ios - 带有 NSBlockOperation 和队列的 NSURLSession

Question

我有一个应用程序目前使用 NSURLConnection 进行绝大多数网络。我想搬到 NSURLSession 因为 Apple 告诉我这是要走的路。

我的应用只是通过 + (NSData *)sendSynchronousRequest:(NSURLRequest *)request returnedResponse:(NSURLResponse **)response error:(NSError 的方式使用 NSURLConnection 的同步版本**)error 类方法。我在运行在 NSOperationQueue 上的 NSBlockOperation 中执行此操作，因此我不会不必要地阻塞主队列。以这种方式做事的一大优势是我可以使操作相互依赖。例如，我可以让请求数据的任务依赖于登录任务的完成。

我没有在 NSURLSession 中看到任何对同步操作的支持。我能找到的所有文章都 mock 我甚至考虑同步使用它，并且我是一个阻塞线程的可怕人。美好的。但我认为没有办法让 NSURLSessionTask 相互依赖。有没有办法做到这一点？

或者有没有描述我将如何以不同的方式做这样的事情？

Answer 1

对同步网络请求最严厉的批评是留给那些从主队列执行的人(因为我们知道永远不应该阻塞主队列)。但是你是在你自己的后台队列上做的，它解决了同步请求中最严重的问题。但是您正在失去异步技术提供的一些出色功能(例如，如果需要，可以取消请求)。

我将在下面回答您的问题(如何使 NSURLSessionDataTask 同步运行)，但我真的鼓励您接受异步模式而不是反对它们。我建议重构您的代码以使用异步模式。具体来说，如果一个任务依赖于另一个任务，只需将依赖任务的启动放在前一个任务的完成处理程序中即可。

如果您在该转换中遇到问题，请发布另一个 Stack Overflow 问题，向我们展示您的尝试，我们可以尝试帮助您。

---

如果您想使异步操作同步，一种常见的模式是使用分派(dispatch)信号量，以便启动异步进程的线程可以在继续之前等待来自异步操作完成 block 的信号。永远不要从主队列执行此操作，但如果您是从某个后台队列执行此操作，这可能是一种有用的模式。

您可以使用以下方法创建信号量:

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

然后您可以让异步进程的完成 block 向信号量发出信号:

dispatch_semaphore_signal(semaphore);

然后您可以让代码在完成 block 之外(但仍在后台队列中，而不是主队列中)等待该信号:

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

因此，使用 NSURLSessionDataTask，将所有内容放在一起，可能看起来像:

[queue addOperationWithBlock:^{

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    NSURLSession *session = [NSURLSession sharedSession]; // or create your own session with your own NSURLSessionConfiguration
    NSURLSessionTask *task = [session dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        if (data) {
            // do whatever you want with the data here
        } else {
            NSLog(@"error = %@", error);
        }

        dispatch_semaphore_signal(semaphore);
    }];
    [task resume];

    // but have the thread wait until the task is done

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    // now carry on with other stuff contingent upon what you did above
]);

使用 NSURLConnection(现已弃用)，您必须跳过一些环节才能从后台队列发起请求，但 NSURLSession 可以优雅地处理它。

---

话虽如此，使用这样的 block 操作意味着操作不会响应取消事件(至少在它们运行时)。因此，我通常会使用 block 操作来避免这种信号量技术，而只是将数据任务包装在异步 NSOperation 子类中。然后，您可以享受运营带来的好处，但您也可以将其取消。这是更多的工作，但更好的模式。

例如:

//
//  DataTaskOperation.h
//
//  Created by Robert Ryan on 12/12/15.
//  Copyright © 2015 Robert Ryan. All rights reserved.
//

@import Foundation;
#import "AsynchronousOperation.h"

NS_ASSUME_NONNULL_BEGIN

@interface DataTaskOperation : AsynchronousOperation

/// Creates a operation that retrieves the contents of a URL based on the specified URL request object, and calls a handler upon completion.
///
/// @param  request                    A NSURLRequest object that provides the URL, cache policy, request type, body data or body stream, and so on.
/// @param  dataTaskCompletionHandler  The completion handler to call when the load request is complete. This handler is executed on the delegate queue. This completion handler takes the following parameters:
///
/// @returns                           The new session data operation.

