发生读取时，Objective C 锁定写入操作

Question

我有一个 ObjC class (Writer)，它有一些属性，它在主线程上写入这些属性。然后还有其他class（Reader）在主线程或后台线程上读取这些属性。

当reader从主线程读取时，它获得了 Writer class 中这些属性的确切值，但是当reader从后台线程读取时，显然没有保证reader 将获得相同的值。这是示例代码。

这里是 Writer class

numberString 都是用逗号分隔的，例如。 1,1,1,1
numberSum 是这些的总和，例如。 4

在Reader class:

我想确保 numberSum 确实是 numberString 变量。

// Writer Class
@interface Writer : NSObject
@property (nonatomic,strong) NSString* numberString;
@property (nonatomic,assign) NSUInteger numberSum;
@end

@implementation Writer

-(instancetype)init {
    if (self = [super init]){
        self.numberString = @"1";
        self.numberSum = 1;

        self.timer = [NSTimer timerWithTimeInterval:0.1f target:self selector:@selector(writeToVariables:) userInfo:nil repeats:YES];
        [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];
    }

    return self;
}

- (void)writeToVariables:(NSTimer *)timer {
    self.numberString = [self.numberString stringByAppendingString:@",1"];    
    self.numberSum += 1;
}

// Reader Class
@interface Reader ()
@property (nonatomic,strong) Writer* writer;
@end

@implementation Reader

-(instancetype)init {
        if (self = [super init]){
            self.writer = [Writer new];
        }
        return self;
    }

-(void) getVariablesOnBackground:(BOOL)background {

    void (^readerBlock)() = ^{

        NSString* numberStr = self.writer.numberString;

        // Introduce delay So background thread get time to update numberSum
        int i=0;
        while (i < 10000000) {
            i += 1;
        }

        NSUInteger numberSum = self.writer.numberSum;

        NSArray *onesArray = [numberStr componentsSeparatedByString:@","];
        NSLog(@"%lu",(unsigned long)onesArray.count);
        NSLog(@"%lu",(unsigned long)numberSum);
    };

    // If background then dispatch on default queue else just call the block
    if (background) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),readerBlock);
    }
    else {
        readerBlock();
    }
}
@end

这是演示代码，仅供示例使用。 reader 块中的两个 NSLog 语句在主线程中执行时将始终打印相同的内容，但在后台线程中它们有时会有所不同，因为在读取 numberString 后在 Reader 中读取 numberSum 时它有已由 Writer 中的主线程更新。

无论调用线程如何，我应该怎么做才能确保始终获得正确的值？

Answer 1

您需要的是并发调度队列和调度屏障功能的组合。只要您需要同时访问 numberString 和 numberSum，就应该将其隐藏并提供一个方法来同时获取它们。像这样

@interface Writer : NSObject {
    NSString* numberString;
    NSUInteger numberSum;

    dispatch_queue_t accessQueue;
}

- (NSDicionary*) readStringAndSum;
@end

@implementation Writer

- (instancetype) init {
    ...
    accessQueue = dispatch_queue_create("Access queue", DISPATCH_QUEUE_CONCURRENT);
    ...
}

- (void)writeToVariables:(NSTimer *)timer {

     dispatch_barrier_async(accessQueue, ^{
           numberString = [self.numberString stringByAppendingString:@",1"];    
           numberSum += 1;
      });

}


- (NSDicionary*) readStringAndSum; {
    __block NSDictionary* response;
    dispatch_sync(accessQueue ^{
        response = @{@"Sum" : [NSNumber numberWithUnsignedInteger: numberSum],
                     @"String" : [numberString copy];
                     }
    });

    return response;
}

@end

发生读取时，Objective C 锁定写入操作

ObjectiveC locking write operation when read happens

multithreading

locking

objective-c

race-condition

grand-central-dispatch