为什么 dispatch_async(dispatch_get_main_queue() 之后的块永远不会被调用？

Question

我想异步执行一些代码，因此开始使用 GCD OSX/iOS。
目前我正在使用函数 dispatch_async().
当我想在另一个线程上同时执行某事时，我使用函数 dispatch_get_global_queue().
当我想将结果分派给主线程时，我使用函数 dispatch_get_main_queue().
但是结果永远不会到达主线程。

在调试器中设置断点时（在 dispatch_async 行），函数 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) 只是有时会执行，但调试器通常会忽略它。
当块确实被执行并且执行流程到达 dispatch_async(dispatch_get_main_queue() 处的断点时，之后的块总是被忽略。

没有dispatch_async(dispatch_get_global_queue)和dispatch_async(dispatch_get_main_queue) 代码按预期执行，尽管是同步执行的。

我的问题是为什么 dispatch_async(dispatch_get_main_queue() 在 dispatch_async(dispatch_get_global_queue() 块永远不会被执行？
同样，为什么 dispatch_async(dispatch_get_global_queue() 块不是每次都执行？

我的开发环境是
OS: OS X 10.11.3
IDE: Xcode 7.2
编译器：Apple LLVM 版本 7.0.2 (clang-700.1.81)
目标：x86_64-apple-darwin15.3.0

这是一个重现不稳定行为的简单示例（OS X 控制台应用程序）：

TestClass.h

#ifndef TestClass_h
#define TestClass_h

@interface TestClass : NSObject {
}

- (void)testMethod:(NSString *)testString withCompletionBlock:(void(^)(NSString *blockResult, NSError __autoreleasing *error))completionBlock;

@end

#endif

TestClass.m

#import <Foundation/Foundation.h>
#import "TestClass.h"

@implementation TestClass

- (void)testMethod:(NSString *)testString withCompletionBlock:(void(^)(NSString *blockResult, NSError __autoreleasing *error))completionBlock {
    __block NSString *stringResult = nil;

    if (completionBlock) {
        __block NSError *resultError = nil;


       dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // THIS BLOCK IS CALLED ONLY SOMETIMES, MOST OF THE TIME IT IS IGNORED.

            if ([testString isEqual: @"Error string"]) {
                NSDictionary *errorUserInfo = @{ NSLocalizedDescriptionKey: @"This is an error.", NSLocalizedFailureReasonErrorKey: @"", NSLocalizedRecoverySuggestionErrorKey: @"" };                
                resultError = [[NSError alloc] initWithDomain:@"com.test.TestErrorDomain" code:10 userInfo:errorUserInfo];
            }
            else {
                stringResult = testString;
            }

            dispatch_async(dispatch_get_main_queue(), ^{
                // THIS BLOCK NEVER GETS EXECUTED.

                completionBlock(stringResult, resultError);
            });
        });
    }
}

@end

main.m

#import <Foundation/Foundation.h>
#import "TestClass.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        __block NSString *resultString = nil;
        TestClass * testObject = [[TestClass alloc] init];

        // Output for this call should be: The result string is: Test string.
        [testObject testMethod:@"Test string" withCompletionBlock:^(NSString *blockString, NSError __autoreleasing *error) {
            resultString = blockString;

            if (resultString) {
                NSLog(@"The result string is: %@.", resultString);
            }
        }];

        // Output for this call should be: Error: This is an error.
        [testObject testMethod:@"Error string" withCompletionBlock:^(NSString *blockString, NSError __autoreleasing *error) {
            resultString = blockString;

            if (!resultString) {
                if (error) {
                    NSLog(@"Error: %@", [error localizedDescription]);
                }
                else {
                    NSLog(@"Error not recognized!");
                }
            }
        }];
    }

    return 0;
}

Answer 1

那是因为一旦 main() 函数退出，您的 TestClass 对象就会被释放。主要是异步的，testMethod: 不会阻塞 main() 中的指针。

尝试在 main() 的末尾添加一个信号量。这个信号量应该在 testMethod dispatch_async(dispatch_get_main_queue(), ^{}); 块中发出信号。

Answer 2

这是带有信号量的工作示例，正如@CharlesThierry 的回答中所建议的那样。我还包含了一些有助于说明执行流程的 NSLog：

TestClass.h

#ifndef TestClass_h
#define TestClass_h

@interface TestClass : NSObject {
}

- (void)testMethod:(NSString *)testString withSemaphore:(dispatch_semaphore_t)sem withCompletionBlock:(void(^)(NSString *blockResult, NSError __autoreleasing *error))completionBlock;

@end

#endif

TestClass.m

#import <Foundation/Foundation.h>
#import "TestClass.h"

@implementation TestClass

- (void)testMethod:(NSString *)testString withSemaphore:(dispatch_semaphore_t)sem withCompletionBlock:(void(^)(NSString *blockResult, NSError __autoreleasing *error))completionBlock {
    __block NSString *stringResult = nil;

    if (completionBlock) {
        __block NSError *resultError = nil;

        NSLog(@"INSIDE TEST METHOD, OUTSIDE DISPATCH ASYNC");

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"INSIDE THE DISPATCH ASYNC TO THE GLOBAL QUEUE");

            if ([testString isEqual: @"Error string"]) {
                NSDictionary *errorUserInfo = @{ NSLocalizedDescriptionKey: @"This is an error.", NSLocalizedFailureReasonErrorKey: @"", NSLocalizedRecoverySuggestionErrorKey: @"" };
                resultError = [[NSError alloc] initWithDomain:@"com.test.TestErrorDomain" code:10 userInfo:errorUserInfo];
            }
            else {
                stringResult = testString;
            }

            dispatch_async(dispatch_get_main_queue(), ^{
                NSLog(@"INSIDE THE DISPATCH ASYNC TO THE MAIN QUEUE!");
                completionBlock(stringResult, resultError);
                dispatch_semaphore_signal(sem);
            });
        });
    }
}

@end

main.m

#import <Foundation/Foundation.h>
#import "TestClass.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSLog(@"BEFORE BLOCK");

        __block NSString *resultString = nil;
        TestClass * testObject = [[TestClass alloc] init];
        dispatch_semaphore_t sem = dispatch_semaphore_create(0);

        [testObject testMethod:@"Test string" withSemaphore:sem withCompletionBlock:^(NSString *blockString, NSError __autoreleasing *error)
         {
             NSLog(@"INSIDE FIRST BLOCK");

             resultString = blockString;

             if (resultString)
             {
                 NSLog(@"The result string is: %@.", resultString);
             }
         }];

        [testObject testMethod:@"Error string" withSemaphore:sem withCompletionBlock:^(NSString *blockString, NSError __autoreleasing *error)
         {
             NSLog(@"INSIDE SECOND BLOCK");

             resultString = blockString;

             if (!resultString)
             {
                 if (error)
                 {
                     NSLog(@"Error: %@", [error localizedDescription]);
                 }
                 else
                 {
                     NSLog(@"Error not recognized!");
                 }
             }
         }];

        NSLog(@"AFTER BLOCK!");

        while (dispatch_semaphore_wait(sem, DISPATCH_TIME_NOW))
            [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:10]];
    }

    return 0;
}

为什么 dispatch_async(dispatch_get_main_queue() 之后的块永远不会被调用？

Why does the block after dispatch_async(dispatch_get_main_queue() never get called?

macos

xcode

objective-c

grand-central-dispatch

dispatch-async