1

我正在尝试从 JSON 请求中获取一个 CLLocation 数组的值，并将它们分配给一个 NSMutable 数组的属性。这里是相关的控制器代码：

- (void)viewDidLoad
{
    [super viewDidLoad];
    //using a background thread to receive the json call so that the UI doesn't stall
    dispatch_async(directionQueue, ^{
        NSData* data = [NSData dataWithContentsOfURL:openMapURL];
        [self performSelectorOnMainThread:@selector(fetchedData:)
                               withObject:data waitUntilDone:YES];
    });

    NSLog(@"%@", self.coordinates);
}

- (void)fetchedData:(NSData *)responseData 
{
    //parse out the json data
    NSError* error;
    NSDictionary* json = [NSJSONSerialization
                          JSONObjectWithData:responseData //turn the data from the json request into a gigantic dictionary
                                     options:0
                                       error:&error];

    NSDictionary* route = [json objectForKey:@"route"]; //created a dictionary out of the contents of route
    NSDictionary* shape = [route objectForKey:@"shape"]; //create a sub-dictionary with the contents of shape
    NSArray* shapePoints = [shape objectForKey:@"shapePoints"]; //turn shapePoints object into an NSArray

    //Loop to turn the array of coordinate strings into CLLocation array
    NSMutableArray* locations = [[NSMutableArray alloc] init];
    NSUInteger i;
    for (i = 0; i < ([shapePoints count])/2 ; i = i+2) 
    {
        [locations addObject: [[CLLocation alloc]
                               initWithLatitude:[[shapePoints objectAtIndex:i] floatValue]
                               longitude:[[shapePoints objectAtIndex:i+1] floatValue]
                               ]];
    }

    //When I NSLog within the function, the array has the correct data. But in viewDidLoad
    //the array is null
    [self.coordinates initWithArray:locations copyItems:YES];   
}

为什么这个数组在 viewDidLoad 中变成了 null？

4

2 回答 2

5

您正在使用 GCD 发出异步请求以从viewDidLoad方法中获取数据。由于它是异步的，它不会阻塞该viewDidLoad方法。一旦异步请求下载并被解析，数组就会被填充。这就是您的数组nil位于viewDidLoad.

如果在数据下载并填充数组之前您的 UI 看起来是空白的，您可以选择显示活动指示器。这将使应用程序用户知道某些活动正在进行。

希望有帮助！

于 2013-09-06T06:31:31.463 回答
0

可能您需要做的是等待该过程完成并且该块实际上不知道您在其中做什么，或者我会说内部函数没有被块捕获。所以使用数组（调用函数仅在块完成后才使用您的数组）。

- (void)fetchedData:(NSData *)responseData 
{
    //parse out the json data
    NSError* error;
    NSDictionary* json = [NSJSONSerialization
                          JSONObjectWithData:responseData //turn the data from the json request into a gigantic dictionary
                          options:0
                          error:&error];

    NSDictionary* route = [json objectForKey:@"route"]; //created a dictionary out of the contents of route
    NSDictionary* shape = [route objectForKey:@"shape"]; //create a sub-dictionary with the contents of shape
    NSArray* shapePoints = [shape objectForKey:@"shapePoints"]; //turn shapePoints object into an NSArray

    //Loop to turn the array of coordinate strings into CLLocation array
    NSMutableArray* locations = [[NSMutableArray alloc] init];
    NSUInteger i;
    for (i = 0; i < ([shapePoints count])/2 ; i = i+2) 
    {
        [locations addObject: [[CLLocation alloc]
                               initWithLatitude:[[shapePoints objectAtIndex:i] floatValue]
                               longitude:[[shapePoints objectAtIndex:i+1] floatValue]
                               ]];
    }

    //When I NSLog within the function, the array has the correct data. But in viewDidLoad
    //the array is null

    [self.coordinates initWithArray:locations copyItems:YES];

//这里调用访问 self.coordinates 数组的方法。

NSLog(@"%@", self.coordinates);

}
于 2013-09-06T06:44:39.120 回答