cocoa - NSOperation 等待异步 block 执行

Question

我需要将异步操作放入操作队列中，但是，它们需要在另一个操作之后执行

self.operationQueue = [NSOperationQueue new];
self.operationQueue.maxConcurrentOperationCount = 1;

[self.operationQueue addOperationWithBlock:^{

    // this is asynchronous
    [peripheral1 connectWithCompletion:^(NSError *error) {

    }];

}];

[self.operationQueue addOperationWithBlock:^{

    // this is asynchronous
    [peripheral2 connectWithCompletion:^(NSError *error) {

    }];

}];

问题是，由于peripheralN connectWithCompletion是异步的，队列中的操作结束并执行下一个，但是我需要模拟，peripheralN connectWithCompletion是同步的并等待操作结束，直到异步 block 执行

所以我需要这样的行为，仅使用操作队列

    [peripheral1 connectWithCompletion:^(NSError *error) {

            [peripheral2 connectWithCompletion:^(NSError *error) {

            }];

    }];

Answer 1

NSBlockOperation 无法处理异步操作，但创建可以...的 NSOperation 子类并不那么困难

基本上，您需要创建一个 NSOperation，它采用一个 block ，该 block 将 另一个 block 作为完成处理程序。该 block 可以这样定义:

typedef void(^AsyncBlock)(dispatch_block_t completionHandler);

然后，在您的 NSOperation 子类的 start 方法中，您需要调用 AsyncBlock 并向其传递 dispatch_block_t执行完成后将被调用。您还需要确保保持 KVO 与 NSOperation 的 isFinished 和 isExecuting 属性(至少)兼容(请参阅 NSOperation 文档中的 KVO-Compliant Properties)；这就是允许 NSOperationQueue 等待异步操作完成的原因。

类似这样的事情:

- (void)start {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = YES;
    [self didChangeValueForKey:@"isExecuting"];

    self.block(^{
        [self willChangeValueForKey:@"isExecuting"];
        _executing = NO;
        [self didChangeValueForKey:@"isExecuting"];
        [self willChangeValueForKey:@"isFinished"];
        _finished = YES;
        [self didChangeValueForKey:@"isFinished"];
    });
}

请注意，_executing 和 _finished 需要在子类中的某个位置定义，并且您需要重写 isExecuting和 isFinished 属性以返回正确的值。

如果将所有这些放在一起，以及一个采用 AsyncBlock 的初始化程序，那么您可以将操作添加到队列中，如下所示:

[self.operationQueue addOperationWithBlock:^(dispatch_block_t completionHandler) {
    [peripheral1 connectWithCompletion:^(NSError *error) {
        // call completionHandler when the operation is done
        completionHandler();
    }];
}];

[self.operationQueue addOperationWithBlock:^(dispatch_block_t completionHandler) {
    [peripheral2 connectWithCompletion:^(NSError *error) {
        // call completionHandler when the operation is done
        completionHandler();
    }];
}];

<小时/>

我在这里整理了一个简单版本的要点:AsyncOperationBlock 。这只是一个最低限度的实现，但它应该可以工作(例如，如果 isCancelled 也实现了，那就太好了)。

为了完整起见，复制到此处:

AsyncBlockOperation.h:

#import <Foundation/Foundation.h>

typedef void(^AsyncBlock)(dispatch_block_t completionHandler);

@interface AsyncBlockOperation : NSOperation

@property (nonatomic, readonly, copy) AsyncBlock block;

+ (instancetype)asyncBlockOperationWithBlock:(AsyncBlock)block;

- (instancetype)initWithAsyncBlock:(AsyncBlock)block;

@end


@interface NSOperationQueue (AsyncBlockOperation)

- (void)addAsyncOperationWithBlock:(AsyncBlock)block;

@end

AsyncBlockOperation.m:

#import "AsyncBlockOperation.h"

@interface AsyncBlockOperation () {
    BOOL _finished;
    BOOL _executing;
}

@property (nonatomic, copy) AsyncBlock block;

@end


@implementation AsyncBlockOperation

+ (instancetype)asyncBlockOperationWithBlock:(AsyncBlock)block {
    return [[AsyncBlockOperation alloc] initWithAsyncBlock:block];
}

- (instancetype)initWithAsyncBlock:(AsyncBlock)block {
    if (self = [super init]) {
        self.block = block;
    }
    return self;
}

- (void)start {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = YES;
    [self didChangeValueForKey:@"isExecuting"];

    self.block(^{
        [self willChangeValueForKey:@"isExecuting"];
        _executing = NO;
        [self didChangeValueForKey:@"isExecuting"];
        [self willChangeValueForKey:@"isFinished"];
        _finished = YES;
        [self didChangeValueForKey:@"isFinished"];
    });
}

- (BOOL)isFinished {
    return _finished;
}

- (BOOL)isExecuting {
    return _executing;
}

- (BOOL)isAsynchronous {
    return YES;
}

@end

@implementation NSOperationQueue (AsyncBlockOperation)

- (void)addAsyncOperationWithBlock:(AsyncBlock)block {
    [self addOperation:[AsyncBlockOperation asyncBlockOperationWithBlock:block]];
}

@end