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锁的用法可以参照上篇文章 Objective-C 加锁：看我就够了！
1、atomic & noatomic
给属性添加atomic 可以保证属性的setter和getter原子性操作，也就是保证setter和getter内部是线程同步的
- 源码 objc4 objc-accessors.mm文件
//setter方法
void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy)
{
bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
bool mutableCopy = (shouldCopy == MUTABLE_COPY);
reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}
static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
...
if (!atomic) { // 如果非原子性 直接赋值
oldValue = *slot;
*slot = newValue;
} else { //如果是原子性,加自旋锁
spinlock_t& slotlock = PropertyLocks[slot];
slotlock.lock();
oldValue = *slot;
*slot = newValue;
slotlock.unlock();
}
...
}
// getter方法
id objc_getProperty(id self, SEL _cmd, ptrdiff_t offset, BOOL atomic) {
...
if (!atomic) return *slot; //如果非原子性 直接返回
// Atomic retain release world
spinlock_t& slotlock = PropertyLocks[slot]; //如果原子性 则加自旋锁
slotlock.lock();
id value = objc_retain(*slot);
slotlock.unlock();
...
// for performance, we (safely) issue the autorelease OUTSIDE of the spinlock.
return objc_autoreleaseReturnValue(value);
}
复制代码- 从源码看 atomic是自旋锁，且只能保证在Setter和getter的时候安全，例如[NSMutableArray addobject]的时候 是不安全的
2、读写锁
为什么会有读写锁，用其他的的锁来实现不可以么？
普通锁可以达到同一时间段只有一个线程对本地文件进行读或者写。效率低。无法保证read的时候可以多线程read，而只是在write的时候 只有一个线程在write。
所以 pthread_rwlock解决这个问题。可以达到 当线程write的时候，只有一个线程写入。read的时候可以多线程read。
- API:
#import <pthread.h>
pthread_rwlock_init(&_lock, NULL);
pthread_rwlock_rdlock(&_lock); //read的时候用的锁
pthread_rwlock_wrlock(&_lock);// write的时候用的锁
pthread_rwlock_unlock(&_lock);
pthread_rwlock_tryrdlock(pthread_rwlock_t *) //尝试read锁
pthread_rwlock_trywrlock(pthread_rwlock_t *) //尝试wirte锁
复制代码- 举个?
#import <pthread.h>
- (void)viewDidLoad {
[super viewDidLoad];
pthread_rwlock_init(&_lock, NULL);
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
dispatch_queue_t queue = dispatch_queue_create("rwqueue", DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 10; i ) {
dispatch_async(queue, ^{
for (int i = 0; i < 10; i ) {
[self read];
}
});
dispatch_async(queue, ^{
for (int i = 0; i < 10; i ) {
[self read];
}
});
dispatch_async(queue, ^{
[self write];
});
}
}
- (void) read {
pthread_rwlock_rdlock(&_lock);
sleep(1);
NSLog(@"read");
pthread_rwlock_unlock(&_lock);
}
- (void) write {
pthread_rwlock_wrlock(&_lock);
sleep(1);
NSLog(@"write");
pthread_rwlock_unlock(&_lock);
}
复制代码- 运行结果及分析:
3、dispatch_barrier_async
举个?
代码语言：javascript复制- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
_queue = dispatch_queue_create("rwqueue", DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 10; i ) {
[self read];
[self write];
[self write];
[self read];
}
}
- (void) read {
dispatch_async(_queue, ^{
sleep(1);
NSLog(@"read");
});
}
- (void) write {
dispatch_barrier_async(_queue, ^{
sleep(1);
NSLog(@"write");
});
}
复制代码- 运行结果分析
- 注意点
必须传入dispatch_queue_create的并发队列 不能传入
dispatch_get_global_queue队列 如果传入串行或者是global 就相当于dispatch_async ,没有栅栏(barrier)效果 - 原理
