初学者在 iPhone 上常犯的多线程错误

Question

我刚刚在我的应用程序中引入了多线程，只是为了让一个愚蠢的 UIActivityIndi​​catorView 工作。好吧，活动指示器可以工作，好吧 - 但现在我的应用程序有时会崩溃，有时不会 - 在其他受控条件下......我需要弄清楚这一点，但不知道从哪里开始寻找......

那么，初学者在 iPhone 上使用多线程时常犯哪些常见错误呢？请具体回答。感谢您抽出时间。

更新：我添加了有问题的来源以供参考。

//--------------------Where the multithreading starts------------------------


-(IBAction)processEdits:(id)sender
{
        //Try to disable the UI to prevent user from launching duplicate threads
    [self.view setUserInteractionEnabled:NO];

        //Initialize indicator (delcared in .h)
    myIndicator = [[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(155, 230, 20, 20)];
    myIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhite;
    [self.view addSubview:myIndicator];
    [self.view bringSubviewToFront:myIndicator];
    [myIndicator startAnimating];


    //Prepare and set properties of the NEXT modal view controller to switch to
    controller = [[EndViewController alloc] initWithNibName:@"EndViewController" bundle:nil];

    controller.delegate = self;

    [self performSelectorInBackground:@selector(threadWork:) withObject:nil];


}



//-----------------------------THE THREAD WORK--------------------------------


-(IBAction)threadWork:(id)sender{

    NSAutoreleasePool * pool;
    NSString *          status;

    pool = [[NSAutoreleasePool alloc] init];
    assert(pool != nil);


        //The image processing work that takes time
    controller.photoImage = [self buildPhoto];

    //Stop the UIActivityIndicatorView and launch next modal view
    [self performSelectorOnMainThread:@selector(stopSpinner:)withObject:nil waitUntilDone:NO];

    [pool drain];


}




//-------------------Most of the WORKLOAD called in above thread ------------------------



-(UIImage*)buildPhoto
{
    /* 
       This is the work performed in the background thread. Process photos that the user has edited and arrange them into a UIView to be finally flattened out into a new UIImage. Problem: UI usually changes for some reason during this work.
       */

    UIView* photoContainerView = [[UIView alloc] initWithFrame:CGRectMake(0,0,975,1300)];
    photoContainerView.backgroundColor = [UIColor whiteColor];
    UIImage* purikuraFlattened;
    int spacerX = 10;
    int spacerY = 10;

    switch (myPattern) {

        case 0:

            photoContainerView.frame = CGRectMake(0, 0, 320, 427);
            layoutSingle = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x,photoContainerView.frame.origin.y,320,427)];
            [photoContainerView addSubview:layoutSingle];
            layoutSingle.image = editPhotoData1;

            break;


        case 1:

            layoutAimg1 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX, photoContainerView.frame.origin.y+spacerY, 427, 320)];
            layoutAimg2 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX+427, photoContainerView.frame.origin.y+spacerY, 427, 320)];
            layoutAimg3 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX, photoContainerView.frame.origin.y+spacerY+320, 427, 320)];
            layoutAimg4 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX+427, photoContainerView.frame.origin.y+spacerY+320, 427, 320)];
            layoutAimg5 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX, photoContainerView.frame.origin.y+spacerY+(320*2), 427, 320)];
            layoutAimg6 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX+427, photoContainerView.frame.origin.y+spacerY+(320*2), 427, 320)];
            layoutAimg7 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX, photoContainerView.frame.origin.y+spacerY+(320*3), 427, 320)];
            layoutAimg8 = [[UIImageView alloc] initWithFrame:CGRectMake(photoContainerView.frame.origin.x+spacerX+427, photoContainerView.frame.origin.y+spacerY+(320*3), 427, 320)];

            [photoContainerView addSubview:layoutAimg1];
            [photoContainerView addSubview:layoutAimg2];
            [photoContainerView addSubview:layoutAimg3];
            [photoContainerView addSubview:layoutAimg4];
            [photoContainerView addSubview:layoutAimg5];
            [photoContainerView addSubview:layoutAimg6];
            [photoContainerView addSubview:layoutAimg7];
            [photoContainerView addSubview:layoutAimg8];


            if(myShots == 1){

            rotPhoto1 = [self rotateImage:editPhotoData1.size:editPhotoData1];

                layoutAimg1.image = rotPhoto1;
                layoutAimg2.image = rotPhoto1;
                layoutAimg3.image = rotPhoto1;
                layoutAimg4.image = rotPhoto1;
                layoutAimg5.image = rotPhoto1;
                layoutAimg6.image = rotPhoto1;
                layoutAimg7.image = rotPhoto1;
                layoutAimg8.image = rotPhoto1;



            }else if(myShots == 2){


            rotPhoto1 = [self rotateImage:editPhotoData1.size: editPhotoData1];
            rotPhoto2 = [self rotateImage:editPhotoData2.size: editPhotoData2];

                layoutAimg1.image = rotPhoto1;
                layoutAimg2.image = rotPhoto2;
                layoutAimg3.image = rotPhoto2;
                layoutAimg4.image = rotPhoto1;
                layoutAimg5.image = rotPhoto1;
                layoutAimg6.image = rotPhoto2;
                layoutAimg7.image = rotPhoto2;
                layoutAimg8.image = rotPhoto1;


