dispatch_semaphore_t

API说明

1、dispatch_semaphore_create ，初始化锁

2、dispatch_semaphore_wait ，阻断，等待信号

3、dispatch_semaphore_signal ，发出信号

下面是自己的理解和例子代码

#import <Foundation/Foundation.h>
@interface NSLockTest : NSObject
- (void)forTest;
@end

#import "NSLockTest.h"
#import <pthread.h>
@interface NSLockTest()
@property (nonatomic,strong) dispatch_semaphore_t semaphore;
@end
@implementation NSLockTest

- (void)forTest
{
    // 创建信号量
    self.semaphore = dispatch_semaphore_create(2);
    
    NSThread *thread1 = [[NSThread alloc]initWithTarget:self selector:@selector(download1) object:nil];
    [thread1 start];
    
    NSThread *thread2 = [[NSThread alloc]initWithTarget:self selector:@selector(download2) object:nil];
    [thread2 start];
    
    NSThread *thread3 = [[NSThread alloc]initWithTarget:self selector:@selector(download3) object:nil];
    [thread3 start];
    
    NSThread *thread4 = [[NSThread alloc]initWithTarget:self selector:@selector(download4) object:nil];
    [thread4 start];
    
    
}
-(void)download1
{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"download1 begin");
    sleep(arc4random()%2);
    NSLog(@"download1 end");
    dispatch_semaphore_signal(self.semaphore);
    [self download1];
}

-(void)download2
{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"download2 begin");
    sleep(arc4random()%2);
    NSLog(@"download2 end");
    dispatch_semaphore_signal(self.semaphore);
    [self download2];
}

-(void)download3
{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"download3 begin");
    sleep(arc4random()%2);
    NSLog(@"download3 end");
    dispatch_semaphore_signal(self.semaphore);
    [self download3];
}

-(void)download4
{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    NSLog(@"download4 begin");
    sleep(arc4random()%2);
    NSLog(@"download4 end");
    dispatch_semaphore_signal(self.semaphore);
    [self download4];
}

@end

注意

dispatch_semaphore_t，使用过信号量来控制线程之间的同步

1、dispatch_semaphore_create(long value); 创建一个信号量，value是初始值，大体意思是同时允许多少线程运行，demo中初始值是2

2、dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout); 这时候信号量会-1，变为1

3、如果有其他线程继续运行，那么继续-1，变为0

4、此时如果还有其他线程想运行，因为此时的value变为了0，该线程就阻塞了，必须等其他进程发出信号量

5、线程运行完调用，dispatch_semaphore_signal(dispatch_semaphore_t dsema); 这时候信号量会+1，变为1.

6、这时候4中的线程得到信号通知，继续执行操作，即信号量-1，变为0.