Objective c 目标C等待异步任务处理完成

在我的代码中，我运行的是一个本地服务器（CocoaHTTPServer）。当服务器收到请求时，它会创建一个线程并将控制权传递给某个方法（

（NSObject*）httpResponseForMethod:（NSString*）方法URI:（NSString*）path

，这里可能不相关）

我需要阅读本地资产列表并返回结果。API调用（

[assetsLibrary enumerateGroupsWithTypes:AlasSetGroupAll usingBlock:^（AlasSetGroup*group，BOOL*stop）.

）是异步的

由于HTTPResponse需要等待API调用完成，因此我创建了一个名为

\u isProcessing

的标志，我在进行API调用之前设置了该标志。调用完成后，我将取消设置该标志并返回HTTP请求。要等待的代码如下所示：

// the API call is non-blocking. hence, wait in a loop until the command has finished
samCommand->isProcessing = YES;
while (samCommand->isProcessing) {
    usleep(100*1000);
}

API调用在完成其任务时调用委托方法，如下所示：

// to be called at the end of an asynch operation (eg: reading local asset list)
- (void) commandDidFinish {
    // flag to open the lock
    isProcessing = NO;
}

    @implementation SAMCommand {
    NSData* resultData;
    dispatch_semaphore_t semaphore; // a lock to establish whether the command has been processed
}

// construct the object, ensuring that the "command" field is present in the jsonString
+(NSData*) createAndExecuteCommandWithJSONParamsAs:(NSString *)jsonString {

    SAMCommand* samCommand = [[SAMCommand alloc] init];

    samCommand.commandParams = [jsonString dictionaryFromJSON];

    if(COMPONENT==nil || COMMAND==nil){
        DDLogError(@"command not found in %@",jsonString);
        return nil;
    }

    samCommand->semaphore = dispatch_semaphore_create(0);
    DDLogInfo(@"created semaphore %p->%p",samCommand,samCommand->semaphore);

    // to execute a command contained in the jsonString, we use reflection. 
    DDLogInfo(@"calling execute with %p",samCommand);
    [NSClassFromString(COMPONENT) performSelectorOnMainThread:NSSelectorFromString([NSString stringWithFormat:@"%@_%@_%@:",COMMAND,MEDIA_SOURCE,MEDIA_TYPE]) withObject:samCommand waitUntilDone:NO];

    // the above calls are non-blocking. hence, wait in a loop until the command has finished
    DDLogInfo(@"starting wait loop %p->%p",samCommand,samCommand->semaphore);
    dispatch_semaphore_wait(samCommand->semaphore, DISPATCH_TIME_FOREVER);
    DDLogInfo(@"ending wait loop %p->%p",samCommand,samCommand->semaphore);
    DDLogInfo(@"");

    // return the data
    return samCommand->resultData;

}

// to be called at the end of an asynch operation (eg: reading local asset list)
- (void) commandDidFinish {

    // flag to release the lock
    DDLogInfo(@"releasing semaphore %p->%p",self,semaphore);
    dispatch_semaphore_signal(semaphore);
    semaphore = nil;

}

@end

这是可行的，但可能需要性能增强。我如何在这里使用任何东西（运行循环等）来提高性能

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使用dispatch_信号量编辑以下weichsel解决方案 按照weichsel的解决方案，我创建了一个信号量。我的代码顺序是：

CocoaHTTPServer接收一个请求，因此创建一个新线程

它调用命令类的静态方法来执行请求

Command类创建一个新的Command对象来调用另一个类（使用反射），该类调用alassetapi并将Command对象传递给它

返回时，ALAsset API调用调用的委托方法 命令类

因此，我在适当的位置嵌入了信号量。但是，信号量的等待循环有时不会结束。正常的输出应该是：

2014-02-07 11:27:23:214 MM2Beta[7306:1103] HTTPServer: Started HTTP server on port 1978
2014-02-07 11:27:23:887 MM2Beta[7306:6303] created semaphore 0x1f890670->0x1f8950a0
2014-02-07 11:27:23:887 MM2Beta[7306:6303] calling execute with 0x1f890670
2014-02-07 11:27:23:887 MM2Beta[7306:6303] starting wait loop 0x1f890670->0x1f8950a0
2014-02-07 11:27:23:887 MM2Beta[7306:907] calling getAssetsList with delegate 0x1f890670
2014-02-07 11:27:24:108 MM2Beta[7306:907] calling delegate [0x1f890670 commandDidFinish]
2014-02-07 11:27:24:108 MM2Beta[7306:907] releasing semaphore 0x1f890670->0x1f8950a0
2014-02-07 11:27:24:109 MM2Beta[7306:6303] ending wait loop 0x1f890670->0x0

