutableArray alloc]init];

    GCDVC2* __weak weakSelf = self;

    self.postGCDBlock = ^{

        GCDVC2* __strong strongSelf2 = weakSelf;

        [strongSelf2.proArray removeObject:@"3"];
        NSLog(@"%@",strongSelf2.proArray);
        [strongSelf2.activityIndicator stopAnimating];
    };

    self.addObjectsBlock = ^{

        GCDVC2* __strong strongSelf = weakSelf;

        [strongSelf.proArray addObject:@"2"];
        [strongSelf.proArray addObject:@"3"];
        [NSThread sleepForTimeInterval:5];

        dispatch_async(dispatch_get_main_queue(),strongSelf.postGCDBlock);
    };

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), self.addObjectsBlock);

}

这段代码工作正常:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.proArray = [[NSMutableArray alloc]init];

    GCDVC2* __weak weakSelf = self;


    //self.postGCDBlock = ;

    self.addObjectsBlock = ^{

        GCDVC2* __strong strongSelf = weakSelf;

        [strongSelf.proArray addObject:@"2"];
        [strongSelf.proArray addObject:@"3"];
        [NSThread sleepForTimeInterval:5];

         GCDVC2* __weak weakSelf2 = strongSelf;

        dispatch_async(dispatch_get_main_queue(),^{

            GCDVC2* __strong strongSelf = weakSelf2;

            [strongSelf.proArray removeObject:@"3"];
            NSLog(@"%@",strongSelf.proArray);
            [strongSelf.activityIndicator stopAnimating];
        });
    };

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), self.addObjectsBlock);

}

有什么方法可以将第二段代码转换为与第一段代码的结构一起工作？我尝试了很多变化，但它总是随机的。我能以某种方式确保 self.postGCDBlock 不会将 self 设置为 nil 吗？

更新:属性(property)声明:

typedef void(^CustomBlock)(void);

@interface GCDVC2 ()
@property (weak, nonatomic) IBOutlet UIActivityIndicatorView *activityIndicator;
@property(nonatomic,strong)NSMutableArray *proArray;
@property (nonatomic, copy) CustomBlock addObjectsBlock;
@property (nonatomic, copy) CustomBlock postGCDBlock;
@end

Answer 1

我认为你的问题在于这一行(我无法用这段代码重现失败案例):

dispatch_async(dispatch_get_main_queue(),strongSelf.postGCDBlock);

此时，在您的 addObjectsBlock 中，strongSelf 持有对 self 的引用，但当您离开该 block 的范围时，它会结束. dispatch_async 将复制 postGCDBlock，但该 block 没有对 self 的强引用。

要让 dispatch_async 持有对 self 的强引用，您需要执行如下操作:

dispatch_async(dispatch_get_main_queue(), ^{
    strongSelf.postGCDBlock();
});

在 block 中包装 strongSelf 将导致 dispatch_async 保留 strongSelf(从而 self)足够长的时间它调用 postGCDBlock。