思考如何实现以下场景

 同一时间，只能有一个线程进行写操作
 同一时间，允许有多个线程读操作
 同一时间，不允许既有写的操作又有读的操作

1.1代码实现一 dispatch_barrier_async

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_queue_create("rwQueue", DISPATCH_QUEUE_CONCURRENT);
    for (int i=0; i<=10; i++) {
        dispatch_async(queue, ^{
            [self read];
        });
        
        dispatch_async(queue, ^{
            [self read];
        });
        
        dispatch_async(queue, ^{
            [self read];
        });
        //这个函数传入的并发队列必须是通过dispatch_queue_create创建的
       //这个传入的是一个串行或是一个全局的并发队列，那么这个函数就等于dispatch_aync函数的效果
        dispatch_barrier_async(queue, ^{
            [self write];
        });
    }
}

- (void)read {
    sleep(1);
    NSLog(@"read");
}

- (void)write {
    sleep(1);
    NSLog(@"write");
}

@end

1.2 原理

dispatch_barrier_async(queue, ^{
[self write];
});

dispatch_barrier_sync: Submits a barrier block object for execution and waits until that block completes.(提交一个栅栏函数在执行中,它会等待栅栏函数执行完)

dispatch_barrier_async: Submits a barrier block for asynchronous execution and returns immediately.(提交一个栅栏函数在异步执行中,它会立马返回)

作者理解:dispatch_barrier_sync 需要等待栅栏执行完才会执行栅栏后面的任务,而dispatch_barrier_async 无需等待栅栏执行完,会继续往下走(保留在队列里)

原因:在同步栅栏时栅栏函数在主线程中执行,而异步栅栏中开辟了子线程栅栏函数在子线程中执行

2.1代码实现 pthread_rwlock_t

#import "ViewController.h"
#import <pthread/pthread.h>

@interface ViewController ()
@property(nonatomic, assign) pthread_rwlock_t lock;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
   
    //初始化锁
    pthread_rwlock_init(&_lock, NULL);
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (int i=0; i<10; i++) {
        dispatch_async(queue, ^{
            [self read];
        });
        dispatch_async(queue, ^{
            [self write];
        });
    }
}

- (void)read {
    pthread_rwlock_rdlock(&_lock);
    sleep(1);
    NSLog(@"read");
    pthread_rwlock_unlock(&_lock);
}

- (void)write {
    pthread_rwlock_wrlock(&_lock);
    sleep(1);
    NSLog(@"write");
    pthread_rwlock_unlock(&_lock);
}

- (void)dealloc
{
    pthread_rwlock_destroy(&_lock);
}

@end

2.2原理：

//初始化锁
pthread_rwlock_t lock;
pthread_rwlock_init(&lock, NULL);
//读加锁
pthread_rwlock_rdlock(&lock);
//读尝试加锁
pthread_rwlock_tryrdlock(&lock);
//写加锁
pthread_rwlock_wrlock(&lock);
//写尝试加锁
pthread_rwlock_trywrlock(&lock);
//解锁
pthread_rwlock_unlock(&lock);
//销毁
pthread_rwlock_destroy(&lock);