iOS底层系列25 -- 线程锁

• 在 iOS底层系列22 -- 多线程基础概念 这篇文章中提到线程安全问题，可以通过线程锁来解决线程安全问题，下面给出卖票与存取款这两个案例来模拟线程安全问题；
• 卖票代码如下：

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
}

• 票数初始为15张，然后开启三个子线程开始卖票；
• 存取款代码如下：

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
}

//取款
- (void)drawMoney{
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
}

• 初始化金额为100元，然后开启两个子线程，一个子线程存钱，另一个子线程取钱；
• 分别执行两个场景的代码，得到如下结果：

image.png image.png

• 下面通过线程锁来解决上述的线程安全问题；
• iOS中常见的线程锁有如下：
  • OSSpinLock
  • os_unfair_lock
  • pthread_mutex_t
  • NSLock
  • NSRecursiveLock
  • NSCondition
  • NSConditionLock
  • dispatch_queue
  • dispatch_semaphore
  • @synchronized

OSSpinLock

• OSSpinLock叫做自旋锁，等待锁的线程会处于忙等状态，一直占用着CPU资源；
• 需要导入头文件#import <libkern/OSAtomic.h>
• 利用OSSpinLock，解决线程安全的代码如下：

#import "ViewController.h"
#import <libkern/OSAtomic.h>

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,assign)OSSpinLock lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.lock = OS_SPINLOCK_INIT;
    //卖票与存取款 每次执行时，两者只运行其中一个
//    [self ticketsTest];
    [self moneyTest];
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //加锁
    OSSpinLockLock(&_lock);
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    OSSpinLockUnlock(&_lock);
}

//取款
- (void)drawMoney{
    //加锁
    OSSpinLockLock(&_lock);
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    OSSpinLockUnlock(&_lock);
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //加锁
    OSSpinLockLock(&_lock);
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    OSSpinLockUnlock(&_lock);
}

@end

• OSSpinLock目前已经不再安全，可能会出现优先级反转的问题，即如果等待锁的线程优先级比较高，那么它会一直占用着CPU资源，导致优先级低的线程无法释放锁；
• 假设现在有两条线程，分别为线程一，其优先级较高，线程二，其优先级较低，若线程二首先获取到锁，然后准备执行线程二中逻辑，此时线程一会处于循环忙等状态，但由于其优先级比较高，那么系统会分配更多的时间给线程一，其会一直占用CPU资源，导致线程二的逻辑执行不了，且无法释放锁，这就出现了线程二虽然获取到锁，却执行不了逻辑且无法释放锁
• 为了探索OSSpinLock的底层实现，将代码做如下改动：

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

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
//@property(nonatomic,assign)pthread_mutex_t lock;

@property(nonatomic,assign)OSSpinLock lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //初始化锁
    self.lock = OS_SPINLOCK_INIT;
 
    [self ticketsTest];
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    //开辟10条子线程，开始卖票
    for (int i = 0; i < 10; i++) {
        [[[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil] start];
    }
}

//卖票
- (void)saleTicket{
    //加锁
    OSSpinLockLock(&_lock);
    
    int oldTicketCount = self.ticketCount;
    sleep(600);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    OSSpinLockUnlock(&_lock);
}

@end

• 在saleTicket函数中加断点，当第一条线程执行saleTicket函数断住时，过掉断点，当第二条线程执行到saleTicket函数时断住，此时第二条线程会检测_lock锁是否被加锁，若已加过就会进入自旋忙等状态，汇编分析如下：
  Snip20210926_61.png

[图片上传中...(Snip20210926_63.png-4e7ac6-1632649343519-0)] Snip20210926_63.png

os_unfair_lock

• os_unfair_lock是用于取代不安全的OSSpinLock，从iOS10才开始支持；
• 从底层调用来看，等待os_unfair_lock锁的线程会处于休眠状态，并非忙等；
• 需要导入头文件#import <os/lock.h>
• 使用os_unfair_lock的代码如下所示：

