iOS循环引用常见场景和解决办法

//ViewController.m
- (IBAction )onRefreshClicked:(id)sender {
    //场景获数据的操作对象
    self.updateOp = [DataUpdateOp new];
    [self.updateOp startUsingDelegate:self withSelector:@selector(onDataAvailable:)]; 
}

- (void)onDataAvailable:(NSArray *)records {
    //任务完成时，将操作对象置nil    
    self.updateOp = nil;
}
//如果控制器 delloc 则取消操作
- (void)delloc {
    //取消  
    if(slef.updateOp  !=nil){
        [self.updateOp cancel];    
    }
}


//DataUpdateOp.h
@protocol  DataUpdateOpDeleate<NSObject>
- (void)onDataAvailable:(NSArray *)records;
@end

@interface DataUpdateOp
@property (nonatomic, weak)id <DataUpdateOpDeleate> delegate;

- (void)startUpdate;
- (void)cancel;
@end
//DataUpdateOp.m
@implementation DataUpdateOp
- (void)startUpdate {
    dispatch_async{
       dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //执行网络请求后获取到结果
            NSArray *records = ...
            dispatch_async(dispatch_get_main_queue(),^{
                //尝试获取委托对象的强引用
                id<DataUpdateOpDeleate> delegate = self.delegate;
                if (!delegate){
                    return;
                }else{//判断原始对象仍然存在吗？
                    //回传数据
                    [delegate onDataAvailable:records];
                }
            })
        };
    }); 
}

//显示的要求废弃回调对象
- (void)cancel {
    //取消执行中的网络请求
    self.delegate = nil;
}

  

-(void)someMethod {
    SomeViewController *vc = [[SomeViewController alloc] init]; 
　　[self presentViewController:vc animated:YES
             completion:^{
                     self.data = vc.data;
                     [self dismissViewControllerAnimated:YES completion:nil];
                 }];
}

-(void)someMethod {
SomeViewController *vc = [[SomeViewController alloc] init];
__weak typeof(self) weakSelf = self; //弱引用self 方便被 completion捕获
         [self presentViewController:vc animated:YES
             completion:^{
            typeof(self) theSelf = weakSelf; //通过一弱引用获取一个强引用
            if(theSelf != nil) { //只在控制器 不为nil的时候才继续
                theSelf.data = vc.data; 
                [theSelf dismissViewControllerAnimated:YES completion:nil];
            }
         }];
} 

@implementation SomeViewController
- (void)startPollingTask {
    self.timer = [NSTimer scheduledTimerWithTimeInterval:120 target:self 
        selector:@selector(updateTask:) userInfo:nil repeats:YES];
}

- (void)updateTask:(NSTimer *)timer {
    //...
}

- (void)delloc {
    [self.timer invalidated];
}

@end

@implementation SomeViewController
- (void)startPollingTask {
    self.timer = [NSTimer scheduledTimerWithTimeInterval:120 target:self 
        selector:@selector(updateTask:) userInfo:nil repeats:YES];
}

- (void)updateTask:(NSTimer *)timer {
    //...
}

- (void)delloc {
    [self.timer invalidate];
}

@end

1、方法一，在用户离开当前视图控制器的时候清理timer

//当视图控制器进入或者离开视图控制器时，调用 该方法
- (void)didMoveToParentViewController:(UIViewController *)parent {
    //如果是离开父控制器， if中判断为YES, 才执行 cleanUp
    if (parent == nil)
    {
        [self cleanUp];
    }
}

- (void)cleanUp {
    [self.timer invalidate];
}

//2、方法二 通过拦截返回按钮 执行清理
- (id)init {
    if (self = [super init])
    {
        self.navigationItem.backBarButtonItem.target = self;
        self.navigationItem.backBarButtonItem.action = @selector(backButtonPressDetected:);
    }
    return sel;
}

- (void)backButtonPressDetected:(id)sender {
    [self cleanUp];
    [self.navigationController popViewControllerAnimated:YES];
}

//NewFeedUpdateTask.h
@interface NewFeedUpdateTask
@property (nonatomic, weak) id target;//target是弱引用，target会在这里实例化任务，并持有它
@property (nonatomic, assign) SEL selector;

@end

//NewFeedUpdateTask.m
@implementation NewFeedUpdateTask
//推荐使用的构造方法 外部最好不要用哪个alloc init了 
- (void)initWithTimerInterval:(NSTimerInterval )interval target:(id)target selector:(SEL)selector{
    if (sellf = [super init])
    {
        self.target = target;
        self.selector = selector;
        self.timer = [NSTimer scheduledTimerWithTimeInterval:interval target:self selector:@selector(fetchAndUpdate:) userInfo:nil repeats:YES];
    }
    return self;
}

//周期性执行的任务
- (void)fetchAndUpdate:(NSTimer *)timer {
    //检索feed
    NewsFeed *feed = [self getFromServerAndCreateModel];
    //用weak修饰，确保，使用异步块的时候不会造成循环引用
    __weak typeof(self) weakSelf = self;
    dispatch_async(dispatch_get_main_queue(),^{
        __strong typeof(self) strongSlef = weakSelf;
        if (!strongSlef){
                return;
        }
        if (strongSlef.target == nil){
            return; 
        }

        /**
            strongSlef.target 和strongSlef.selector 是控制器传过来的，也就是可能有不同的控制器创建本对象，进而初始化 target 和 selector
            使用本地变量 target 和 selector 有一个好处：
            避免了在以下执行序列中发生竞争的情况
            1】在某一个线程A中调用 [target responsToSelector:selector];
            2】在线程B中修改 target 或者 selector
            3】在线程A中调用[target performSelector:selector withObject:feed];
            有了这个代码，即使 target 或者 selector 此刻已经发生了变化，performSelector  仍然会被正确的 target 和 selecctor所调用
        **/
        id target = strongSlef.target;
        SEL selector = strongSlef.selector;

        if ([target respondsToSelector:selector]){
            [target performSelector:selector withObject:feed];;
        }
    });
}

- (void)shutDown {
    [self.timer invalidate];
    self.timer = nil;
}


//viewController.m
@implementation viewController
- (void)viewDidLoad {
    //初始化 定时执行任务的对象 ，内部会触发计时器
    self.updateTask = [NewFeedUpdateTask initWithTimerInterval:120 target:self selector:@selector(updateUsingFeed:)];

}

//是 NewFeedUpdateTask 对象周期性的回调方法。
- (void)updateUsingFeed:(NewsFeed *)feed {
    //根据返回的数据 feed 更新ui
}

//调用 任务对象的 shutDown方法，其内部会销毁定时器
- (void)delloc {
    [self.updateTask shutDown];
}    
@end