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 != -){
         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 != -){
             block();
             *predicate = -;
         }

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