iOS多线程同步之NSLock实战例子

场景：用户账户初始有1000元钱，开10个线程同时进行存钱和取钱操作，10个线程同时执行，其中5个线程负责存钱操作，5个线程负责取钱操作。
(1) 5个存钱线程，每个线程每隔0.1毫秒存入100元，执行10W次；
(1) 5个取钱线程，每个线程每隔0.1毫秒取出100元，执行10W次；
可以预期的结果是：存钱线程和取钱线程存入和取出刚好相互抵消，当所有线程执行完的时候，用户账号余额仍然是1000元。
上代码，账户类Accounter.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN
/**
 账户类
 */
@interface Accounter : NSObject

@property (nonatomic, assign) int balance;  // 余额

@end

NS_ASSUME_NONNULL_END

账户类Accounter.m

#import "Accounter.h"

@implementation Accounter

@end

ViewController.m
ViewController.m开5个线程存钱，5个线程取钱。当线程全部执行完毕的时候，点击视图查看账户余额。

#import "ViewController.h"
#import "Accounter.h"         // 引入账户类

@interface ViewController ()

@property (nonatomic, strong) Accounter *accounter;  // 账户

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.accounter = [[Accounter alloc] init];
    _accounter.balance = 1000;  // 账户初始余额1000
    [self testMultiThread];     // 多线程存钱/取钱
}

// 存钱线程函数
- (void)saveThreadFunc:(id)obj {
    for (int i = 0; i < 100000; i++) {
        _accounter.balance = _accounter.balance + 100;
        [NSThread sleepForTimeInterval:0.0001];
    }
    
    NSLog(@"存钱线程[%@] 执行结束", obj);
}

// 取钱线程函数
- (void)withdrawThreadFunc:(id)obj {
    for (int i = 0; i < 100000; i++) {
        _accounter.balance = _accounter.balance - 100;
        [NSThread sleepForTimeInterval:0.0001];
    }
    
    NSLog(@"取钱线程[%@] 执行结束", obj);
}

// 多线程存钱/取钱
- (void)testMultiThread {
    for (int i = 0; i < 10; i++) {
        NSNumber *number = [NSNumber numberWithInt:i]; // 线程编号
        
        if (i % 2 == 0) {
            [NSThread detachNewThreadSelector:@selector(saveThreadFunc:) toTarget:self withObject:number];
        } else {
            [NSThread detachNewThreadSelector:@selector(withdrawThreadFunc:) toTarget:self withObject:number];
        }
    }
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSLog(@"账户余额 = %d", _accounter.balance);
}

@end

打印输出：

2020-04-05 12:25:58.269626+0800 MultiThreadLockDemo[2394:47732] 存钱线程[6] 执行结束
2020-04-05 12:25:58.269632+0800 MultiThreadLockDemo[2394:47733] 取钱线程[7] 执行结束
2020-04-05 12:25:58.271439+0800 MultiThreadLockDemo[2394:47727] 取钱线程[1] 执行结束
2020-04-05 12:25:58.274413+0800 MultiThreadLockDemo[2394:47729] 取钱线程[3] 执行结束
2020-04-05 12:25:58.274990+0800 MultiThreadLockDemo[2394:47730] 存钱线程[4] 执行结束
2020-04-05 12:25:58.275353+0800 MultiThreadLockDemo[2394:47734] 存钱线程[8] 执行结束
2020-04-05 12:25:58.276267+0800 MultiThreadLockDemo[2394:47726] 存钱线程[0] 执行结束
2020-04-05 12:25:58.277008+0800 MultiThreadLockDemo[2394:47731] 取钱线程[5] 执行结束
2020-04-05 12:25:58.278642+0800 MultiThreadLockDemo[2394:47735] 取钱线程[9] 执行结束
2020-04-05 12:25:58.282194+0800 MultiThreadLockDemo[2394:47728] 存钱线程[2] 执行结束
2020-04-05 12:27:39.032068+0800 MultiThreadLockDemo[2394:47529] 账户余额 = -13300