- (instancetype)initWithRequest:(NSURLRequest *)request dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler;

/// Creates a operation that retrieves the contents of a URL based on the specified URL request object, and calls a handler upon completion.
///
/// @param  url                        A NSURL object that provides the URL, cache policy, request type, body data or body stream, and so on.
/// @param  dataTaskCompletionHandler  The completion handler to call when the load request is complete. This handler is executed on the delegate queue. This completion handler takes the following parameters:
///
/// @returns                           The new session data operation.

- (instancetype)initWithURL:(NSURL *)url dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler;

@end

NS_ASSUME_NONNULL_END

和

//
//  DataTaskOperation.m
//
//  Created by Robert Ryan on 12/12/15.
//  Copyright © 2015 Robert Ryan. All rights reserved.
//

#import "DataTaskOperation.h"

@interface DataTaskOperation ()

@property (nonatomic, strong) NSURLRequest *request;
@property (nonatomic, weak) NSURLSessionTask *task;
@property (nonatomic, copy) void (^dataTaskCompletionHandler)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error);

@end

@implementation DataTaskOperation

- (instancetype)initWithRequest:(NSURLRequest *)request dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler {
    self = [super init];
    if (self) {
        self.request = request;
        self.dataTaskCompletionHandler = dataTaskCompletionHandler;
    }
    return self;
}

- (instancetype)initWithURL:(NSURL *)url dataTaskCompletionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))dataTaskCompletionHandler {
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    return [self initWithRequest:request dataTaskCompletionHandler:dataTaskCompletionHandler];
}

- (void)main {
    NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithRequest:self.request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        self.dataTaskCompletionHandler(data, response, error);
        [self completeOperation];
    }];

    [task resume];
    self.task = task;
}

- (void)completeOperation {
    self.dataTaskCompletionHandler = nil;
    [super completeOperation];
}

- (void)cancel {
    [self.task cancel];
    [super cancel];
}

@end

地点:

//
//  AsynchronousOperation.h
//

@import Foundation;

@interface AsynchronousOperation : NSOperation

/// Complete the asynchronous operation.
///
/// This also triggers the necessary KVO to support asynchronous operations.

- (void)completeOperation;

@end

和

//
//  AsynchronousOperation.m
//

#import "AsynchronousOperation.h"

@interface AsynchronousOperation ()

@property (nonatomic, getter = isFinished, readwrite)  BOOL finished;
@property (nonatomic, getter = isExecuting, readwrite) BOOL executing;

@end

@implementation AsynchronousOperation

@synthesize finished  = _finished;
@synthesize executing = _executing;

- (instancetype)init {
    self = [super init];
    if (self) {
        _finished  = NO;
        _executing = NO;
    }
    return self;
}

- (void)start {
    if ([self isCancelled]) {
        self.finished = YES;
        return;
    }

    self.executing = YES;

    [self main];
}

- (void)completeOperation {
    self.executing = NO;
    self.finished  = YES;
}

#pragma mark - NSOperation methods

- (BOOL)isAsynchronous {
    return YES;
}

- (BOOL)isExecuting {
    @synchronized(self) {
        return _executing;
    }
}

- (BOOL)isFinished {
    @synchronized(self) {
        return _finished;
    }
}

- (void)setExecuting:(BOOL)executing {
    @synchronized(self) {
        if (_executing != executing) {
            [self willChangeValueForKey:@"isExecuting"];
            _executing = executing;
            [self didChangeValueForKey:@"isExecuting"];
        }
    }
}

- (void)setFinished:(BOOL)finished {
    @synchronized(self) {
        if (_finished != finished) {
            [self willChangeValueForKey:@"isFinished"];
            _finished = finished;
            [self didChangeValueForKey:@"isFinished"];
        }
    }
}

@end