iOS 锁

锁被使用在多线程中，避免多个线程对同一公共资源的操作

测试对象

LockObj

• LockObj.h

@interface LockObj : NSObject

@property (nonatomic, strong) NSMutableArray *array;
@property (nonatomic, strong) NSLock *lock;
@property (nonatomic, assign) int num3;

+ (int)num1;
+ (int)num2;
+ (int)num4;

@end

• LockObj.m

static int _kNum1 = 0;
static int _kNum2 = 0;
static int _kNum4 = 0;
static NSLock *_klock = nil;

@implementation LockObj

+ (void)load{
    _klock = [[NSLock alloc] init];
}

- (instancetype)init{
    self = [super init];
    if (self) {
        self.array = [[NSMutableArray alloc] init];
        self.lock = [[NSLock alloc] init];
        self.num3 = 0;
    }
    return self;
}

+ (int)num1{
    return _kNum1;
}
+ (int)num2{
    return _kNum2;
}
+ (int)num4{
    return _kNum4;
}

num3是对象的属性，_kNum1,_kNum2分别是两个全局静态变量

测试函数

• 循环函数

static void _cycleDo(dispatch_block_t block){
    for (int i = 0; i < 3000; i++) {
        if(block) {block();}
    }
}

• 多线程函数

static void _asyncDo(dispatch_block_t block, dispatch_block_t last) {
    __block UInt16 finish = 0x0000;
    dispatch_async(dispatch_queue_create("1", DISPATCH_QUEUE_CONCURRENT), ^{
        _cycleDo(block);
      `if(block) {block()
        finish = finish | 0x001;
    });
    dispatch_async(dispatch_queue_create("2", DISPATCH_QUEUE_SERIAL), ^{
        _cycleDo(block);
        finish = finish | 0x010;
    });
    _cycleDo(block);
    finish = finish | 0x100;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10*NSEC_PER_MSEC), dispatch_get_main_queue(), ^{
        NSLog(@"finish:%x", finish);
        if(last) {last();}
    });
}

NSLock

一个互斥锁，不能被递归调用，性能优秀

lock & unlock

同一个对象，不加锁的多线程访问全局静态变量

// LockObj
- (void)unlockNum2{
    _kNum2+=1;
}

void lockCommonTest2() {
    LockObj *obj = [[LockObj alloc] init];
    _asyncDo(^{
        [obj unlockNum2];
    }, ^{
        NSLog(@"num2:%d", LockObj.num2);
    });
}

• 日志输出

 // 1
 Hello, World!
 finish:111
 num2:9000
 // 2
 Hello, World!
 finish:111
 num2:8331

• 结论
  • 多线程对全局静态变量的修改不能保证正确性

同一个对象，锁使用对象的lock访问全局静态变量

// LockObj
- (void)lockNum3 {
    [self.lock lock];
    self.num3+=1;
    [self.lock unlock];
}

void lockNum3ParamLock() {
    LockObj *obj = [[LockObj alloc] init];
    _asyncDo(^{
        [obj lockNum3];
    }, ^{
        NSLog(@"num3:%d", obj.num3);
    });
}

• 日志输出

 // 多次输出相同
 Hello, World!
 finish:111
 num3:9000

• 结论
  • 对象的属性的锁，成功多对象的其他属性就行锁，保证了其他属性的正确性

多个对象，锁使用对象的lock访问属性变量

// LockObj
- (void)unlockNum3 {
    self.num3+=1;
}

void lockMulpNum3ParamLock() {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 lockNum3];
        [obj2 unlockNum3];
        [obj3 lockNum3];
    }, ^{
        NSLog(@"num3:%d, num3:%d, num3:%d", obj1.num3, obj2.num3, obj3.num3);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num3:9000, num3:8963, num3:9000

结论

• obj1和obj3的锁没有影响到obj2的属性修改，obj2属性的num3修改不能保证正确性

多个对象，锁使用对象的lock访问全局静态变量

// LockObj
- (void)lockNum1{
    [self.lock lock];
    _kNum1+=1;
    [self.lock unlock];
}

void lockMulpNum1ParamLock() {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 lockNum1];
        [obj2 lockNum1];
        [obj3 lockNum1];
    }, ^{
        NSLog(@"num1:%d", LockObj.num1);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num1:26566

结论

• 多个NSLock锁之间并不会互斥，既obj1的锁只会影响obj1对_kNum1的操作，不影响obj2，ob3对_kNum1的操作

多个对象使用全局锁访问全局静态变量

// LockObj
- (void)klockNum1 {
    [_klock lock];
    _kNum1+=1;
    [_klock unlock];
}

void lockMulpNum1kLock() {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 klockNum1];
        [obj2 klockNum1];
        [obj3 klockNum1];
    }, ^{
        NSLog(@"num1:%d", LockObj.num1);
    });
}

• 日志输出

 // 多次输出相同
 Hello, World!
 finish:111
 num1:27000

• 结论
  • 全局静态变量的多线程访问，需要使用全局的NSLock锁才能保证操作的正确性

临界区

// LockObj
- (void)klockNum124{
    _kNum1+=1;
    [_klock lock];
    _kNum2+=1;
    [_klock unlock];
    _kNum4+=1;
}

