尝试在iOS中使用Singleton模式重构网络方法

我的应用程序中有以下方法,它使用NSURLSession从 JSON格式的Web服务中提取电影数据:

- (void) downloadMovieData {

    //this is just a visual cue to show that processing is being done
    [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

    //creating the request
    NSURL *url = [NSURL URLWithString:kMovieURL];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];

    //creating the session
    self.config = [NSURLSessionConfiguration defaultSessionConfiguration];
    self.session = [NSURLSession sessionWithConfiguration:self.config];

    //the object that makes the call to the web service using the request and the session, and returns a response or an error
    NSURLSessionDataTask *task = [self.session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

        //At this point a response has been received, so we can turn the indicator off
        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

        //I am casting the response to an NSHTTPURLResponse so I can check the status code of the response
        NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;

        //a status code of 200 means a successful connection with the web service
        if (httpResponse.statusCode == 200) {

            dispatch_async(dispatch_get_main_queue(), ^{
                NSLog(@"Success!");
                //I send the data that was received from the response to the method so that the JSON data is extracted
                [self populateArray:data];
            });

        } else {
            NSString *result = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            NSLog(@"Received HTTP %ld: %@", (long)httpResponse.statusCode, result);
        }
    }];

    [task resume];

}

- (void) populateArray: (NSData *)data {

    NSError *jsonError;
    NSDictionary *response =[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&jsonError];

    if (response) {
        self.movieObjects = response[@"movies"];
        NSLog(@"The movie objects are: %@", self.movieObjects);
        [self.tableView reloadData];
    } else {
        NSLog(@"ERROR: %@", jsonError);
    }
}

上面的代码工作正常.没有问题.但是,我现在要做的是重构我的代码,以便在包含我的UITableView委托方法的类中包含所有网络代码,我想将代码移动到一个单独的类中,该类使用Singleton方法更好地分离代码我有以下框架代码,如下所示:

#import "Networker.h"

@implementation Networker

+ (NSURLSession *)dataSession {
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    });
    return session;
}

+ (void)fetchContentsOfURL:(NSURL *)url completion:(void (^)(NSData *data, NSError *error)) completionHandler {

    NSURLSessionDataTask *dataTask = [[self dataSession] dataTaskWithURL:url
                      completionHandler: ^(NSData *data, NSURLResponse *response, NSError *error) {

         if (completionHandler == nil) return;

         if (error) {
             completionHandler(nil, error);
             return;
         }
         completionHandler(data, nil);
     }];

    [dataTask resume];
}

我完全理解Singleton类没有问题.这不是问题.我的问题是了解如何理解此方法的“完成处理程序”部分:

+ (void)fetchContentsOfURL:(NSURL *)url completion:(void (^)(NSData *data, NSError *error)) completionHandler {}

我想做的是将我在方法中的代码“downloadMovieData”移动到“fetchContentsOfURL”中,然后返回一个NSData对象,然后我可以使用它来填充我的调用类中的UITableView.但是,在这样做的过程中,我想确保我理解这个新方法的“completionHandler”部分发生了什么.我怎样才能做到这一点?

最佳答案 你不能移动它以便它返回,因为它是异步的.从技术上讲你可以,但你在等待时阻止主线程这是坏事.

相反,您只需用对单例的调用替换当前代码,在完成块中调用其他方法来处理数据.

有几点需要注意:

>在调用某人完成块之前调出main中的main是个好主意,这样你就不需要函数的所有用户记住这样做了
>它不需要是单例,您可以在每次需要时实例化类的副本或使用依赖注入,这两者通常都提供更好的体系结构

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