线程安全

当多个线程同时访问同一块资源时,会造成资源的读写错误,意思就是同一个文件,同时有人在读,又有人在写入,肯定是要出错的,那么这就涉及到线程安全,线程安全我们使用互斥锁来防止,代码如下

#import "ViewController.h"

@interface ViewController ()
/** A */
@property (nonatomic, strong) NSThread *threadA;
/** B */
@property (nonatomic, strong) NSThread *threadB;
/** C */
@property (nonatomic, strong) NSThread *threadC;

@property (nonatomic, assign) NSInteger totalCount;
@end

@implementation ViewController

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    
    //设置中票数
    self.totalCount = 100;
    
     self.threadA = [[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
     self.threadB = [[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
     self.threadC = [[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
    
    self.threadA.name = @"A";
    self.threadB.name = @"B";
    self.threadC.name = @"C";
    
    //启动线程
    @synchronized(self) {
        [self.threadA start];
        [self.threadB start];
        [self.threadC start];
    }
    
}

-(void)saleTicket
{
    while (1) {
        
    //锁:必须是全局唯一的
    //1.注意枷锁的位置
    //2.注意枷锁的前提条件,多线程共享同一块资源
    //3.注意加锁是需要代价的,需要耗费性能的
    //4.加锁的结果:线程同步
        
    @synchronized(self) {
        //线程1
        //线程2
        //线程3
        NSInteger count = self.totalCount;
        if (count >0) {
            
            for (NSInteger i = 0; i<1000000; i++) {
            }
            
            self.totalCount = count - 1;
            //卖出去一张票
            NSLog(@"%@卖出去了一张票,还剩下%zd张票", [NSThread currentThread].name,self.totalCount);
        }else
        {
            NSLog(@"不要回公司上班了");
            break;
        }
        }
    }
    
}

@end

• 我们在创建单列时有时候为了考虑线程安全的问题,也会使用互斥锁

static PayManager *_instance = nil;


/**
 * alloc 方法内部会调用这个方法
 */
+(id)allocWithZone:(struct _NSZone *)zone{
    if (_instance == nil) {//防止频繁加锁
        @synchronized(self) {
            if (_instance == nil) {//防止创建多次
                _instance = [super allocWithZone:zone];
            }
        }
    }
    return _instance;
}

/**
 * 实例化类
 */
+(instancetype)sharedMusicTool{
    if (_instance == nil) {//防止频繁枷锁
        @synchronized(self) {
            if (_instance == nil) {//防止创建多次
                _instance = [[self alloc] init ];
            }
        }
    }
    return _instance;
}

/**
 * 重写copy方法，保证copy出来的也是单例类
 */
-(id)copyWithZone:(NSZone *)zone{
    return _instance;
}