#import "ViewController.h"
#import <os/lock.h>

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,assign)os_unfair_lock lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.lock = OS_UNFAIR_LOCK_INIT;
    //卖票与存取款 每次执行时，两者只运行其中一个
    [self ticketsTest];
//    [self moneyTest];
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //加锁
    os_unfair_lock_lock(&_lock);
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    os_unfair_lock_unlock(&_lock);
}

//取款
- (void)drawMoney{
    //加锁
    os_unfair_lock_lock(&_lock);
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    os_unfair_lock_unlock(&_lock);
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //加锁
    os_unfair_lock_lock(&_lock);
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    os_unfair_lock_unlock(&_lock);
}

@end

pthread_mutex_t

• pthread_mutex叫做互斥锁，等待锁的线程会处于休眠状态；
• 需要导入头文件#import <pthread.h>
• 使用pthread_mutex的代码如下：

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

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,assign)pthread_mutex_t lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //属性初始化
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    //锁的初始化
    pthread_mutex_init(&_lock, &attr);
    //属性销毁
    pthread_mutexattr_destroy(&attr);
    
    //卖票与存取款 每次执行时，两者只运行其中一个
//    [self ticketsTest];
    [self moneyTest];
}

//锁的销毁
- (void)dealloc{
    pthread_mutex_destroy(&_lock);
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //加锁
    pthread_mutex_lock(&_lock);
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    pthread_mutex_unlock(&_lock);
}

//取款
- (void)drawMoney{
    //加锁
    pthread_mutex_lock(&_lock);
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    pthread_mutex_unlock(&_lock);
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //加锁
    pthread_mutex_lock(&_lock);
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    pthread_mutex_unlock(&_lock);
}

@end

• pthread_mutex_t递归锁，实现同一线程可给同一把锁重复加锁

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

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,assign)pthread_mutex_t lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //属性初始化
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    //锁的初始化
    pthread_mutex_init(&_lock, &attr);
    //属性销毁
    pthread_mutexattr_destroy(&attr);
    
    [self otherTest1];
}

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

- (void)otherTest1{
    //加锁
    pthread_mutex_lock(&_lock);
    NSLog(@"%s",__func__);
    
    [self otherTest2];
    
    //解锁
    pthread_mutex_unlock(&_lock);
}

- (void)otherTest2{
    //加锁
    pthread_mutex_lock(&_lock);
    
    NSLog(@"%s",__func__);
    
    //解锁
    pthread_mutex_unlock(&_lock);
}

@end

• 出现死锁，函数otherTest1中执行加锁，然后执行到otherTest2函数，由于_lock锁已经被加过了，所以主线程会等待_lock锁而处于休眠状态，也就是说[self otherTest2]必须等待pthread_mutex_unlock(&_lock) 执行完才会执行，而pthread_mutex_unlock(&_lock)也必须等待[self otherTest2]执行完才会执行，两者相互等待，造成死锁；
• 解决方案：修改pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE) 其中参数PTHREAD_MUTEX_RECURSIVE就是所谓的递归锁
• 递归锁允许同一个线程对同一把锁进行重复加锁，注意⚠️两个同一，在otherTest1和otherTest2函数中都是主线程对_lock进行加锁，不会造成主线程的休眠阻塞，所以不会出现死锁；
• 为了探索pthread_mutex互斥锁的底层实现，将代码做如下改动：

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

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,assign)pthread_mutex_t lock;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //属性初始化
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
    //锁的初始化
    pthread_mutex_init(&_lock, &attr);
    //属性销毁
    pthread_mutexattr_destroy(&attr);
    
    //卖票与存取款 每次执行时，两者只运行其中一个
    [self ticketsTest];
}

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

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    for (int i = 0; i < 10; i++) {
        [[[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil] start];
    }
}

//卖票
- (void)saleTicket{
    //加锁
    pthread_mutex_lock(&_lock);
    
    int oldTicketCount = self.ticketCount;
    sleep(600);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    pthread_mutex_unlock(&_lock);
}

@end

• 在saleTicket函数中加断点，当第一条线程执行saleTicket函数断住时，过掉断点，当第二条线程执行到saleTicket函数时断住，此时第二条线程会检测_lock锁是否被加锁，若已加过就会进入休眠状态，汇编分析如下：
  image.png

image.png

• 会执行如下汇编：

pthread_mutex_lock_init_slow
pthread_mutex_firstfit_lock_wait
psynch_mutexwait
syscall

• 最终当前子线程会进入休眠状态；
• 也就是说互斥锁的当前子线程进入休眠状态时，最终会调用系统的syscall函数方法；
• pthread_mutex_t可实现条件锁，见如下代码：

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

@interface ViewController ()
//线程锁
@property(nonatomic,assign)pthread_mutex_t lock;
//条件
@property(nonatomic,assign)pthread_cond_t cond;
//数据源
@property(nonatomic,strong)NSMutableArray *dataArr;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //属性初始化
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
    //锁的初始化
    pthread_mutex_init(&_lock, &attr);
    //属性销毁
    pthread_mutexattr_destroy(&attr);
    //条件初始化
    pthread_cond_init(&_cond, NULL);
    
    self.dataArr = [NSMutableArray array];
    
    [self otherTest];
}

- (void)dealloc{
    pthread_mutex_destroy(&_lock);
    pthread_cond_destroy(&_cond);
}

- (void)otherTest{
    //开辟两条子线程 分别添加与删除 数组元素
    [[[NSThread alloc]initWithTarget:self selector:@selector(removeElement) object:nil] start];
    [[[NSThread alloc]initWithTarget:self selector:@selector(addElement) object:nil] start];
}

//线程1 删除元素
- (void)removeElement{
    //加锁
    pthread_mutex_lock(&_lock);
    
    if (self.dataArr.count == 0) {
        //1.线程1进入休眠
        //2.放开锁 唤醒线程2
        //3.线程1设置自己被唤醒的条件_cond
        //4.线程1被唤醒之后 会再次加锁 然后执行下面的罗
        pthread_cond_wait(&_cond, &_lock);
    }
    
    [self.dataArr removeLastObject];
    NSLog(@"删除了元素");
    
    //解锁
    pthread_mutex_unlock(&_lock);
}

//线程2 添加元素
- (void)addElement{
    //加锁
    pthread_mutex_lock(&_lock);
    
    [self.dataArr addObject:@"test"];
    NSLog(@"添加了元素");
    //唤醒线程1
    pthread_cond_signal(&_cond);
    NSLog(@"-----");
    //解锁
    pthread_mutex_unlock(&_lock);
}
@end

• 若线程1删除元素首先获取到锁_lock，那么线程2添加元素会阻塞进入休眠状态，然后线程1发现数组元素为空，执行pthread_cond_wait(&_cond, &_lock)，此行代码包含四层含义：
  • 1.线程1阻塞进入休眠状态；
  • 2.线程1释放锁_lock，那么线程二获取到锁(加锁)，然后开始执行自己的逻辑；
  • 3.线程1会设置自己被唤醒重新执行的条件_cond；
  • 4.当线程1被唤醒时，会重新再次加锁；
• 线程2获取到锁(加锁)，然后执行添加元素，然后执行pthread_cond_signal(&_cond)发信号给线程1，接着往下执行释放锁，然后线程1收到信号，解除休眠，并再次加锁，然后执行删除元素，最后解锁；
• 上述的条件锁的执行逻辑，实现了两个子线程之间的相互通信以及线程之间的依赖逻辑

NSLock

• NSLock是对pthread_mutex_t普通锁的封装；
  image.png

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,strong)NSLock *lock;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.lock = [[NSLock alloc]init];
    
    //卖票与存取款 每次执行时，两者只运行其中一个
    [self ticketsTest];
//    [self moneyTest];
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //加锁
    [self.lock lock];
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    [self.lock unlock];
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //加锁
    [self.lock lock];
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    [self.lock unlock];
}

//取款
- (void)drawMoney{
    //加锁
    [self.lock lock];
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    [self.lock unlock];
}
@end

NSRecursiveLock

• NSRecursiveLock是对pthread_mutex_t递归锁的封装；
  image.png

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,strong)NSRecursiveLock *lock;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.lock = [[NSRecursiveLock alloc]init];
    
