GCD實現多讀單寫

#import "WHObject.h"

@interface WHObject()

@property (nonatomic, strong) dispatch_queue_t dictQueue;//並發隊列
@property (nonatomic, strong) NSMutableDictionary *dict;//可變字典

@end

@implementation WHObject

- (instancetype)init {
    if(self = [super init]) {
        _dictQueue = dispatch_queue_create("com.huangwenhong.queue", DISPATCH_QUEUE_CONCURRENT);
        _dict = [NSMutableDictionary dictionary];
    }
    return self;
}

- (void)huangwenhongmethod {
    self.target = [NSObject new];
}

- (id)valueForKey:(NSString *)key {
    id __block obj;
    dispatch_sync(_dictQueue, ^{//因為有數據 return，所以這里是同步調用
        obj = [self.dict valueForKey:key];
    });
    return obj;
}

- (void)setObject:(id)obj forKey:(id<NSCopying>)key {
    //重點：dispatch_barrier_async()，異步柵欄調用；
    //只有提交到並行隊列才有意義
    dispatch_barrier_async(_dictQueue, ^{
        [self.dict setObject:obj forKey:key];
    });
}
@end

總結：首先棧欄函數的作用，是執行到當前位置，前面的全部任務都要等待，等待block內部任務執行完成后，繼續執行其他任務。（面試時候的問題，用鎖不知道能不能實現）