NSBlockOperation 在执行前不等待依赖答案

【问题标题】：NSBlockOperation is not waiting on dependency before executingNSBlockOperation 在执行前不等待依赖
【发布时间】：2017-04-21 14:36:22
【问题描述】：

我正在学习NSOperations & NSOperationQueue。

我有一组NSBlockOperation：“上传”和“DELETE”。删除必须等待上传完成后再执行。

我希望完成一组操作，然后再进行下一组操作。

我用NSThread sleepForTimeInterval模拟了上传等待和删除延迟时间。

但是这些操作并没有等待集合完成。

我将 maxConcurrentOperationCount 设置为 1，但这似乎不起作用。

您可以从输出中看到 Set 1 完成得很好。

但在 Delete 2 完成之前，第二组 Upload 3 就盯着看。然后上传/删除 4 就可以了。然后从那里开始变得更加混乱。

有什么帮助吗？

输出：

Start UPLOAD 1
Completed UPLOAD 1
Start DELETE 1
Completed DELETE 1

Start UPLOAD 2
Completed UPLOAD 2
Start DELETE 2
Start UPLOAD 3
Completed DELETE 2
Start DELETE 3
Completed UPLOAD 3
Completed DELETE 3

Start UPLOAD 4
Start DELETE 4
Completed UPLOAD 4
Completed DELETE 4

Start UPLOAD 5
Start DELETE 5
Completed UPLOAD 5
Start UPLOAD 6
Completed DELETE 5
Start DELETE 6
Completed UPLOAD 6
Start UPLOAD 7
Completed DELETE 6

代码：

- (void)viewDidLoad {
    [super viewDidLoad];


    NSOperationQueue *operationQueue = [NSOperationQueue mainQueue];
    operationQueue.maxConcurrentOperationCount = 1;


//pretend there are 100 images that need to be uploaded and information in a SQLite DB waiting to be deleted upon successful upload of the image.

//Upload 1 image at a time, upon successful upload, delete the respective DB info then move to the next image 
    for (__block int i = 1; i < 100; i++){
        NSOperation * UPLOAD = [self createNewOperationWithInt:i Message:@"UPLOAD"];
        NSOperation * DELETE = [self createNewOperationWithInt:i Message:@"DELETE"];
        [DELETE addDependency:UPLOAD];
        [operationQueue addOperation:UPLOAD];
        [operationQueue addOperation:DELETE];

    }

}

- (NSBlockOperation *) createNewOperationWithInt:(int)i Message:(NSString*)message {
    NSBlockOperation * operation = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"Start %@ %i",message , i);

        if ([message containsString:@"UPLOAD"]) {
             [NSThread sleepForTimeInterval:1]; //Pretend there is Network latency on upload
        }

        if ([message containsString:@"DELETE"]) {
            [NSThread sleepForTimeInterval:0.5]; //Pretend the SQLDB is being accessed
        }

    }];
    operation.queuePriority = NSOperationQueuePriorityNormal;
    operation.qualityOfService  = NSOperationQualityOfServiceUtility;
    operation.completionBlock = ^{
       NSLog(@"Completed %@ %i",message , i);
    };

    return operation;
}

【问题讨论】：

阅读NSOperation 的completionBlock 属性的文档。该讨论解释了您的输出。

标签： ios objective-c nsoperation nsoperationqueue nsblockoperation

【解决方案1】：

这种行为似乎很好。 “问题”是完成块和依赖项都在 upload 任务完成后运行。这就是为什么您有时会在“完成上传 N”之前得到“开始删除 N+1”。

尝试在块操作结束时添加一条消息，以检查它是否在开始下一个操作之前完成：

NSBlockOperation * operation = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"Start %@ %i",message , i);

    if ([message containsString:@"UPLOAD"]) {
         [NSThread sleepForTimeInterval:1]; //Pretend there is Network latency on upload
    }

    if ([message containsString:@"DELETE"]) {
        [NSThread sleepForTimeInterval:0.5]; //Pretend the SQLDB is being accessed
    }

    NSLog(@"Finished %@ %i",message , i);
}];

【讨论】：