    //卖票与存取款 每次执行时，两者只运行其中一个
    [self ticketsTest];
//    [self moneyTest];
}


//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //加锁
    [self.lock lock];
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    
    //解锁
    [self.lock unlock];
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //加锁
    [self.lock lock];
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    [self.lock unlock];
}

//取款
- (void)drawMoney{
    //加锁
    [self.lock lock];
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    //解锁
    [self.lock unlock];
}
@end

NSCondition

• NSCondition是对pthread_mutex_t和pthread_cond_t的封装；
  image.png

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,strong)NSCondition *lock;
//数据源
@property(nonatomic,strong)NSMutableArray *dataArr;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.lock = [[NSCondition alloc]init];
    
    self.dataArr = [[NSMutableArray array]init];
    [self otherTest];
}

- (void)otherTest{
    //开辟两条子线程 分别添加与删除 数组元素
    [[[NSThread alloc]initWithTarget:self selector:@selector(removeElement) object:nil] start];
    [[[NSThread alloc]initWithTarget:self selector:@selector(addElement) object:nil] start];
}

//线程1 删除元素
- (void)removeElement{
    //加锁
    [self.lock lock];

    if (self.dataArr.count == 0) {
        //1.线程1进入休眠
        //2.放开锁 唤醒线程2
        //3.线程1设置自己被唤醒的条件_cond
        //4.线程1被唤醒之后 会再次加锁 然后执行下面的罗
        [self.lock wait];
    }

    [self.dataArr removeLastObject];
    NSLog(@"删除了元素");

    //解锁
    [self.lock unlock];;
}

//线程2 添加元素
- (void)addElement{
    //加锁
    [self.lock lock];

    [self.dataArr addObject:@"test"];
    NSLog(@"添加了元素");
    //唤醒线程1
    [self.lock signal];
    
    //解锁
    [self.lock unlock];
}
@end

NSConditionLock

• NSConditionLock是对NSCondition的进一步封装，其可以设置具体的条件值；
  image.png

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//线程锁
@property(nonatomic,strong)NSConditionLock *lock;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.lock = [[NSConditionLock alloc]initWithCondition:1];
}
    
- (void)otherTest{
    //开辟两条子线程 分别添加与删除 数组元素
    [[[NSThread alloc]initWithTarget:self selector:@selector(one) object:nil] start];
    [[[NSThread alloc]initWithTarget:self selector:@selector(two) object:nil] start];
    [[[NSThread alloc]initWithTarget:self selector:@selector(three) object:nil] start];
}

//线程1
- (void)one{
    //加锁
    [self.lock lockWhenCondition:1];
    
    NSLog(@"one");

    //1.解锁
    //2.将锁内部的condition条件值置为2
    [self.lock unlockWithCondition:2];
}

//线程2
- (void)two{
    //加锁
    [self.lock lockWhenCondition:2];
    
    NSLog(@"two");
    
    //1.解锁
    //2.将锁内部的condition条件值置为3
    [self.lock unlockWithCondition:3];
}

//线程3
- (void)three{
    //加锁
    [self.lock lockWhenCondition:3];
    
    NSLog(@"three");
    
    //解锁
    [self.lock unlock];
}

@end

• NSConditionLock初始化的时候，内部的条件值设置为1，也就是说当条件为1时，子线程才会获取锁，执行子线程的内部逻辑；
• [self.lock unlockWithCondition:2]，释放锁，且将锁内部的条件值设置为2；
• NSConditionLock实现了3条子线程的顺序执行；

dispatch_queue

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//串行队列
@property(nonatomic,strong)dispatch_queue_t queue;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //串行队列
    self.queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
    
    //卖票与存取款 每次执行时，两者只运行其中一个
//    [self ticketsTest];
    [self moneyTest];
}

//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    dispatch_sync(self.queue, ^{
        int oldTicketCount = self.ticketCount;
        sleep(0.2);
        oldTicketCount--;
        self.ticketCount = oldTicketCount;
        NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    });
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    dispatch_sync(self.queue, ^{
        int oldMoney = self.money;
        sleep(0.2);
        oldMoney+=50;
        self.money = oldMoney;
        NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    });
}

