[iOS]dispatch_after()中self和weakself的使用

来源：cnblogs　　作者：EverNight　　时间：2021/4/6 10:05:47　　对本文有异议

苹果对编译器在不断优化，GCD方法中的block基本都不需要使用weakself，并不会造成循环引用。

dispatch_after官方文档中对block部分的说明:

一：使用self

从ViewControllerA push 到 ViewControllerB。ViewControllerB中代码：

#import "ViewControllerB.h"
 
@interface ViewControllerB ()
@end
 
@implementation ViewControllerB
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.view.backgroundColor = [UIColor blueColor];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self log];
    });
    
    [self.navigationController popViewControllerAnimated:YES];
}
- (void)log
{
    NSLog(@"Finish");
}
- (void)dealloc
{
    NSLog(@"dealloc");
}

输出结果

pop事件后，ViewControllerB关闭，但并没有立刻调用dealloc方法。而是等待倒计时结束后，log方法执行完成，然后再调用dealloc方法。

整个过程不会发生循环引用，在dispatch_after时间结束之前，block强持有self，时间结束后，block被释放，随后ViewControllerB被释放。

二：使用weakself

将代码修改为

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    NSLog(@"pop action");
    
    __weak ViewControllerB *weakself = self;
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [weakself log];
    });
    
    [self.navigationController popViewControllerAnimated:YES];
}

输出结果

在pop之后，立即调用了dealloc方法，ViewControllerB被释放。不会调用log方法。

三：使用weak-strong dance

将代码修改为

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    NSLog(@"pop action");
    
    __weak ViewControllerB *weakself = self;
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        __strong ViewControllerB *strongself = weakself;
        [strongself log];
    });
    
    [self.navigationController popViewControllerAnimated:YES];
}