Question

#import "ViewController.h"

@implementation A

- (instancetype)init
{
    self = [super init];

    if (self)
    {
        self.databaseQueue = dispatch_queue_create("someQueue", DISPATCH_QUEUE_SERIAL);
    }

    return self;
}
- (void) privateLogMethod
{
    NSLog(@"private log method called");
    [NSThread sleepForTimeInterval:1];
}

- (void) performSomeAction
{
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_async(_databaseQueue, ^{
            NSLog(@"inside for loop");
            [self privateLogMethod];
        });
    }
}

- (void) dealloc
{
    NSLog(@"removing A from memory");
}

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.someClassA = [[A alloc] init];

    [self.someClassA performSomeAction];

    [self performSelector:@selector(removeA) withObject:nil afterDelay:5];
}

- (void) removeA
{
    NSLog(@"call to remove A from memory");
    self.someClassA = nil;
}

@end


#import <UIKit/UIKit.h>

@interface A: NSObject
- (void) performSomeAction;
@property (nonatomic, strong) dispatch_queue_t databaseQueue;
@end

@interface ViewController : UIViewController
@property (nonatomic, strong) A* someClassA;
@end

如果我们看到上面的代码，则会有一个保留周期，因为class A持有databaseQueue且databaseQueue持有self。因此，当ViewController在5秒后询问dealloc A时，class A会在deallocating之前完成循环。以下是输出。

2017-09-14 14:21:06.774517+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:06.774768+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:07.775218+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:07.775480+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:08.778805+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:08.779251+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:09.784467+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:09.785089+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:10.790469+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:10.790929+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:11.775522+0530 testdealloc[72021:1812575] **call to remove A from memory**
2017-09-14 14:21:11.796196+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:11.796659+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:12.802018+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:12.802483+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:13.804953+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:13.805432+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:14.806252+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:14.806604+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:15.807852+0530 testdealloc[72021:1812626] inside for loop
2017-09-14 14:21:15.808306+0530 testdealloc[72021:1812626] private log method called
2017-09-14 14:21:16.809550+0530 testdealloc[72021:1812626] **removing A from memory**

我的问题是：我们在代码中有一个保留周期，但是，这不会导致内存泄漏。在这种情况下，是否可以保留代码[因为它确实不会导致内存泄漏]？或者我应该在块中使用__weak weakSelf = self然后weakSelf来确保根本没有保留周期？

Answer 1

你问：

这可以吗？

很大程度上，是的。

更准确地说，这取决于您的需求/意图。如果你需要这个继续运行让你的应用程序正常运行（例如，你可能正在转换并保存一些图像），那么你肯定需要一个强大的参考，以便它完成）。

但如果您不需要继续执行，那么您将使用weakSelf模式。例如，假设您在视图控制器中有一系列调度任务，这些任务仅用于以后触发UI更新。在这种情况下，如果视图控制器被解除，您可能不需要运行这些块，并且您当然不希望它挂起到视图控制器以查看已被解雇的视图。（您甚至可能希望在取消视图控制器时取消这些已分派的项目，但这是另一个主题。）

底线，这取决于你的意图。

Answer 2

我是第二个@Rob。但寻找任何避免保留周期的机会。这对于一个简单的项目是可以的，但是在处理复杂的项目时...它会节省大量的时间来尝试调试/解决你不确定根本原因的问题。

这是一篇很好的文章，解释了不同类型的保留周期并避免它们。

https://www.cocoawithlove.com/2009/07/rules-to-avoid-retain-cycles.html

在不导致内存泄漏的情况下在代码中保留周期

2 个答案: