如何从 Swift 3 中的闭包中返回

Question

我曾经在 Objective-C 中创建一个单例类来执行我代码中的所有服务调用。但是，swift 使用闭包，我无法在 Swift 中获得相同的结果。有没有办法在 Swift 3 中做同样的事情？

@implementation ServiceManager

+(id)sharedManager {
      static ServiceManager *sharedMyManager = nil;
      static dispatch_once_t onceToken;
      dispatch_once(&onceToken, ^{
          sharedMyManager = [[self alloc] init];
      });
      return sharedMyManager;
}

-(NSURLSession *)urlSession {
if (_urlSession) {
    return _urlSession;
}

NSURLSessionConfiguration *defaultConfigObject =
 [NSURLSessionConfiguration defaultSessionConfiguration];
_urlSession = [NSURLSession sessionWithConfiguration:defaultConfigObject delegate:nil delegateQueue:[NSOperationQueue mainQueue]];
_urlSession = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
_urlSession.sessionDescription = @"net.socialInvesting.nsurlsession";

return _urlSession;
}

-(void)ExecuteRequest:(NSURLRequest *)request withCompletion:(void (^)(id result, NSError *error))completionBlock {

NSURLSessionDataTask * dataTask =[[self urlSession] dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if(error == nil) {
        id result = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&error];
        completionBlock(result,error);
    } else {
        NSLog(@"Error: %@", error.localizedDescription);
        completionBlock(nil, error);
    }
}];
[dataTask resume];
}

@end

我在下面的代码 sn-p 中调用这个函数。

[sharedServiceManager ExecuteRequest:urlRequest withCompletion:^(id result, NSError *error) {
    if (result) {
        LoginModel *model = [[LoginModel alloc]initWithDictionary:result error:NULL];
        ResponseManager *manager = [ResponseManager sharedManager];
        manager.loginResponse = model;
        completionBlock(YES,NULL);
    } else {
        completionBlock(NO,error);
    }
}];

这就是我尝试在 swift 中执行类似执行的方式。但无法返回值。

import UIKit

class ServiceManager: UIView {

var session = URLSession.shared
static let sharedSessionManager = ServiceManager()

class func sharedManager() -> ServiceManager {
    return sharedSessionManager
}

func executeGetRequest(with urlString: String, inputDictionary:[String : Any], completionHandler: @escaping () -> (Error?, [[String : Any]])) {
    let url = URL.init(string: urlString)
    let urlRequest = URLRequest(url: url!)
    let task = session.dataTask(with: urlRequest) { (data, response, error) in
        if error != nil {
            print("ERROR: could not execute request")
        } else {
            do {
                let responseDict = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any]
                if let results = responseDict!["results"] as? [[String:Any]] {
                    completionHandler // How to return results and Error from here to the calling function
                }
            } catch {
                    print("ERROR: could not retrieve response")
                }
        }
    }
    task.resume()
    }
}

任何帮助将不胜感激。

Answer 1

您只需编写completionHandler(arg1, arg2,...) 并将您想要返回的值放在括号内，就像调用函数时一样。您应该更改完成处理程序以返回其两个参数的可选值，因为如果您收到错误，最好返回 nil 而不是空字典。

do {
    let responseDict = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any]
    if let results = responseDict!["results"] as? [[String:Any]] {
        completionHandler(nil, responseDict)
    }
 } catch {
     completionHandler(error, nil)
 }

Answer 2

感谢您的帮助。

通过对闭包的更多研究，我得到了结果。

下面是我创建的 Singleton ServiceManager。

class ServiceManager: NSObject {

    //  Static Instance variable for Singleton
    static var sharedSessionManager = ServiceManager()

    //  Preventing initialisation from any other source.
    private init() {
    }

    //  Function to execute GET request and pass data from escaping closure
    func executeGetRequest(with urlString: String, completion: @escaping (Data?) -> ()) {

        let url = URL.init(string: urlString)
        let urlRequest = URLRequest(url: url!)

        URLSession.shared.dataTask(with: urlRequest) { (data, response, error) in
        //  Log errors (if any)
            if error != nil {
                print(error.debugDescription)
            } else {
                //  Passing the data from closure to the calling method
                completion(data)
            }
        }.resume()  // Starting the dataTask
    }

    //  Function to perform a task - Calls executeGetRequest(with urlString:) and receives data from the closure.
    func downloadMovies(from urlString: String, completion: @escaping ([Movie]) -> ()) {
        //  Calling executeGetRequest(with:)
        executeGetRequest(with: urlString) { (data) in  // Data received from closure
            do {
                //  JSON parsing
                let responseDict = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any]
                if let results = responseDict!["results"] as? [[String:Any]] {
                    var movies = [Movie]()
                    for obj in results {
                        let movie = Movie(movieDict: obj)
                        movies.append(movie)
                    }
                //  Passing parsed JSON data from closure to the calling method.
                    completion(movies)
                }
            } catch {
                print("ERROR: could not retrieve response")
            }
        }
    }
}

下面的用法示例。

ServiceManager.sharedSessionManager.downloadMovies(from: urlBase) { (movies : [Movie]) in   // Object received from closure
    self.movies = movies
    DispatchQueue.main.async {
         //  Updating UI on main queue
        self.movieCollectionView.reloadData()
    }
}

我希望这可以帮助任何寻找类似解决方案的人。