objective-c - NSOperationQueue 内的 NSOperation 导致应用程序卡住 waitUntilFinished :YES

Question

我有 NSOperation 和 AFHTTPClient 请求。在操作结束时，我需要对请求执行另外 N 个操作，并等待请求完成以将主要操作标记为已完成

@interface MyOperation : OBOperation

@end

@implementation MyOperation

- (id)init
{
    if (self = [super init]) {
        self.state = OBOperationReadyState;
    }

    return self;
}

- (void)start
{
    self.state = OBOperationExecutingState;

    AFHTTPClient *client = [[AFHTTPClient alloc] initWithBaseURL:[NSURL URLWithString:@"http://google.com"]];
    [client getPath:@"/"
         parameters:nil
            success:^(AFHTTPRequestOperation *operation, id responseObject) {
                NSOperationQueue *queue = [NSOperationQueue new];
                queue.maxConcurrentOperationCount = 1;

                NSMutableArray *ops = [NSMutableArray array];
                for (int i = 1; i < 10; i++) {
                    MyInnerOperation *innerOp = [[MyInnerOperation alloc] initWithNumber:@(i)];
                    [ops addObject:innerOp];
                }

                [queue addOperations:ops waitUntilFinished:YES];

                self.state = OBOperationFinishedState;
            } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
                self.state = OBOperationFinishedState;
                NSLog(@"error");
            }];
}

@end

链接到问题末尾的 OBOperation 来源。这是一个简单的类，添加了有用的方法来控制 NSOperation 流

内部操作示例:

@interface MyInnerOperation : OBOperation

- (id)initWithNumber:(NSNumber *)number;

@end

@implementation MyInnerOperation

- (id)initWithNumber:(NSNumber *)number
{
    if (self = [super init]) {
        _number = number;
        self.state = OBOperationReadyState;
    }

    return self;
}

- (void)start
{
    self.state = OBOperationExecutingState;

    NSLog(@"begin inner operation: %@", _number);

    AFHTTPClient *client = [[AFHTTPClient alloc] initWithBaseURL:[NSURL URLWithString:@"http://google.com"]];
    [client getPath:@"/"
         parameters:nil
            success:^(AFHTTPRequestOperation *operation, id responseObject) {
                NSLog(@"inner operation success: %@", _number);
                self.state = OBOperationFinishedState;
            } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
                self.state = OBOperationFinishedState;
                NSLog(@"inner operation error: %@", _number);
            }];
}

@end

所以如果我开始我的手术:

MyOperation *op = [MyOperation new];
[_queue addOperation:op];

我在控制台中看到 begin inner operation: 1 就这样!我的应用程序完全卡住(甚至 UI)

经过一些探索后，我确定卡住是由 [queue addOperations:ops waitUntilFinished:YES]; 引起的。如果我不等待完成，我的内部操作会按预期工作，但 MyOperation 会在子操作完成之前完成。

所以现在我有了依赖 block 操作的解决方法:

NSBlockOperation *endOperation = [NSBlockOperation blockOperationWithBlock:^{
    self.state = OBOperationFinishedState;
}];

NSMutableArray *ops = [NSMutableArray arrayWithObject:endOperation];
for (int i = 1; i < 10; i++) {
    MyInnerOperation *innerOp = [[MyInnerOperation alloc] initWithNumber:@(i)];
    [ops addObject:innerOp];

    [endOperation addDependency:innerOp];
}

[queue addOperations:ops waitUntilFinished:NO];

但我仍然完全不明白这次卡住的真正问题是什么。任何解释都会非常有用。

OBOperaton 类来源:https://dl.dropboxusercontent.com/u/1999619/issue/OBOperation.h https://dl.dropboxusercontent.com/u/1999619/issue/OBOperation.m

整个项目:https://dl.dropboxusercontent.com/u/1999619/issue/OperationsTest.zip

Answer 1

您陷入僵局的原因是 AFNetworking 将完成 block 分派(dispatch)到主队列。因此，第一个 success 处理程序中的 waitUntilFinished 将阻塞主队列，直到从属请求完成。但是那些从属请求无法完成，因为它们需要将它们的完成 block 分派(dispatch)到主队列，而第一个操作仍在阻塞。

很明显，无论如何你都不想阻塞主队列，但是如果你阻塞主队列等待操作，你就会遇到死锁，而这些操作本身就需要主队列。