ios - 通过引用传递的对象不起作用？

Question

iOS 开发新手。所以我传递了一个带有对象地址的方法，认为这样做会改变同一个对象:

_posts = [[NSMutableArray alloc]init];

FetchPosts *dataControl = [[FetchPosts alloc]init];
[dataControl accessPosts: _posts];

下面的代码接收传入的对象。

-(void)accessPosts: (NSMutableArray *)transition {
    //access the posts here.

    _readablePosts = transition;

    NSURL *url = [NSURL URLWithString: @"private"];
    NSURLRequest *request = [[NSURLRequest alloc]initWithURL:url];
    [NSURLConnection connectionWithRequest:request delegate:self];

}

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    _jsonPosts = [[NSMutableData alloc]init];
    [_jsonPosts appendData: data];
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection {

    NSError *error;
    NSArray *jsonArray = [NSJSONSerialization JSONObjectWithData:_jsonPosts options:NSJSONReadingAllowFragments error:&error];

    for(int i = 0; i<jsonArray.count; i++) {

        NSDictionary *element = jsonArray[i];

        Posts *newPost = [[Posts alloc]init];

        newPost.title = element[@"Title"];
        newPost.discovery = element[@"Discovery"];
        newPost.summary = element[@"Description"];
        newPost.contact = element[@"Contact"];

        [_readablePosts addObject:newPost];

    }
}

在我看来，一切似乎都已就位。然而，当我返回到我原来的方法时，_posts 并没有像它应该的那样保存正确的对象。我在这里的指针有问题吗？

Answer 1

你调用这个方法-

[dataControl accessPosts: _posts];

调用

[NSURLConnection connectionWithRequest:request delegate:self];

这是一个非阻塞方法，因此它会立即返回并在检索到要处理的数据后调用委托(delegate)方法。

如果您在 accessPosts 返回时立即访问 _posts，您将在从网络检索数据之前执行此操作。

只有在 connectionDidFinishLoading: 被调用后，您才能访问 _posts 中的数据

一种解决方案是使用委托(delegate)模式。您的外部代码会将自己设置为 FetchPosts 对象的委托(delegate)，并从 connectionDidFinishLoading 调用委托(delegate)方法。这不仅可以解决您的异步问题，还可以避免 NSMutableArray 的潜在不安全更新，并避免使用副作用。

您需要在 FetchPosts.h 中创建一个适当的协议(protocol)，然后您可以使用类似 -

-(void) requestPosts {
    FetchPosts *dataControl = [[FetchPosts alloc]init];
    dataControl.delegate=self;
    [dataControl accessPosts];
}

-(void) didReceiveNewPosts:(NSArray *)posts {
   // Do something with posts
}

FetchPosts.m

-(void)connectionDidFinishLoading:(NSURLConnection *)connection {

    NSError *error;
    NSArray *jsonArray = [NSJSONSerialization JSONObjectWithData:_jsonPosts options:NSJSONReadingAllowFragments error:&error];

    for(int i = 0; i<jsonArray.count; i++) {

        NSDictionary *element = jsonArray[i];

        Posts *newPost = [[Posts alloc]init];

        newPost.title = element[@"Title"];
        newPost.discovery = element[@"Discovery"];
        newPost.summary = element[@"Description"];
        newPost.contact = element[@"Contact"];

        [_readablePosts addObject:newPost];

    }

    if ([self.delegate respondsToSelector:@selector(didReceiveNewPosts:)]) {
        [self.delegate didReceiveNewPosts:_readablePosts];
    }
}