ios – NSProgress奇怪的行为

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我有一个由几个子任务组成的大任务.我想为这项重大任务添加进度报告.
为此,我想使用NSProgress,并根据类文档,我可以通过使用其子 – 父机制来做这种子任务进展.

所以为了简化它,让我说我有一个由一个子任务组成的大任务(当然在现实生活中会有更多的子任务).这就是我所做的:

#define kFractionCompletedKeyPath @"fractionCompleted"  

- (void)runBigTask {
    _progress = [NSProgress progressWithTotalUnitCount:100]; // 100 is arbitrary 

    [_progress addObserver:self
                forKeyPath:kFractionCompletedKeyPath
                   options:NSKeyValueObservingOptionNew
                   context:NULL];

    [_progress becomeCurrentWithPendingUnitCount:100]; 
    [self subTask];
    [_progress resignCurrent];
} 

- (void)subTask {
    NSManagedObjectContext *parentContext = self.managedObjectContext; // self is AppDelegate in this example
    NSManagedObjectContext *bgContext = [[NSManagedObjectContext alloc]initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [bgContext setParentContext:parentContext];

    [bgContext performBlockAndWait:^{
        NSInteger totalUnit = 1000;
        NSInteger completedUnits = 0;
        NSProgress *subProgress = [NSProgress progressWithTotalUnitCount:totalUnit];

        for (int i=0; i < totalUnit; i++) {   

            // run some Core Data related code...  

            completedUnits++;
            subProgress.completedUnitCount = completedUnits;
        }
    }];
}      

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:kFractionCompletedKeyPath]) {
        if ([object isKindOfClass:[NSProgress class]]) {
            NSProgress *progress = (NSProgress *)object;
            NSLog(@"progress… %f",progress.fractionCompleted);
        }
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

如您所见,子任务使用背景上下文来运行一些与Core Data相关的代码,而后台上下文使用主上下文作为其父上下文.
这会导致进度的“fractionCompleted”属性出现一些奇怪的KVO.

这是印刷品:

progress… 1.000000 // why???
progress… 0.500000 // why?????
progress… 1.000000 // why???????
progress… 0.666650 // why???????????
progress… 0.666990
progress… 0.667320
progress… 0.667660
progress… 0.667990
progress… 0.668320
...  
progress… 1.000000

如你所见,打印以1.0,0.5和1.0开始,然后是0.66?
从这里它正常,并像我期望的那样进入1.0.

我试图理解为什么会发生这种情况,我注意到如果我从背景上下文中删除父上下文,它工作正常!我从0.0升到1.0.

任何想法为什么会发生这种情况?我该如何解决这个问题?

添加了一个非常simple project以演示此问题(您可以删除setParentContext:调用以查看它没有它可以正常工作)

