iOS 使用 NSBlockOperation 限制异步 API 调用

Question

我想将进行中的 API 调用次数限制为 2。我可以创建一个 NSOperationQueue 并将块添加到队列中，但是每个 API 调用都有一个完成块，所以初始调用是有限的，但我不知道如何根据完成块的执行来限制队列的处理。

在下面的代码中，任何时候都可能有超过 2 个调用 API 正在运行。

NSOperationQueue *requestQueue = [[NSOperationQueue alloc] init];
service.requestQueue.maxConcurrentOperationCount = 2;

for (int i = 0; i < 100; i++)
{
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{

        [self invokeAPI:kAPIName completion:^BOOL(APIResult *result) {

            // Do stuff
        }

         [requestQueue addOperation:operation];
    }
}

非常感谢任何指向正确使用模式的指示。

编辑 - 基于 Marc-Alexandre 的回答

已经创建了这个 class 来封装操作，从内存的角度来看，这种方法是安全的，因为这个 class 将从 dataAccessService 创建并注入，以及完成块引用了自身，并且完成调用 before 完成块执行？

@interface MAGApiOperation : NSOperation

@property (nonatomic, strong) id<MAGDataAccessServiceProtocol> dataAccessService;
@property (nonatomic, copy) NSString *apiName;
@property (nonatomic, copy) BOOL (^onCompletion)(APIResult *);

+ (instancetype)apiOperationWithName:(NSString *)apiName dataAccessService:(id<MAGDataAccessServiceProtocol>)dataAccessService completion:(BOOL (^)(APIResult *))onCompletion;

@implementation MAGApiOperation

@synthesize executing = _isExecuting;
@synthesize finished = _isFinished;

#pragma mark - Class methods

/// Creates a new instance of MAGApiOperation
+ (instancetype)apiOperationWithName:(NSString *)apiName dataAccessService:(id<MAGDataAccessServiceProtocol>)dataAccessService completion:(BOOL (^)(APIResult *))onCompletion {

    MAGApiOperation *operation = [[self alloc] init];
    operation.apiName = apiName;
    operation.dataAccessService = dataAccessService;
    operation.onCompletion = onCompletion;

    return operation;
}

#pragma mark - NSOperation method overrides

- (void)start {

    [self willChangeValueForKey:@"isExecuting"];
    _isExecuting = YES;
    [self didChangeValueForKey:@"isExecuting"];

    if (!self.isCancelled)
    {
        [self invokeApiWithName:self.apiName completion:self.onCompletion];
    }
}

- (void)finish {

    [self willChangeValueForKey:@"isExecuting"];
    [self willChangeValueForKey:@"isFinished"];

    _isExecuting = NO;
    _isFinished = YES;

    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

#pragma mark - Private methods

/// Invokes the api with the name then executes the completion block
- (void)invokeApiWithName:(NSString *)apiName completion:(BOOL (^)(VAAInvokeAPIResult *))onCompletion {

        [self.dataAccessService invokeAPI:kAPIName completion:^BOOL(APIResult *result) { {

            [self finish];

            return onCompletion(result);
        }];
}

Answer 1

您需要子类化 NSOperation 才能做到这一点。

这里是完整的文档，解释了如何将 NSOperation 子类化：https://developer.apple.com/reference/foundation/operation

快速笔记：

您的 "start" 操作将是 invokeAPI 调用。
然后在您的 invokeAPI 完成块中，您将自己的操作标记为已完成（见下文，注意 willChange 和 didChange 调用非常重要）

[self willChangeValueForKey:@"isExecuting"];
[self willChangeValueForKey:@"isFinished"];
self.isExecuting = NO;
self.isFinished = YES;
[self didChangeValueForKey:@"isExecuting"];
[self didChangeValueForKey:@"isFinished"];

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