ios - 变量不在 block 外更新

Question

下面是我的代码。我正在尝试根据分数对可变数组进行排序。我已经在字典中为每个酒吧的标题分配了分数。但是当我调用 keysortedbyvalue 时，它​​对它们进行了很好的排序。如日志(下方)所示，但由于某种原因，此更改未反射(reflect)在 block 外的变量 sortedPubArray 中。有什么想法吗？

注意:我为 sortedPubArray 创建了一个单独的变量，因此更容易看出它没有反射(reflect)在 block 外的变量中。

//Method to get us the pubs for a specific user
+(NSMutableArray *)PubsForUser:(PFUser *)passedUser
{
//First things first let's create our array to represent the array of pubs which should be returned when our query finally executes (down there somewhere)
__block NSMutableArray *pubArray = [[NSMutableArray alloc] init];
//And the sorted version, which we will ultimately return
__block NSMutableArray *sortedPubArray = [[NSMutableArray alloc] init];

//Get the user passed in so we can get there preferences
PFUser *currentUser = passedUser;
//Get all the keys into a local array so we can traverse through and find out what user likes
NSMutableArray *allTypes = [NSMutableArray arrayWithArray:[currentUser allKeys]];
//first we have to remove the username and email keys from the array
[allTypes removeObject:@"username"];
[allTypes removeObject:@"email"];

NSMutableArray *userTypes = [[NSMutableArray alloc] init];

//Now traverse through array and if the type is set to true add it to our local userTypes array
//For each category in the user
for (NSString *typeKey in allTypes) {
    //If the user has selected this category as one of their choices
    if ([currentUser[typeKey]  isEqual: @YES]) {
        //Then add the category name (ie the key) to our local property representing the users choosen style of pubs
        [userTypes addObject:typeKey];
    }
}

//Create our array of queries
NSMutableArray *queryArray = [[NSMutableArray alloc] init];

//Traverse through our array of user categories and create a query for each one.
for (NSString *style in userTypes) {

    //Set up Parse query
    PFQuery *pubQuery = [PFQuery queryWithClassName:@"Pub"];
    [pubQuery whereKey:style equalTo:@YES];

    //Add query to array of queries
    [queryArray addObject:pubQuery];

}



//Now create final query which will contain each of our subqueries
PFQuery *totalQuery = [PFQuery orQueryWithSubqueries:queryArray];
[totalQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    //Do error checking
    if (error) {
        //Log out error message to user
        UIAlertView *queryErrorAlert = [[UIAlertView alloc] initWithTitle:@"Whoops!" message:@"Houston there's a problem! Try again in 5 minutes or drop us an email if this keep's happening at gordon@outerapp.com" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
        [queryErrorAlert show];
    } //It worked!
    else {
        //So now we have an array of PFObjects ie Pubs!
        [pubArray addObjectsFromArray:objects];

        //Now sort the array by number of hits, ie number of categories the same. So that the pub/club most tailored to the users tastes is top of the array

        //First create array to contain all pub categories so we're not comparing user's restaurant categories with the pub (this could crash the app as you can see below, we'd be trying toaccessing properties of the pub which don't exist
        NSMutableArray *pubTypes = [NSMutableArray arrayWithArray:[[objects objectAtIndex:0] allKeys]];
        //And set up a dictionary to keep the score of each pub (score is how many of the same types it has as user
        NSMutableDictionary *pubScores = [[NSMutableDictionary alloc] init];


        //This requires us to iterate through the array assinging a value to the variable representing the "likeness" of the pub to the user. So the higher the score, the more hits
        for (PFObject *pub in pubArray) {

            int pubScore = 0;

            //So now we should calculate the total score, by iterating through and adding 1 each time it's true
            for (NSString *category in pubTypes) {

                //Test if the pub and the user's category choice is the same. ie this will iterate through student, theme, gastropub etc and everytime they are the same we add 1, and different -1
                if (pub[category] == currentUser[category]){

                    //They're the same so add to the score
                    pubScore++;

                } //If they're not the same
                else {

                    //Subtract one
                    pubScore--;

                }
            }

            //Now store the score of the pub in our dictionary
            [pubScores setValue:[NSNumber numberWithInt:pubScore] forKey:[pub objectForKey:@"PubName"]];
        }



        //And now finally simply sort the array by score (the first with the highest score), so that the pub with the best rating is at the top of the feed
        //To do this, we can use an inbuilt NSMutableDictionary method to output our keys in descending order of magnitude
        sortedPubArray = [NSMutableArray arrayWithArray:[pubScores keysSortedByValueUsingComparator:^NSComparisonResult(id obj1, id obj2) {

            //If it's greater, put it above
            if ([obj1 integerValue] > [obj2 integerValue])
                return (NSComparisonResult)NSOrderedAscending;
            //If lower, then below
            if ([obj1 integerValue] < [obj2 integerValue])
                return (NSComparisonResult)NSOrderedDescending;

            return (NSComparisonResult)NSOrderedSame;

        }]];

        NSLog(@"FinishedBlockLog: %@", sortedPubArray);
    }
}];

NSLog(@"FinishedMethodLog: %@", sortedPubArray);
return pubArray;
}

日志:

2013-11-09 16:35:27.722 外部 [5037:70b] FinishedMethodLog: ( )

2013-11-09 16:35:27.723 Outer[5037:70b] 分配给方法返回的变量:( )

2013-11-09 16:35:27.925 外部 [5037:70b] FinishedBlockLog: ( 测试发布， “蜡质奥康纳”， “方舟”， “帐房”， radio )

2013-11-09 16:35:32.590 Outer[5037:70b] 我创建的按钮用于注销变量，该变量返回分配给它的方法:( )

所以我们可以看到，当 block 完成时，当我注销 sortedPubArray 时，它仍然没有反射(reflect)在 block 外的 sortedPubArray 中。任何帮助将不胜感激

Answer 1

首先，没有理由为pubArray 使用__block。您没有更改 block 中的任何一个变量(对变量引用的对象调用方法不会更改变量值)。对于 sortedPubArray，没有理由为该变量分配一个空的可变数组，因为您只是稍后在代码中重新分配给所述变量。

顺便说一句:PubsForUser: 应该是 pubsForUser:。方法总是以小写字母开头。

不过，问题的真正根源在于并发模型。您正在后台执行某些操作，但希望结果立即在前台可用。考虑您的日志语句:

2013-11-09 16:35:27.722 Outer[5037:70b] FinishedMethodLog: ( )

2013-11-09 16:35:27.723 Outer[5037:70b] Variable assigned to the return of the method: ( )

2013-11-09 16:35:27.925 Outer[5037:70b] FinishedBlockLog: ( TestPub, "Waxy O'Connors", "The Ark", "The Counting House", Radio )

注意时间戳；您的 pubsForUserMethod: 在 16:35:27.722 完成，但后台执行直到 16:35:27.925 才完成，~200ms稍后。

您需要同步执行此操作或在后台 block 的末尾放置一个回调，告诉您的主线程东西已准备就绪(最有可能)。

dispatch_async(dispatch_get_main_queue(), ^{ [myUIThingy youManYourStuffIsReady: sortedPubArray]; });

请注意，简单地将内容扔到后台执行并不是一个很好的并发模型。您需要非常仔细地考虑您的数据、同步点和线程之间的一致性。

---

现在，如果您真的正在使用返回的 pubsArray(这回避了为什么 sortedPubsArray 存在的问题)并且仍然想知道为什么您的 UI 状态不会更新...

...您的 UI 状态不会自行更新。您仍然需要在该后台任务中进行回调，让 UI 知道它应该重新加载显示所述数组内容的 TableView 。

并且您需要确保没有任何内容复制 pubsArray 并丢弃原始值。让 pubsForUser: 返回 (void) 并可能将一个 block 作为参数，在一切都说完之后执行。