解决方法

发生这种情况时的堆栈跟踪如下所示:
(lldb) bt
* thread #1: tid = 0x81f2,0x0000000105bffcda Foundation`-[NSProgress setTotalUnitCount:],queue = 'com.apple.main-thread',stop reason = breakpoint 1.1
  * frame #0: 0x0000000105bffcda Foundation`-[NSProgress setTotalUnitCount:]
    frame #1: 0x0000000105bfeb1b Foundation`+[NSProgress progressWithTotalUnitCount:] + 87
    frame #2: 0x0000000105a31213 Foundation`_NSReadBytesFromFileWithExtendedAttributes + 287
    frame #3: 0x0000000105a3109d Foundation`-[NSData(NSData) initWithContentsOfFile:] + 89
    frame #4: 0x0000000105a30b40 Foundation`+[NSDictionary(NSDictionary) newWithContentsOf:immutable:] + 101
    frame #5: 0x0000000105a5622a Foundation`+[NSDictionary(NSDictionary) dictionaryWithContentsOfFile:] + 45
    frame #6: 0x00000001043c4560 CoreData`-[NSManagedObjectModelBundle initWithPath:] + 224
    frame #7: 0x00000001043c42ed CoreData`-[NSManagedObjectModel initWithContentsOfURL:] + 205
    frame #8: 0x00000001040f723f CDProgress`-[AppDelegate managedObjectModel](self=0x00007fbe48c21f90,_cmd=0x000000010459b37b) + 223 at AppDelegate.m:127
    frame #9: 0x00000001040f7384 CDProgress`-[AppDelegate persistentStoreCoordinator](self=0x00007fbe48c21f90,_cmd=0x000000010459c1cb) + 228 at AppDelegate.m:142
    frame #10: 0x00000001040f708c CDProgress`-[AppDelegate managedObjectContext](self=0x00007fbe48c21f90,_cmd=0x0000000104598f0d) + 92 at AppDelegate.m:111
    frame #11: 0x00000001040f6bdb CDProgress`-[AppDelegate subTask](self=0x00007fbe48c21f90,_cmd=0x00000001040f7997) + 43 at AppDelegate.m:45
    frame #12: 0x00000001040f6b89 CDProgress`-[AppDelegate runTask](self=0x00007fbe48c21f90,_cmd=0x00000001040f7928) + 233 at AppDelegate.m:40
    frame #13: 0x00000001040f6a4b CDProgress`-[AppDelegate application:didFinishLaunchingWithOptions:](self=0x00007fbe48c21f90,_cmd=0x0000000104f5dba9,application=0x00007fbe48f00fb0,launchOptions=0x0000000000000000) + 571 at AppDelegate.m:26
    frame #14: 0x000000010477c5a5 UIKit`-[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 234
    frame #15: 0x000000010477d0ec UIKit`-[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 2463
    frame #16: 0x000000010477fe5c UIKit`-[UIApplication _runWithMainScene:transitionContext:completion:] + 1350
    frame #17: 0x000000010477ed22 UIKit`-[UIApplication workspaceDidEndTransaction:] + 179
    frame #18: 0x00000001088092a3 FrontBoardServices`__31-[FBSSerialQueue performAsync:]_block_invoke + 16
    frame #19: 0x000000010615fabc CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    frame #20: 0x0000000106155805 CoreFoundation`__CFRunLoopDoBlocks + 341
    frame #21: 0x00000001061555c5 CoreFoundation`__CFRunLoopRun + 2389
    frame #22: 0x0000000106154a06 CoreFoundation`CFRunLoopRunSpecific + 470
    frame #23: 0x000000010477e799 UIKit`-[UIApplication _run] + 413
    frame #24: 0x0000000104781550 UIKit`UIApplicationMain + 1282
    frame #25: 0x00000001040f7793 CDProgress`main(argc=1,argv=0x00007fff5bb09308) + 115 at main.m:16
    frame #26: 0x000000010686f145 libdyld.dylib`start + 1
(lldb)

这里发生的是当加载模型时,它正在读取一个plist文件.读取plist文件调用 – [NSData initWithContentsOfFile:],它在主线程上调用[NSProgress progressWithTotalUnitCount:].作为release notes point out,这将创建一个NSProgress,它是当前进步的孩子. initWithContentsOfFile:实际上正在执行此操作,并创建您创建的NSProgress的新子项:

<NSProgress: 0x7f9353596f80> : Parent: 0x0 / Fraction completed: 0.0000 / Completed: 0 of 1  
   <_NSProgressGroup: 0x7f935601a0d0> : Portion of parent: 100 Children: 1
      <NSProgress: 0x7f935600bf50> : Parent: 0x7f9353596f80 / Fraction completed: 0.0000 / Completed: 0 of 0

这里发生的事情是在你面前增加了额外的工作.此时,它对您要添加的其他工作一无所知. initWithContentsOfFile添加的子项:完成,从树中删除,然后开始添加您的工作.

当前进度从0开始,并且变为100%.您看到100%是因为您的KVO选项不包含NSKeyValueObservingOptionInitial.

NSData添加从0开始的子进度,并转到100%.

您的核心数据任务会添加一个从0开始并且(最终)达到100%的子项.

然而,这里的一个关键点是你正在使用performBlockAndWait:.虽然块本身在专用队列上运行,但此方法将阻止调用线程,这将延迟您的KVO通知. performBlockAndWait:如果可能的话,还将重用调用线程,这是需要注意的事情.

如果您编辑subTask方法以使用NSProgress包装自身作为整个工作单元的父级,最后重新调整当前状态,您可能会获得更接近您期望的行为:

- (void)subTask {
    NSProgress  *progress   = [NSProgress progressWithTotalUnitCount:1];
    NSManagedObjectContext *parentContext = self.managedObjectContext;
    NSManagedObjectContext *bgContext = [[NSManagedObjectContext alloc]initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [bgContext setParentContext:parentContext];

    [progress becomeCurrentWithPendingUnitCount:1];
    [bgContext performBlock:^{

    ... stuff

    [progress resignCurrent];
}

NSProgress可能有点困难,但有了一些经验,它会变得更容易.我承诺!

原文链接:https://www.f2er.com/iOS/328411.html

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