Question

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

当读者从主线程中读取时，它会在Writer类中保存这些属性的确切值，但是当读者从后台线程中读取时，显然没有保证读者会得到相同的值。这是示例代码。

这里是Writer类

numberString是以逗号分隔的所有内容，例如。 1,1,1,1
numberSum是这些的总和，例如。 4

在Reader课程中：

我想确保numberSum确实是所有人的总和 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，仅用于示例目的。当在主线程中执行时，readerBlock中的两个NSLog语句将始终打印相同，但是在后台线程中它们将在某个时间不同，因为在读取numberString之后读取numberString时读取它的时间已经由Writer中的主线程更新。

我应该怎样做以确保无论调用线程如何都能获得正确的值？

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，您应该隐藏它并提供单一方法来同时获取它们。喜欢这个

@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

当读取发生时，ObjectiveC锁定写操作

1 个答案: