使用NSCondition等待异步方法

Question

我正在通过互联网异步下载四个plist文件。我需要等到所有四个文件都下载完毕，直到我在第一次运行时推送UIViewController，或者在所有后续运行中刷新数据并重新加载我的所有UITableViews。

在第一次运行时，一切都运行得很完美。在刷新时，所有四个url请求都会被调用并启动，但永远不会调用它们的完成块或失败块，并且UI会冻结。这很奇怪，因为我在后台线程中执行所有操作。我还不能弄清楚为什么会发生这种情况。

第一个load和refresh方法以相同的方式调用四个“更新”方法，并以相同的方式使用NSCondition。

对于第一次运行：

- (void)loadContentForProgram:(NSString *)programPath
{
    NSLog(@"Start Load Program");
    AppDelegate *myDelegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
    hud = [[MBProgressHUD alloc] initWithView:myDelegate.window];
    [myDelegate.window addSubview:hud];
    hud.labelText = @"Loading...";
    hud.detailsLabelText = @"Loading Data";
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        //Do stuff here to load data from files

        //Update From online files
        hud.detailsLabelText = @"Updating Live Data";
        resultLock = NO;
        progressLock = NO;
        recallLock = NO;
        stageLock = NO;

        condition = [[NSCondition alloc] init];
        [condition lock];

        [self updateCurrentCompsText];
        [self updateCompetitionResults];
        [self updateCompetitionRecalls];
        [self updateCompetitionProgress];


        while (!resultLock) {
            [condition wait];
        }
        NSLog(@"Unlock");
        while (!stageLock) {
            [condition wait];
        }
        NSLog(@"Unlock");
        while (!recallLock) {
            [condition wait];
        }
        NSLog(@"Unlock");
        while (!progressLock) {
            [condition wait];
        }
        NSLog(@"Unlock");
        [condition unlock];
        updateInProgress = NO;
        //Reset Refresh controls and table views
        self.refreshControlsArray = [[NSMutableArray alloc] init];
        self.tableViewsArray = [[NSMutableArray alloc] init];
        NSLog(@"Finished Loading Program");
        [[NSNotificationCenter defaultCenter] postNotificationName:@"WMSOFinishedLoadingProgramData" object:nil]; //Pushes view controller
        dispatch_async(dispatch_get_main_queue(), ^{
            [MBProgressHUD hideHUDForView:myDelegate.window animated:YES];
        });
    });
}

刷新数据时：

- (void)updateProgramContent
{
    if (!updateInProgress) {
        updateInProgress = YES;
        for (int i = 0; i < self.refreshControlsArray.count; i++) {
            if (!((UIRefreshControl *)self.refreshControlsArray[i]).refreshing) {
                [self.refreshControlsArray[i] beginRefreshing];
                [self.tableViewsArray[i] setContentOffset:CGPointMake(0.0, 0.0) animated:YES];
            }
        }

        resultLock = NO;
        stageLock = NO;
        recallLock = NO;
        progressLock = NO;
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

            condition = [[NSCondition alloc] init];
            [condition lock];

            [self updateCompetitionProgress];
            [self updateCompetitionRecalls];
            [self updateCompetitionResults];
            [self updateCurrentCompsText];

            while (!resultLock) {
                [condition wait];
            }
            NSLog(@"Unlock");
            while (!stageLock) {
                [condition wait];
            }
            NSLog(@"Unlock");
            while (!recallLock) {
                [condition wait];
            }
            NSLog(@"Unlock");
            while (!progressLock) {
                [condition wait];
            }
            NSLog(@"Unlock");
            [condition unlock];
        });

        for (int i = 0; i < self.refreshControlsArray.count; i++) {
            [self.refreshControlsArray[i] performSelector:@selector(endRefreshing) withObject:nil afterDelay:1.0];
            [self.tableViewsArray[i] performSelector:@selector(reloadData) withObject:nil afterDelay:1.0];
        }
        updateInProgress = NO;
    }
}

下面的块出现在上面的每个加载方法中，对应于将下载和更新特定数据的方法。

[self updateCompetitionProgress];
[self updateCompetitionRecalls];
[self updateCompetitionResults];
[self updateCurrentCompsText];

它运行：

- (void)updateCompetitionResults
{
    __block NSDictionary *competitionResultsData = nil;
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"Some URL",[self.programName stringByReplacingOccurrencesOfString:@" " withString:@"%20"]]] cachePolicy:NSURLCacheStorageNotAllowed timeoutInterval:20.0];
    AFPropertyListRequestOperation *operation = [AFPropertyListRequestOperation propertyListRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id propertyList) {
        competitionResultsData = (NSDictionary *)propertyList;
        [competitionResultsData writeToFile:[@"SOME LOCAL PATH"] atomically:NO];
        [self updateCompetitionResultsWithDictionary:competitionResultsData];
    } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id propertyList) {
        competitionResultsData = [NSDictionary dictionaryWithContentsOfFile:[@"SOME LOCAL PATH"]];
        NSLog(@"Failed to retreive competition results: %@", error);
        [self updateCompetitionResultsWithDictionary:competitionResultsData];
    }];
    [operation start];
}

完成块和失败块调用相同的方法来更新数据

- (void)updateCompetitionResultsWithDictionary:(NSDictionary *)competitionResultsData
{
    //Do Stuff with the data here
    resultLock = YES;
    [condition signal];
}

那么，为什么这在第一次运行时有效，但在随后的任何一次运行中都不起作用？

Answer 1

正如我在上面的评论中提到的，最明显的问题是您在初始化condition之前调用了使用condition的方法。确保在开始调用updateCompetitionResults之前初始化condition，等等。

就更彻底的变化而言，我可能建议完全淘汰NSCondition，并使用操作队列：

1. 我可以使用NSOperationQueue (或者您也可以使用调度组，如果您愿意，但我喜欢操作队列的功能，它可以配置您可以操作的并发操作数……此外，如果你想取消操作，我认为NSOperationQueue也提供了一些很好的功能)。然后，您可以将每个下载和处理定义为一个单独的NSOperation (每个下载应该同步进行，因为它们在一个操作队列中运行，因此您可以获得异步操作的好处，但您可以在下载完成后立即启动后处理)。然后，您只需将它们排队以异步运行，但定义一个依赖于其他四个的最终操作将在四个下载完成后立即启动。(顺便说一句，我使用的是NSBlockOperation，它为NSOperation对象提供了块功能，但是您可以按照自己的意愿来做。)
2. 和您的updateProgramContent可能会异步下载，但它会依次处理四个下载的文件。因此，如果第一个下载需要一段时间才能下载，它将阻止其他下载的后处理。相反，我喜欢将四个plist文件中每个文件的下载和后处理都封装在一个NSOperation中。
3. 而不是使用AFNetworking (我通常是它的铁杆粉丝)与plist相关的方法，我可能倾向于使用NSDictionary和NSArray特性，它们允许你从web上下载一个plist并将它们加载到适当的结构中。这些dictionaryWithContentsOfURL和arrayWithContentsOfURL是同步运行的，但是因为我们是在后台操作中执行的，所以一切都像您所希望的那样异步运行。这也绕过了将它们保存到文件的步骤。如果您希望将它们保存到Documents目录下的文件中，也可以很容易地完成。显然，如果你在plist文件的下载中做了一些复杂的事情(例如，你的服务器正在进行一些挑战-响应身份验证)，你就不能使用方便的NSDictionary和NSArray方法。但是如果你不需要所有这些，简单的NSDictionary和NSArray方法，___WithContentsOfURL会让你的生活变得非常简单。

综合起来，它可能看起来像这样：

@interface ViewController ()

@property (nonatomic, strong) NSArray *competitions;
@property (nonatomic, strong) NSDictionary *competitionResults;
@property (nonatomic, strong) NSDictionary *competitionRecalls;
@property (nonatomic, strong) NSDictionary *competitionProgress;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    [self transfer];
}

