如何通过 MVVMcross 中的绑定将视图模型中的更改反映到 tableviewcell 视图

Question

当在MvxBindableTableViewCell 中使用时，我有点难以将更改从 ViewModel 反映到 View。我在 iOS 上使用 MvvmCross 的 vNext 分支。

一切都已正确设置，并且在首次加载/显示列表时可以看到初始值。该列表是一个ObservableCollection<T>，ViewModels 继承自MvxViewModel（因此实现了 INotifyPropertyChanged）。

主视图模型如下所示：

public abstract class BaseViewModel : MvxViewModel, IMvxServiceConsumer
{
    //... just regular implementation
}

public class UploadListViewModel: BaseViewModel
{
    private readonly IUploadItemTasks uploadItemTasks;
    private readonly IPhotoPickerService photoPickerService;

    public IObservableCollection<UploadItemViewModel> Uploads { get { return this.LoadUploadItems(); }  }

    public UploadListViewModel()
    {
        this.uploadItemTasks = this.GetService<IUploadItemTasks>();
        this.photoPickerService = this.GetService<IPhotoPickerService>();
    }

    private IObservableCollection<UploadItemViewModel> LoadUploadItems()
    {
        using (var unitOfWork = UnitOfWork.Start ())
        {
            return new SimpleObservableCollection<UploadItemViewModel>(uploadItemTasks.GetAll());
        }
    }

    public void StartUpload ()
    {
        if (this.Uploads == null || this.Uploads.Count == 0) {
            ReportError("Error", "No images to upload");
            return;
        }

        this.Uploads.ForEach (uploadItem => PostCallback (uploadItem));
    }

    private void PostCallback (UploadItemViewModel uploadAsset)
    {
        IProgressReporter progressReporter = uploadAsset;

        this.photoPickerService.GetAssetFullImage(uploadAsset.ImageUrl,
                                                  (image) => {
            UIImage fullImage = image;
            NSData jpeg = fullImage.AsJPEG();

            byte[] jpegBytes = new byte[jpeg.Length];           
            System.Runtime.InteropServices.Marshal.Copy(jpeg.Bytes, jpegBytes, 0, Convert.ToInt32(jpeg.Length));

            MemoryStream stream = new MemoryStream(jpegBytes);
            Uri destinationUrl = new Uri(uploadAsset.DestinationUrl + "&name=" + uploadAsset.Name + "&contentType=image%2FJPEG");

            //TO DO: Move this to plugin
            var uploader = new Uploader().UploadPicture (destinationUrl, stream, UploadComplete, progressReporter);
            uploader.Host = uploadAsset.Host;

            ThreadPool.QueueUserWorkItem (delegate {
                uploader.Upload ();                 
                jpeg = null;
            });
        });
    }

    private void UploadComplete (string name)
    {
        if (name == null){
            ReportError("Error","There was an error uploading the media.");
        } else 
        {
            //ReportError("Succes", name);
        }
    }

项目 ViewModel 看起来像：

public interface IProgressReporter
{
    float Progress { get; set;} 
}

public abstract class BaseAssetViewModel: BaseViewModel, IBaseAssetViewModel
{
    //... just regular properties 
}

public class UploadItemViewModel: BaseAssetViewModel, IProgressReporter
{
    public UploadItemViewModel(): base()
    {
    }

    private float progress;
    public float Progress {
        get {
            return this.progress;
        }
        set {
            this.progress = value;
            this.RaisePropertyChanged(() => Progress);
        }
    }
}

项目的视图继承自 MvxBindableTableViewCell 并具有以下属性：

private float progress;
public float ProgressMarker {
    get {
        return progress;
    }
    set {
        progress = value;
        // change progressbar or textfield here
    }
}

tableviewcell 通过 BindingText 绑定到UploadItemViewModel：

public const string BindingText = @"ProgressMarker Progress, Converter=Float;";

UploadListViewModel 的 sn-p 中提到的 Uploader 类实现了一个尝试在 IProgressReporter 上设置进度的私有方法。

    float progressValue;
    void SetProgress (float newvalue)
    {
        progressValue = newvalue;

        this.dispatcher.InvokeOnMainThread (delegate {
            if (ProgressReporter != null)
                ProgressReporter.Progress = progressValue;
        });
    }

在第一次查看列表期间，我可以看到 ViewModel 和 View 中的属性都被击中，但是当我通过接口 IProgressReporter 使用 Progress 中的新值更新 ViewModel 时，tableviewcell 中的 View没有更新，也没有调用该属性。

我做错了什么或者我在这里错过了什么？

更新：检查此问题的答案。

Answer 1

我找到了绑定不起作用的原因。我一遍又一遍地替换 ObservableCollection。我按如下所述更改了那段代码，现在它反映了对单元格视图中的 UploadItemViewModel 所做的更改。

    private IObservableCollection<UploadItemViewModel> uploads;
    private IObservableCollection<UploadItemViewModel> LoadUploadItems()
    {
        if (uploads == null)
        {
            using (var unitOfWork = UnitOfWork.Start ())
            {
                uploads = new SimpleObservableCollection<UploadItemViewModel>(uploadItemTasks.FindAll());
            }
        }
        return uploads;
    }