iOS中的几种锁

作者: 无敌大闸蟹 | 来源:发表于2018-12-13 15:06 被阅读10次

多个线程访问同一块资源的时候，很容易引发数据混乱问题所以我们就需要给我们的任务加上锁网上大多的例子都是卖票的

@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    self.count --;
    NSLog(@"%ld",(long)self.count);
}

可以看到打印的

image.png
这里就是抢占了资源当两个线程同时访问count时都为96 这里就存在数据错误和线程安全的问题了
使用线程同步技术，按照预定的先后次序依次进行，常见的线程同步技术就是加锁

锁分为互斥锁和自旋锁

大概的区别就是当B线程在访问加锁的内容时线程A也来访问的话
互斥锁：线程A会被阻塞如果线程A运行在X处理器上的话 X会被加入等待队列而X可以去处理其他的任务等B访问完了之后A继续访问
自旋锁：自旋锁一直占用CPU，他在未获得锁的情况下，一直运行，所以占用着CPU，如果不能在很短的时间内获得锁相当耗性能

所以对比下很容易得出结论如果锁的任务不怎么耗时的话选择自旋锁耗时的话选择互斥锁

然后说几种常用的锁
os_unfair_lock
os_unfair_lock是自旋锁

//初始化
os_unfair_lock lock = OS_UNFAIR_LOCK_INIT;
//加锁
os_unfair_lock_lock(&lock);
//解锁
os_unfair_lock_unlock(&lock);

需要导入#import <os/lock.h>
代码如下

#import "ViewController.h"
#import <os/lock.h>
@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@property (nonatomic, assign) os_unfair_lock lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    self.lock = OS_UNFAIR_LOCK_INIT;
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    os_unfair_lock_lock(&_lock);
    self.count --;
    NSLog(@"%ld",(long)self.count);
    os_unfair_lock_unlock(&_lock);
}

pthread_mutex 属于互斥锁

pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);

需要导入#import <pthread.h>
初始化锁是

pthread_mutex_init(mutex, &attr);

初始化锁结束以后，销毁属性

pthread_mutexattr_destroy(&attr)

加锁解锁销毁锁

pthread_mutex_lock(&_mutex);
pthread_mutex_unlock(&_mutex);
pthread_mutex_destroy(&_mutex);

代码真尼玛麻烦

#import <pthread.h>
@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@property (nonatomic, assign) pthread_mutex_t lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    pthread_mutex_init(&_lock, &attr);
    pthread_mutexattr_destroy(&attr);
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    pthread_mutex_lock(&_lock);
    self.count --;
    NSLog(@"%ld",(long)self.count);
    pthread_mutex_unlock(&_lock);
    
}

NSLock是基于pthread的封装用起来也是很方便也是最常见的锁

@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@property (nonatomic, strong) NSLock *lock;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    self.lock = [[NSLock alloc] init];
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    [self.lock lock];
    self.count --;
    NSLog(@"%ld",(long)self.count);
    [self.lock unlock];
    
}

NSRecursiveLock和NSCondition 也是基于pthread_ mutex的封装用法和NSLock一致就不写了
NSConditionLock是对NSCondition的封装可以设定锁和解锁时候的值
1、initWithCondition：初始化Condition，并且设置状态值
2、lockWhenCondition:(NSInteger)condition:当状态值为condition的时候加锁
3、unlockWithCondition:(NSInteger)condition当状态值为condition的时候解锁

dispatch_semaphoreGCD里面的信号量主要是三个方法
dispatch_semaphore_create(1) 1代表最大并发量如果信号量的值 > 0就让信号量的值减1，然后继续往下执行代码如果信号量的值 <= 0，就会休眠等待，直到信号量的值变成>0，就让信号量的值减1，然后继续往下执行代码
dispatch_semaphore_wait 等待
dispatch_semaphore_signal 让信号量+1
代码如下

@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@property (nonatomic, strong) dispatch_semaphore_t semaphore;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    self.semaphore = dispatch_semaphore_create(1);
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    self.count --;
    NSLog(@"%ld",(long)self.count);
    dispatch_semaphore_signal(self.semaphore);
}

dispatch_queue GCD里面的这里我们可以用串行队列的形式

@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    dispatch_queue_t queue = dispatch_queue_create("sold", DISPATCH_QUEUE_SERIAL);
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            dispatch_async(queue, ^{
                for (int j = 0; j < 10; j ++) {
                    [self soldTicket];
                }
            });
        });
    }
}

- (void)soldTicket
{
    self.count --;
    NSLog(@"%ld",(long)self.count);
}

@synchronized也是我们常见的锁

@interface ViewController ()

@property (nonatomic, assign) NSInteger count;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            for (int j = 0; j < 10; j ++) {
                [self soldTicket];
            }
        });
    }
}

- (void)soldTicket
{
    @synchronized([self class]){
        self.count --;
        NSLog(@"%ld",(long)self.count);
    }
}

dispatch_barrier_async栏栅块这个例子好像不太适合用这个·········
这样吧强行栏栅块一波···

- (void)viewDidLoad {
    [super viewDidLoad];
    self.count = 100;
    dispatch_queue_t queue = dispatch_queue_create("123", DISPATCH_QUEUE_CONCURRENT);
    for (int i = 0; i < 10; i ++) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            dispatch_barrier_async(queue, ^{
                for (int j = 0; j < 10; j ++) {
                    [self soldTicket];
                }
            });
        });
    }
}

- (void)soldTicket
{
    self.count --;
    NSLog(@"%ld",(long)self.count);
}

后面贴上其他地方抄过来的性能对比我也不知道真的还是假的
锁的性能比较
性能从高到低排序

os_unfair_lock
dispatch_semaphore
pthread_mutex
dispatch_queue(DISPATCH_QUEUE_SERIAL)
NSLock
NSCondition
pthread_mutex(recursive)
NSRecursiveLock
NSConditionLock
@synchronized

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