ios – 使用[NSThread sleep：]解决死锁问题

我只是“解决”了似乎是死锁或同步问题：

[NSThread sleepForTimeInterval:0.1];

在一个应用程序中,将来自IPOD库的MPMediaItem(音乐/图像)属性引用附加到对象实例,并通过CoreData对这些对象进行反向存储.我的兴趣是要准确了解发生了什么,以及在这种情况下最佳做法是什么.开始：

每次复制此内容的方法如下：

>用户创建一个新项目.

doc = [[UIManagedDocument alloc] initWithFileURL:docURL];

if (![[NSFileManager defaultManager] fileExistsAtPath:[docURL path]]) {
    [doc saveToURL:docURL forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success) {
    if (success) {
        completionBlock(doc);
    }
    else {
        DLog(@"Failed document creation: %@",doc.localizedName);
    }
}];

>稍后,managedObjectContext用于关联对象实例并对CoreData模型进行水合

TheProject *theProject = [TheProject projectWithInfo:theProjectInfo
                              inManagedObjectContext:doc.managedObjectContext];

>用户稍后创建“CustomAction”对象,向其添加“ChElement”并将“MusicElement”与ChElement相关联. (这些是CoreData模型对象的假名). MusicElement通过IPOD库添加.

#define PLAYER [MPMusicPlayerController iPodMusicPlayer]

>用户保存此项目,然后切换到已创建一个CustomAction对象的现有项目,其中包含ChElement和MusicElement.
>用户从tableView中选择该ChElement并导航到detailView.
当导航离开ChElementTVC(类似于Apple文档中的CoreData TableViewController类的子类)时,这是必需的：

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    self.fetchedResultsController.delegate = nil;
}

>在详细信息视图中,用户更改ChElement对象的属性并保存项目. detailView调用其委托(ChElementTVC)来进行保存.保存是保存NSManagedObject的UIManagedDocument实例.

#define SAVEDOC(__DOC__) [ProjectDocumentHelper saveProjectDocument:__DOC__]

// Delegate

- (void)chAddElementDetailViewController:(ChDetailViewController *)sender didPressSaveButton:(NSString *)message
{
    SAVEDOC(THE_CURRENT_PROJECT_DOCUMENT);

    [self.navigationController popViewControllerAnimated:YES];
 }


// Helper Class

+ (void)saveProjectDocument:(UIManagedDocument *)targetDocument
{
    NSManagedObjectContext *moc = targetDocument.managedObjectContext;
    [moc performBlockAndWait:^{
        DLog(@" Process Pending Changes before saving : %@,Context = %@",targetDocument.description,moc);

        [moc processPendingChanges];
        [targetDocument saveToURL:targetDocument.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];
    }];
}

>由于委托(ChElementTVC)从导航堆栈弹出detailView,
调用其viewWillAppear,并恢复fetchedResultsController.delegate.

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    if (!self.fetchedResultsController.delegate) {

        DLog(@"Sleep Now %@",self);

        //http://mobiledevelopertips.com/core-services/sleep-pause-or-block-a-thread.html

       [NSThread sleepForTimeInterval:0.1];

       DLog(@"Wake up %@",self);

       [self fetchedResultsControllerWithPredicate:_savedPredicate]; // App Hangs Here ... This is sending messages to CoreData objects.

       [self.tableView reloadData];
}

没有[NSThread sleepForTimeInterval：0.1];该应用程序挂起.当我通过Xcode发送SIGINT时,我得到调试器并显示以下内容：

(lldb)bt

* thread #1: tid = 0x1c03,0x30e06054 libsystem_kernel.dylib semaphore_wait_trap + 8,stop reason = signal SIGINT
    frame #0: 0x30e06054 libsystem_kernel.dylib semaphore_wait_trap + 8
    frame #1: 0x32c614f4 libdispatch.dylib _dispatch_thread_semaphore_wait$VARIANT$mp + 12   
    frame #2: 0x32c5f6a4 libdispatch.dylib _dispatch_barrier_sync_f_slow + 92   
    frame #3: 0x32c5f61e libdispatch.dylib dispatch_barrier_sync_f$VARIANT$mp + 22
    frame #4: 0x32c5f266 libdispatch.dylib dispatch_sync_f$VARIANT$mp + 18
    frame #5: 0x35860564 CoreData _perform + 160

(lldb)帧选择5

frame #5: 0x35860564 CoreData _perform + 160
    CoreData _perform + 160:
    -> 0x35860564:  add    sp,#12
       0x35860566:  pop    {r4,r5,r7,pc}

    CoreData -[NSManagedObjectContext(_NestedContextSupport) executeRequest:withContext:error:]:
       0x35860568:  push   {r4,r6,lr}
       0x3586056a:  add    r7,sp,#12

(lldb)反汇编-f

CoreData _perform:
        0x358604c4:  push   {r4,lr}

    ... snipped ...

        0x35860560:  blx    0x35938bf4                ; symbol stub for: dispatch_sync_f

    -> 0x35860564:  add    sp,pc}

另一种解决方法是可行的.在 – [ChElementTVC viewDidAppear：]中编码fetchedResultsController.delegate恢复也有效地延迟了主队列上的此设置.

另一种解决方法是在项目保存完成后在完成块中执行导航弹出：

#define SAVEDOCWITHCOMPLETION(__DOC__,__COMPLETION_BLOCK__)[ProjectDocumentHelper saveProjectDocument:__DOC__ completionHandler:__COMPLETION_BLOCK__]

    void (^completionBlock)(BOOL) = ^(BOOL success) {
        [self.navigationController popViewControllerAnimated:YES];
    };

    SAVEDOCWITHCOMPLETION(THE_CURRENT_PROJECT_DOCUMENT,completionBlock);

我认为保存操作在后台与主队列上的委托恢复同时运行,但我不知道如何检查/证明/反驳该理论.

那么,有人可以解释发生了什么,在这种情况下最好的做法是什么？此外,赞赏研究参考.

解决方法

我最终实现了第三种方法,即使用完成块保存文档以序列化与CoreData存储交互的事务.