//取款
- (void)drawMoney{
    dispatch_sync(self.queue, ^{
        int oldMoney = self.money;
        sleep(0.2);
        oldMoney-=30;
        self.money = oldMoney;
        NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    });
}
@end

• 将所有的任务全部放入串行队列中，然后任务依次执行；

dispatch_semaphore

• dispatch_semaphore是信号量，其可以控制多线程的并发量；

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
//信号量
@property(nonatomic,strong)dispatch_semaphore_t semaphore;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.lock = [[NSConditionLock alloc]initWithCondition:1];
    //串行队列
    self.queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
    
    self.semaphore = dispatch_semaphore_create(1);
    
    //卖票与存取款 每次执行时，两者只运行其中一个
//    [self ticketsTest];
    [self moneyTest];
}


//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    //信号量值-1
    //若信号量值>0,继续执行
    //若信号量值<=0,当前子线程阻塞休眠
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    
    int oldTicketCount = self.ticketCount;
    sleep(0.2);
    oldTicketCount--;
    self.ticketCount = oldTicketCount;
    NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    //信号量值+1
    dispatch_semaphore_signal(self.semaphore);
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    //信号量值-1
    //若信号量值>0,继续执行
    //若信号量值<=0,当前子线程阻塞休眠
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney+=50;
    self.money = oldMoney;
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    
    //信号量值+1
    dispatch_semaphore_signal(self.semaphore);
}

//取款
- (void)drawMoney{
    //信号量值-1
    //若信号量值>0,继续执行
    //若信号量值<=0,当前子线程阻塞休眠
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    
    int oldMoney = self.money;
    sleep(0.2);
    oldMoney-=30;
    self.money = oldMoney;
    NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);

    //信号量值+1
    dispatch_semaphore_signal(self.semaphore);
}
@end

• dispatch_semaphore_t信号量的初始值为1，保证同一时间只有一条线程在执行任务，就能保证线程安全问题；

@synchronized

• @synchronized是对pthread_mutex_t递归锁的封装，支持递归锁；

#import "ViewController.h"

@interface ViewController ()
//金额
@property(nonatomic,assign)int money;
//票数
@property(nonatomic,assign)int ticketCount;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //串行队列
    self.queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
    
    self.semaphore = dispatch_semaphore_create(1);
    
    //卖票与存取款 每次执行时，两者只运行其中一个
    [self ticketsTest];
//    [self moneyTest];
}


//卖票测试
- (void)ticketsTest{
    self.ticketCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });

    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}

//卖票
- (void)saleTicket{
    @synchronized (self) {
        int oldTicketCount = self.ticketCount;
        sleep(0.2);
        oldTicketCount--;
        self.ticketCount = oldTicketCount;
        NSLog(@"还剩%d张票 -- %@",oldTicketCount,[NSThread currentThread]);
    }
}

//存取款测试
- (void)moneyTest{
    self.money = 100;
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self drawMoney];
        }
    });
}

//存款
- (void)saveMoney{
    @synchronized (self) {
        int oldMoney = self.money;
        sleep(0.2);
        oldMoney+=50;
        self.money = oldMoney;
        NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    }
}

//取款
- (void)drawMoney{
    @synchronized (self) {
        int oldMoney = self.money;
        sleep(0.2);
        oldMoney-=30;
        self.money = oldMoney;
        NSLog(@"还剩%d元 -- %@",oldMoney,[NSThread currentThread]);
    }
}
@end

• 参数self与锁对象是以key-value的形式存储在底层的哈希表中；

线程锁的性能比较

• 性能从高到低的顺序如下所示：
  • os_unfair_lock
  • OSSpinLock
  • dispatch_semaphore
  • pthread_mutex_t
  • dispatch_queue(串行队列)
  • NSLock
  • NSCondition
  • NSRecursiveLock
  • NSConditionLock
  • @synchronized