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

2 回答

• 1

  这是因为一旦main（）函数退出，就会释放TestClass对象 . 主要是异步，testMethod：不会阻塞main（）中的指针 .

  尝试在main（）的末尾添加一个信号量 . 应在testMethod dispatch_async(dispatch_get_main_queue(), ^{}); 块中发信号通知此信号量 .

  回复于 2024-04-25T18:33:37+08:00
• 3

  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 . 我还提供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
  

  的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;
  }
  

  回复于 2024-04-25T18:33:37+08:00