            }else if(myShots == 4){

                rotPhoto1 = [self rotateImage:editPhotoData1.size: editPhotoData1];
                rotPhoto2 = [self rotateImage:editPhotoData2.size: editPhotoData2];
                rotPhoto3 = [self rotateImage:editPhotoData3.size: editPhotoData3];
                rotPhoto4 = [self rotateImage:editPhotoData4.size: editPhotoData4];

                layoutAimg1.image = rotPhoto1;
                layoutAimg2.image = rotPhoto2;
                layoutAimg3.image = rotPhoto3;
                layoutAimg4.image = rotPhoto4;
                layoutAimg5.image = rotPhoto1;
                layoutAimg6.image = rotPhoto2;
                layoutAimg7.image = rotPhoto3;
                layoutAimg8.image = rotPhoto4;


            }
            break;

      }


    UIGraphicsBeginImageContext(photoContainerView.bounds.size);
    [purikuraContainerView.layer renderInContext:UIGraphicsGetCurrentContext()];
    photoFlattened = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext(); 


    NSEnumerator *enumerator = [[photoContainerView subviews] objectEnumerator];
    id object;

    while ((object = [enumerator nextObject])) {

        [object removeFromSuperview];

    }


    [photoContainerView release];

    photoContainerView = nil;

    if(rotPhoto1 != nil){
    [rotPhoto1 release];
        rotPhoto1 = nil;
    }
    if(rotPhoto2 != nil){
    [rotPhoto2 release];
    rotPhoto2 = nil;
    }
    if(rotPhoto3 != nil){
    [rotPhoto3 release];
    rotPhoto3 = nil;
    }
    if(rotPhoto4 != nil){
    [rotPhoto4 release];
    rotPhoto4 = nil;
    }

    if(rotPhotoSm1 != nil){
    [rotPhotoSm1 release];
    rotPhotoSm1 = nil;
    }
    if(rotPhotoSm2 != nil){
    [rotPhotoSm2 release];
    rotPhotoSm2 = nil;
    }
    if(rotPhotoSm3 != nil){
    [rotPhotoSm3 release];
    rotPhotoSm3 = nil;
    }
    if(rotPhotoSm4 != nil){
    [rotPhotoSm4 release];
    rotPhotoSm4 = nil;
    }

    return photoFlattened;

}



//-----------------------------STOP THE UIACTIVITYINDICATORVIEW---------------------



-(IBAction)stopSpinner:(id)sender
{

    [self.view setUserInteractionEnabled:YES];
    [myIndicator stopAnimating];
    [myIndicator release];
    myIndicator = nil;

    if(myPattern == 0){
        NSLog(@"SINGLE-SHOT MODE");
        controller.isSingleShot = TRUE;

    }else{

        NSLog(@"MULTI-SHOT MODE");
        controller.isSingleShot = FALSE;

    }

    controller.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
    [self presentModalViewController:controller animated:YES];

    [controller release];

    [allStamps removeAllObjects];
    [imageFrames removeAllObjects];


    switch (myShots) {
        case 1:
            [editPhotoData1 release];
            break;

        case 2:
            [editPhotoData1 release];
            [editPhotoData2 release];
            break;

        case 4:
            [editPhotoData1 release];
            [editPhotoData2 release];
            [editPhotoData3 release];
            [editPhotoData4 release];
            break;

    }

        /* This is the edited photo that has been onscreen. Processing is now done so it is okay to release it. The UI should be updated and now have a blank, black background instead of the image.
*/
        editedPhoto.image = nil;
    [editedPhoto release];
    editedPhoto = nil;


}

Answer 1

这个问题有一些关于可可多线程的好资源：“在哪里可以找到有关 iPhone/Objective c 多线程的优秀教程？”

我还强烈建议您阅读新的 并发编程指南（但是，请忽略块和调度队列，因为 Grand Central Dispatch 在 iPhone OS 上尚不可用 iOS 4.0 刚刚添加了块和 GCD），因为它为使用 NSOperation 和 NSOperationQueue 等结构作为手动创建线程的替代方案提供了强有力的理由。有关手动创建线程的信息，请参阅 线程编程指南。

正如 RC 提到的，多线程 Cocoa 应用程序崩溃的最大根源是同时访问共享资源。 @synchronized 指令不是最快的，如 Colin Wheeler 指出，因此您可能需要使用 NSLock 来保护对共享资源的访问。然而，任何类型的锁定都可能代价高昂，这就是为什么我一直将我的应用程序迁移到使用单宽 NSOperationQueues 来访问这些资源。性能改进非常显着。

Cocoa 和多线程的另一个问题来自用户界面更新。 Cocoa 中的所有 UI 更新都必须在主线程上完成，否则可能会导致不稳定。如果您有后台线程执行计算，请确保将更新 UI 的任何方法包装在 -performSelectorOnMainThread:withObject:waitUntilDone: 方法调用中。

Answer 2

初学者（在任何语言中）使用线程时最常见的错误可能是允许在没有防护/互斥锁的情况下访问可变共享资源。您可以保护以下资源：

@synchronized(sharedData)
{
   // modify sharedData safely
}

您需要限制线程之间共享的数据量，如果必须共享数据，则更喜欢不可变对象，以减少同步引起的争用。

管理线程是另一个可能出现问题的地方。这是专门针对 iPhone 中线程的使用的文档参考。

http://developer.apple.com/iphone/library/documentation/cocoa/概念/多线程/CreatingThreads/CreatingThreads.html。

如果不提供代码，任何人都可以猜测您的应用程序出了什么问题，但我首先要确保您正确管理线程的创建和终止，并特别注意线程尝试访问的任何共享资源。