模拟dispatch_once

dispatch_once

 

dispatch_once可以保证一段代码只被执行一次，因此出现之后使用最多的场景就是实现单例。本文来模拟实现dispatch_once的功能。

模拟dispatch_once

直接上代码

static NSMutableDictionary 
*lockMapping = nil;
void do_once(int *predicate, void(^block)()){
    if(*predicate != -1){   
        NSLock *lock = nil;        
        @synchronized ([NSObject class]) { 
            NSLog(@"get lock");   
            if(lockMapping == nil){
                lockMapping = [NSMutableDictionary dictionary];
            }            
            NSString *key = [NSString stringWithFormat:@"%p", predicate];     
            if(![lockMapping objectForKey:key]){
                [lockMapping setObject:[[NSLock alloc] init] forKey:key];
            }
            
            lock = [lockMapping objectForKey:key];
        }
        
        [lock lock];        
        if(*predicate != -1){
            block();
            *predicate = -1;
        }
        
        [lock unlock];
    }
}
 

原理很简单，就不做过多解释了。使用：

 

@interface Test : NSObject
 (instancetype)shareInstance;
 @end

@implementation Test

 (instancetype)shareInstance{ 
    static int token;  
    static Test *t = nil;
    do_once(&token, ^{
        t = [[Test alloc] init];        
        NSLog(@"execute once");
    });    
    NSLog(@"execute every time");  
    return t;
}@end

测试：

int main()
{    for(int i = 0; i < 100; i++){
        dispatch_async(dispatch_queue_create(0, DISPATCH_QUEUE_CONCURRENT), ^{
            [Test shareInstance];
        });
    }    
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]];    
    return 0;
}