• 日志输出

 Hello, World!
 finish:111
 num1:26910, num2:27000, num4:26949
 Hello, World!
 finish:111
 num1:26910, num2:27000, num4:26825
 Hello, World!
 finish:111
 num1:26902, num2:27000, num4:26924

结论

• 只有在lock和unlock之间的代码才会被限制多线程访问

tryLock

tryLock 会加锁，在没有获得锁的时候可以做其他事。

// LockObj
- (void)trylockNum12{
    if ([_klock tryLock]) {
        _kNum1+=1;
        [_klock unlock];
    }else {
        [self.lock lock];
        _kNum2+=1;
        [self.lock lock];
    }
}

void lockNum123trylock() {
    LockObj *obj = [[LockObj alloc] init];
    _asyncDo(^{
        [obj trylockNum12];
    }, ^{
        NSLog(@"num1:%d, num2:%d", LockObj.num1, LockObj.num2);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num1:7700, num2:1300

• 结论
  • 👍

@synchronized

@synchronized长得挺别致，到底是个什么东西？？

@synchronized (<#token#>) {
    <#statements#>
}

两个参数，token更像是一种标识，并且在测试过程中，输出的日志也确实符合这个规律。不考虑在递归调用上的问题，更像是这种

static NSMutableDictionary *dic;
NSLock *lock = [dic objectForKey:token];
if (lock) {
    [lock lock];
    //do something
    [lock unlock];
}

仿照之前的测试代码，开始准备测试

token使用一个self来对自身属性做改变

// LockObj
- (void)syncLockNum3 {
    @synchronized (self) {
        self.num3+=1;
    }
}

void syncLockMulpNum3ParamLock (void) {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 syncLockNum3];
        [obj2 unlockNum3];
        [obj3 syncLockNum3];
    }, ^{
        NSLog(@"num3:%d, num3:%d, num3:%d", obj1.num3, obj2.num3, obj3.num3);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num3:9000, num3:8980, num3:9000

 Hello, World!
 finish:111
 num3:9000, num3:8974, num3:9000

• 结论
  • @synchronized很好的避免了多线程对num3修改

token使用一个self来对全局静态变量做改变

// LockObj
- (void)syncLockNum1 {
    @synchronized (self) {
        _kNum1 +=1;
    }
}

void syncLockMulpNum1ParamLock () {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 syncLockNum1];
        [obj2 syncLockNum1];
        [obj3 syncLockNum1];
    }, ^{
        NSLog(@"num1:%d", LockObj.num1);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num1:25618

• 结论
  • @synchronized并没有防止多线程对_kNum1修改

token使用一个字符串来对全局静态变量做改变

// LockObj
- (void)ksyncLockNum124 {
    @synchronized (@"num1") {
        _kNum1+=1;
    }
    @synchronized (@"num2") {
        _kNum2+=1;
    }
    _kNum4 += 1;
}

void syncLockMulpNum124Lock () {
    LockObj *obj1 = [[LockObj alloc] init];
    LockObj *obj2 = [[LockObj alloc] init];
    LockObj *obj3 = [[LockObj alloc] init];
    _asyncDo(^{
        [obj1 ksyncLockNum124];
        [obj2 ksyncLockNum124];
        [obj3 ksyncLockNum124];
    }, ^{
        NSLog(@"num1:%d, num2:%d, num4:%d", LockObj.num1, LockObj.num2, LockObj.num4);
    });
}

• 日志输出

 Hello, World!
 finish:111
 num1:27000, num2:27000, num4:26738
   
 Hello, World!
 finish:111
 num1:27000, num2:27000, num4:26724

• @synchronized的字符串token很好的防止多线程对_kNum1、_kNum2修改

递归锁

不加锁

// LockObj
- (void)arrAdd{
    [self.array addObject:[NSString stringWithFormat:@"%d", arc4random()%1000000]];
}
- (void)arrEnum{
    for (NSString *str in self.array) {
        [str stringByAppendingString:@"***"];
    }
}

void lockArrayUnlock() {
    LockObj *obj = [[LockObj alloc] init];
    [obj arrAdd];
    _asyncDo(^{
        [obj arrAdd];
        [obj arrEnum];
    }, nil);
}

• 日志输出

*** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x100606410> was mutated while being enumerated.'

加锁

// LockObj
- (void)syncLockArrAdd{
    @synchronized (self) {
        [self.array addObject:[NSString stringWithFormat:@"%d", arc4random()%1000000]];
    }
}
- (void)syncLockArrEnum{
    @synchronized (self) {
        for (NSString *str in self.array) {
            [str stringByAppendingString:@"***"];
        }
    }
}

void lockArraySyncRecLock(void) {
    LockObj *obj = [[LockObj alloc] init];
    [obj arrAdd];
    _asyncDo(^{
        [obj syncLockArrAdd];
        [obj syncLockArrEnum];
    }, nil);
}

• 日志输出

 Hello, World!
 finish:111
   
 Hello, World!
 finish:110 

• 结论

可变数组，如果涉及到多线程操作，加锁