Question

我是初学者，第一次使用 ReactiveCocoa 和Swift。我正在构建一个显示电影列表的应用程序，并且我正在使用MVVM模式。我的ViewModel看起来像这样：

#include <fstream>
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int func(int a)
{
        //assume the data is big and writing takes a long time
        int data = a;
        return data;
}
int main()
{
        ifstream in("numbers.txt");
        int a;
        while(in >> a)
        {
                stringstream ss;
                ss << a%10;
                string str;
                ss >> str;
                str += "_File.txt";
                ofstream out(str.c_str(), fstream::in | fstream::out | fstream::trunc);
                //This is blocking, if write takes long
                //but can be rewritten in a multi-thread fashion
                // to allow upto 10 simultaneous file write
                out << func(a) << endl;
        }
        return 0;
}

我想在UITableView中配置我的单元格，如下所示：

class HomeViewModel {

    let title:MutableProperty<String> = MutableProperty("")
    let description:MutableProperty<String> = MutableProperty("")
    var image:MutableProperty<UIImage?> = MutableProperty(nil)

    private var movie:Movie

    init (withMovie movie:Movie) {

        self.movie = movie

        title.value = movie.headline
        description.value = movie.description

        Alamofire.request(.GET, movie.pictureURL)
            .responseImage { response in

                if let image = response.result.value {
                    print("image downloaded: \(image)")
                    self.image.value = image
                }
        }

    }
}

这是Reactive Cocoa的正确方法吗？我是否需要将标题和描述声明为可变或仅仅是图像（是唯一的变化）。我想我可以使用绑定，但我不确定如何继续。

Answer 1

使用Reactive Cocoa + MVVM模式执行此操作我将首先移动所有逻辑以将单元格从其viewmodel配置到单元类本身。然后从viewModel中删除MutableProperties（它们实际上并不可变，我们不需要那些信号）。并且为图像公开一个信号生成器，它将在调用start()时执行网络请求以获取图像，而不是在ViewModel上调用init时隐式获取它，给我们类似的东西

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
  let cell = tableView.dequeueReusableCellWithIdentifier("MovieCell", forIndexPath: indexPath) as! MovieCell
  cell.viewModel = self.viewModelForIndexPath(indexPath)
  return cell
}

private func viewModelForIndexPath(indexPath: NSIndexPath) -> MovieCellViewModel {
  let movie: Movie = movieList[indexPath.row]
  return HomeViewModel(movie: movie)
}

然后

class MovieCell: UITableViewCell
  @IBOutlet weak var titleLabel: UILabel
  @IBOutlet weak var descriptionLabel: UILabel
  @IBOutlet weak var imageView: UIImageView

  var viewModel: MovieCellViewModel {
    didSet {
      self.configureFromViewModel()
    }
  }

  private func configureFromViewModel() {
    self.titleLabel.text = viewModel.title
    self.descriptionLabel.text = viewModel.description
    viewModel.fetchImageSignal()
      .takeUntil(self.prepareForReuseSignal()) //stop fetching if cell gets reused
      .startWithNext { [weak self] image in
        self?.imageView.image = image
      }
  }

  //this could also go in a UITableViewCell extension if you want to use it other places
  private func prepareForReuseSignal() -> Signal<(), NoError> {
    return Signal { observer in
      self.rac_prepareForReuseSignal // reactivecocoa builtin function
        .toSignalProducer() // obj-c RACSignal -> swift SignalProducer
        .map { _ in () } // AnyObject? -> Void
        .flatMapError { _ in .empty } // NSError -> NoError
        .start(observer)
    }
  }
}

并在ViewModel中

struct HomeViewModel {
  private var movie: Movie

  var title: String {
    return movie.headline
  }

  var description: String {
    return movie.description
  }

  func fetchImageSignal() -> SignalProducer<UIImage, NSError> {
    return SignalProducer { observer, disposable in
      Alamofire.request(.GET, movie.pictureURL)
        .responseImage { response in
          if let image = response.result.value {
            print("image downloaded: \(image)")
            observer.sendNext(image) //send the fetched image on the signal
            observer.sendCompleted()
          } else {
            observer.sendFailed( NSError(domain: "", code: 0, userInfo: .None)) //send your error
          }
        }
  }
}

使用ReactiveCocoa（4.2.1）和Swift加载异步图像

1 个答案: