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iOS中的范围和线程(Scope and threading in iOS)

 我对iOS中的Web服务调用和线程都不熟悉。 我的应用程序中有一个包含tableview控件的ViewController 。 我使用通过JSON Web服务获得的数据填充表。 JSON Web服务在其自己的线程上调用，在此期间我填充了NSArray和NSDictionary 。  
 我的数组和字典似乎超出了范围，因为我的NSLog语句为数组计数返回零，即使在fetchedData中数组已完全填充。  
 有人可以解释为什么我的数组和字典对象在线程外是空的吗？  

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSString *serviceEndpoint = [NSString stringWithFormat:
                                 @"http://10.0.1.12:8888/platform/services/_login.php?un=%@&pw=%@&ref=%@",
                                 [self incomingUsername], [self incomingPassword], @"cons"];
    NSURL *url = [NSURL URLWithString:serviceEndpoint];
    dispatch_async(kBgAdsQueue, ^{
        NSData *data = [NSData dataWithContentsOfURL:url];
        [self performSelectorOnMainThread:@selector(fetchedData:) withObject:data waitUntilDone:YES];
    });

    NSLog(@"ARRAY COUNT: %d\n", [jsonArray count]);
}

-(void)fetchedData:(NSData*)responseData{
    NSError *error;
    jsonDict = [NSJSONSerialization JSONObjectWithData:responseData options:kNilOptions error:&error];
    jsonArray = [[jsonDict allKeys]sortedArrayUsingSelector:@selector(compare:)];
    for(NSString *s in jsonArray){
        NSLog(@"%@ = %@\n", s, [jsonDict objectForKey:s]);
    }
}

I am kind of new to web service calls and threading in iOS. I have a ViewController in my app that contains a tableview control. I am populating the table with data obtained via a JSON web service. The JSON web service is called on its own thread, during which I am populating an NSArray and NSDictionary.  
My array and dictionary seem like they are going out of scope since my NSLog statement is returning zero for the array count even though while in fetchedData the array is fully populated.  
Can someone offer an explanation as to why my array and dictionary objects are empty outside of the thread?  

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSString *serviceEndpoint = [NSString stringWithFormat:
                                 @"http://10.0.1.12:8888/platform/services/_login.php?un=%@&pw=%@&ref=%@",
                                 [self incomingUsername], [self incomingPassword], @"cons"];
    NSURL *url = [NSURL URLWithString:serviceEndpoint];
    dispatch_async(kBgAdsQueue, ^{
        NSData *data = [NSData dataWithContentsOfURL:url];
        [self performSelectorOnMainThread:@selector(fetchedData:) withObject:data waitUntilDone:YES];
    });

    NSLog(@"ARRAY COUNT: %d\n", [jsonArray count]);
}

-(void)fetchedData:(NSData*)responseData{
    NSError *error;
    jsonDict = [NSJSONSerialization JSONObjectWithData:responseData options:kNilOptions error:&error];
    jsonArray = [[jsonDict allKeys]sortedArrayUsingSelector:@selector(compare:)];
    for(NSString *s in jsonArray){
        NSLog(@"%@ = %@\n", s, [jsonDict objectForKey:s]);
    }
}

原文：https://stackoverflow.com/questions/16170779

更新时间：2022-02-05 20:02

最满意答案

 哇，真是个噩梦。 我找到了罪魁祸首。  
 我创建了一个TypeConverter来将List<Integer>转换为字符串（并返回），以便它可以存储在房间DB中的单个列中，而无需修改现有的DTO。 但是，当我切换到使用Long类型作为ID时，我无法在转换器中转换下面的单个泛型参数; 仔细查看以下代码：  
public class IdsListConverter {

    @TypeConverter
    public List<Long> idsFromString(String value) {
        Gson gson = new Gson();
        if (value == null || value.isEmpty()) {
            return null;
        } else {
            Type resultType = new TypeToken<List<Integer>>(){}.getType();
            return gson.fromJson(value, resultType);
        }
    }

    @TypeConverter
    public String idsToString(List<Long> ids) {
        if (ids == null) {
            return null;
        } else {
            Gson gson = new Gson();
            return gson.toJson(ids);
        }
    }

}

Wow, what a nightmare. I have found the culprit. 
I had created a TypeConverter to turn a List<Integer> to into a string (and back) so that it can be stored in a single column in the DB in room without having to modify the existing DTOs. However, when I switched over to using Long types as IDs, I failed to convert a single generic argument below in the converter; look carefully at the following code: 
public class IdsListConverter {

    @TypeConverter
    public List<Long> idsFromString(String value) {
        Gson gson = new Gson();
        if (value == null || value.isEmpty()) {
            return null;
        } else {
            Type resultType = new TypeToken<List<Integer>>(){}.getType();
            return gson.fromJson(value, resultType);
        }
    }

    @TypeConverter
    public String idsToString(List<Long> ids) {
        if (ids == null) {
            return null;
        } else {
            Gson gson = new Gson();
            return gson.toJson(ids);
        }
    }

}

iOS中的范围和线程(Scope and threading in iOS)

最满意答案

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