余额竟然是-13300？
问题在于这两个语句
(1)_accounter.balance = _accounter.balance + 100;
(2)_accounter.balance = _accounter.balance - 100;
当一个线程取得_accounter.balance的值，准备进行加减操作的时候，_accounter.balance实际是已经被别的线程赋上新的值了，而该线程拿到了旧的值。
那么如何保证线程操作_accounter.balance的时候，其他线程不能操作。除了@synchronized之外，咱么可以使用NSLock加锁
重新修改一下ViewController.m，加入锁操作

#import "ViewController.h"
#import "Accounter.h"         // 引入账户类

@interface ViewController ()

@property (nonatomic, strong) Accounter *accounter;  // 账户
@property (nonatomic, strong) NSLock *accounterLock; // 账户锁

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.accounter = [[Accounter alloc] init];
    _accounter.balance = 1000;               // 账户初始余额1000
    _accounterLock = [[NSLock alloc] init];  // 账户锁
    [self testMultiThread];                  // 多线程存钱/取钱
}

// 存钱线程函数
- (void)saveThreadFunc:(id)obj {
    for (int i = 0; i < 100000; i++) {
        [_accounterLock lock];
        _accounter.balance = _accounter.balance + 100;
        [_accounterLock unlock];
        [NSThread sleepForTimeInterval:0.0001];
    }
    
    NSLog(@"存钱线程[%@] 执行结束", obj);
}

// 取钱线程函数
- (void)withdrawThreadFunc:(id)obj {
    for (int i = 0; i < 100000; i++) {
        [_accounterLock lock];
        _accounter.balance = _accounter.balance - 100;
        [_accounterLock unlock];
        [NSThread sleepForTimeInterval:0.0001];
    }
    
    NSLog(@"取钱线程[%@] 执行结束", obj);
}

// 多线程存钱/取钱
- (void)testMultiThread {
    for (int i = 0; i < 10; i++) {
        NSNumber *number = [NSNumber numberWithInt:i]; // 线程编号
        
        if (i % 2 == 0) {
            [NSThread detachNewThreadSelector:@selector(saveThreadFunc:) toTarget:self withObject:number];
        } else {
            [NSThread detachNewThreadSelector:@selector(withdrawThreadFunc:) toTarget:self withObject:number];
        }
    }
}

- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSLog(@"账户余额 = %d", _accounter.balance);
}

@end

打印输出：

2020-04-05 12:41:40.592584+0800 MultiThreadLockDemo[2643:55872] 存钱线程[8] 执行结束
2020-04-05 12:41:40.592859+0800 MultiThreadLockDemo[2643:55869] 取钱线程[5] 执行结束
2020-04-05 12:41:40.593439+0800 MultiThreadLockDemo[2643:55865] 取钱线程[1] 执行结束
2020-04-05 12:41:40.595160+0800 MultiThreadLockDemo[2643:55868] 存钱线程[4] 执行结束
2020-04-05 12:41:40.595194+0800 MultiThreadLockDemo[2643:55873] 取钱线程[9] 执行结束
2020-04-05 12:41:40.597746+0800 MultiThreadLockDemo[2643:55866] 存钱线程[2] 执行结束
2020-04-05 12:41:40.597930+0800 MultiThreadLockDemo[2643:55871] 取钱线程[7] 执行结束
2020-04-05 12:41:40.600022+0800 MultiThreadLockDemo[2643:55870] 存钱线程[6] 执行结束
2020-04-05 12:41:40.600148+0800 MultiThreadLockDemo[2643:55867] 取钱线程[3] 执行结束
2020-04-05 12:41:40.611717+0800 MultiThreadLockDemo[2643:55864] 存钱线程[0] 执行结束
2020-04-05 12:42:21.068827+0800 MultiThreadLockDemo[2643:55634] 账户余额 = 1000

OK，账户余额 = 1000