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与它们作斗争。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。
如果您想让异步操作同步，一种常见的模式是使用分派信号量，这样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块的信号。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块向信号量发出信号：
dispatch_semaphore_signal(semaphore);
然后您可以让代码在完成块之外（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);
所以，NSURLSessionDataTask 将所有这些放在一起，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，使用这样的块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，我通常会通过块操作避开这种信号量技术，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