- (void)allTransfersComplete
{
    BOOL success;

    if (self.competitions == nil)
    {
        success = FALSE;
        NSLog(@"Unable to download competitions");
    }

    if (self.competitionResults == nil)
    {
        success = FALSE;
        NSLog(@"Unable to download results");
    }

    if (self.competitionRecalls == nil)
    {
        success = FALSE;
        NSLog(@"Unable to download recalls");
    }

    if (self.competitionProgress == nil)
    {
        success = FALSE;
        NSLog(@"Unable to download progress");
    }

    if (success)
    {
        NSLog(@"all done successfully");
    }
    else
    {
        NSLog(@"one or more failed");
    }
}

- (void)transfer
{
    NSURL *baseUrl = [NSURL URLWithString:@"http://insert.your.base.url.here/competitions"];
    NSURL *competitionsUrl = [baseUrl URLByAppendingPathComponent:@"competitions.plist"];
    NSURL *competitionResultsUrl = [baseUrl URLByAppendingPathComponent:@"competitionresults.plist"];
    NSURL *competitionRecallsUrl = [baseUrl URLByAppendingPathComponent:@"competitionrecalls.plist"];
    NSURL *competitionProgressUrl = [baseUrl URLByAppendingPathComponent:@"competitionprogress.plist"];

    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    queue.maxConcurrentOperationCount = 4; // if your server doesn't like four concurrent requests, you can ratchet this back to whatever you want

    // create operation that will be called when we're all done

    NSBlockOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{

        // any stuff that can be done in background should be done here

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{

            // any user interface stuff should be done here; I've just put this in a separate method so this method doesn't get too unwieldy

            [self allTransfersComplete];
        }];
    }];

    // a variable that we'll use as we create our four download/process operations

    NSBlockOperation *operation;

    // create competitions operation

    operation = [NSBlockOperation blockOperationWithBlock:^{

        // download the competitions and load it into the ivar
        //
        // note, if you *really* want to download this to a file, you can 
        // do that when the download is done

        self.competitions = [NSArray arrayWithContentsOfURL:competitionsUrl];

        // if you wanted to do any post-processing of the download
        // you could do it here.            
        NSLog(@"competitions = %@", self.competitions);
    }];
    [completionOperation addDependency:operation];

    // create results operation

    operation = [NSBlockOperation blockOperationWithBlock:^{

        self.competitionResults = [NSDictionary dictionaryWithContentsOfURL:competitionResultsUrl];

        NSLog(@"competitionResults = %@", self.competitionResults);
    }];
    [completionOperation addDependency:operation];

    // create recalls operation

    operation = [NSBlockOperation blockOperationWithBlock:^{

        self.competitionRecalls = [NSDictionary dictionaryWithContentsOfURL:competitionRecallsUrl];

        NSLog(@"competitionRecalls = %@", self.competitionRecalls);
    }];
    [completionOperation addDependency:operation];

    // create progress operation

    operation = [NSBlockOperation blockOperationWithBlock:^{

        self.competitionProgress = [NSDictionary dictionaryWithContentsOfURL:competitionProgressUrl];

        NSLog(@"competitionProgress = %@", self.competitionProgress);
    }];
    [completionOperation addDependency:operation];

    // queue the completion operation (which is dependent upon the other four)

    [queue addOperation:completionOperation];

    // now queue the four download and processing operations

    [queue addOperations:completionOperation.dependencies waitUntilFinished:NO];
}

@end

现在，我不知道您的plist中哪些是数组，哪些是字典(在我的示例中，我将竞赛设为数组，其余的是以竞赛id为关键字的字典)，但希望您能理解我的目的。最大限度的并发，消除NSCondition逻辑，真正充分利用NSOperationQueue，等等。

这可能很难理解，但我只是将其作为NSCondition的替代方案。如果你目前的技术还行得通，那就太好了。但以上概述了我将如何应对这样的挑战。