在每几次运行中，最后一步（

结束等待循环0x1f890670->0x0

不会发生）。因此，等待循环永远不会结束。有时代码也会在同一点崩溃。这里有什么错误的线索吗

我的代码如下：

// to be called at the end of an asynch operation (eg: reading local asset list)
- (void) commandDidFinish {
    // flag to open the lock
    isProcessing = NO;
}

    @implementation SAMCommand {
    NSData* resultData;
    dispatch_semaphore_t semaphore; // a lock to establish whether the command has been processed
}

// construct the object, ensuring that the "command" field is present in the jsonString
+(NSData*) createAndExecuteCommandWithJSONParamsAs:(NSString *)jsonString {

    SAMCommand* samCommand = [[SAMCommand alloc] init];

    samCommand.commandParams = [jsonString dictionaryFromJSON];

    if(COMPONENT==nil || COMMAND==nil){
        DDLogError(@"command not found in %@",jsonString);
        return nil;
    }

    samCommand->semaphore = dispatch_semaphore_create(0);
    DDLogInfo(@"created semaphore %p->%p",samCommand,samCommand->semaphore);

    // to execute a command contained in the jsonString, we use reflection. 
    DDLogInfo(@"calling execute with %p",samCommand);
    [NSClassFromString(COMPONENT) performSelectorOnMainThread:NSSelectorFromString([NSString stringWithFormat:@"%@_%@_%@:",COMMAND,MEDIA_SOURCE,MEDIA_TYPE]) withObject:samCommand waitUntilDone:NO];

    // the above calls are non-blocking. hence, wait in a loop until the command has finished
    DDLogInfo(@"starting wait loop %p->%p",samCommand,samCommand->semaphore);
    dispatch_semaphore_wait(samCommand->semaphore, DISPATCH_TIME_FOREVER);
    DDLogInfo(@"ending wait loop %p->%p",samCommand,samCommand->semaphore);
    DDLogInfo(@"");

    // return the data
    return samCommand->resultData;

}

// to be called at the end of an asynch operation (eg: reading local asset list)
- (void) commandDidFinish {

    // flag to release the lock
    DDLogInfo(@"releasing semaphore %p->%p",self,semaphore);
    dispatch_semaphore_signal(semaphore);
    semaphore = nil;

}

@end

我让它工作了：）

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最后，似乎稳定工作的是创建信号量，并将其传递给ALAsset asynch API调用，并在调用结束时释放它。之前，我在调用创建信号量的类的委托方法，信号量对象以某种方式得到释放。不确定那里到底发生了什么。

NSRunLoop有一个名为RunUntelDate的方法：该方法应该适用于您在不久的将来使用的日期，例如提前1秒左右。这样，您可以用以下方法替换while循环中的睡眠调用：

[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeintervalSinveNow:1]

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您可以使用信号量阻止当前队列的执行，直到另一个队列返回。
基本模式是：

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[assetsLibrary enumerateAssetsUsingBlock^(ALAsset *result, NSUInteger index, BOOL *stop):^{
    ...
    dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_release(semaphore);

您可以在Apple的MTAudioProcessingTap Xcode项目中找到此方法的完整示例：

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相关的行从MYViewController开始。m:86

非常感谢。那么原则上，usleep（）和而你的方法呢？他们不是都在创建一个等待循环，轮询给定的时间量……在这种情况下为100毫秒吗？那么性能增强从何而来？睡眠确实会阻止运行循环的处理。我的方法不会这样做。在GUI应用程序中，你会在调用睡眠时看到旋转的等待光标。非常感谢。我使用请查看我的编辑，看看你是否能发现错误。