我想异步执行一些代码,因此开始使用GCD for 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些操作时,我使用函数 dispatch_get_global_queue()。
当我想将结果发送到主线程时,我使用函数 dispatch_get_main_queue()。
但结果永远不会到达主线。
在调试器中设置断点(在 dispatch_async 行)时,函数 dispatch_async之后的块(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0) 有时只执行,但调试器通常会忽略它 当块执行并且执行流程到达 dispatch_async(dispatch_get_main_queue()的断点时,之后的块总是被忽略。
如果没有 dispatch_async(dispatch_get_global_queue)和 dispatch_async(dispatch_get_main_queue),代码将按原样执行,尽管是同步的。
我的问题是为什么 dispatch_async(dispatch_get_global_queue()块中的 dispatch_async(dispatch_get_main_queue()永远不会被执行?
同样,为什么每次都不会执行 dispatch_async(dispatch_get_global_queue()块?
我的发展环境是
操作系统: 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
这是一个简单的例子,它可以重现不稳定的行为(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
的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;
}
答案 0 :(得分:4)
这是因为main()函数退出后会立即释放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 #/:param
块中发信号通知此信号量。
答案 1 :(得分:2)
这是带有信号量的工作示例,如@ CharlesThierry的回答中所述。 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;
}