使用基于类型(或上一个ViewModel)的现有委托工厂获取实例

基于此页面，我们创建了一个包含三个步骤的向导。一切都很好，但我们对链接中给出的代码有一个问题，那就是它如何创建下一步实例（从链接粘贴的副本）：

protected override IScreen DetermineNextItemToActivate(IList<IScreen> list, int lastIndex)
{
    var theScreenThatJustClosed = list[lastIndex] as BaseViewModel;
    var state = theScreenThatJustClosed.WorkflowState;
    var nextScreenType = TransitionMap.GetNextScreenType(theScreenThatJustClosed);
    var nextScreen = Activator.CreateInstance(nextScreenType, state);
    return nextScreen as IScreen;
}

目前，在我们的项目中是这样的：

protected override IWizardScreen DetermineNextItemToActivate(IList<IWizardScreen> list, int lastIndex)
{
    var theScreenThatJustClosed = list[lastIndex];
    if (theScreenThatJustClosed == null) throw new Exception("Expecting a screen here!");
    if (theScreenThatJustClosed.NextTransition == WizardTransition.Done)
    {
        TryClose(); // Close the entire Wizard
    }
    var state = theScreenThatJustClosed.WizardAggregateState;
    var nextScreenType = _map.GetNextScreenType(theScreenThatJustClosed);
    if (nextScreenType == null) return null;
    // TODO: CreateInstance requires all constructors for each WizardStep, even if they aren't needed. This should be different!
    var nextScreen = Activator.CreateInstance(nextScreenType, state, _applicationService, _wfdRegisterInstellingLookUp,
        _adresService, _userService, _documentStore, _windowManager, _fileStore, _fileUploadService, _dialogService,
        _eventAggregator, _aanstellingViewModelFactory);
    return nextScreen as IWizardScreen;
}

正如你所看到的，我们在一些步骤中需要相当多的参数。在步骤1中，我们只需要两个，但由于Activator.CreateInstance(nextScreenType, state, ...);，我们仍然需要通过所有这些。

我想要的是使用delegate Factory。我们在项目中的更多地方使用它们，并让AutoFac处理其余的参数。对于三个步骤中的每一个，我们只需要一个使用state的delegate Factory。

因为这三个都使用相同的delegate Factory，只有state，所以我把这个工厂放在了它们的基类中：

public delegate WizardBaseViewModel<TViewModel> Factory(AggregateState state);

我想如何更改DetermineNextItemToActivate方法：

protected override IWizardScreen DetermineNextItemToActivate(IList<IWizardScreen> list, int lastIndex)
{
    var theScreenThatJustClosed = list[lastIndex];
    if (theScreenThatJustClosed == null) throw new Exception("Expecting a screen here!");
    if (theScreenThatJustClosed.NextTransition == WizardTransition.Done)
    {
        TryClose(); // Close the entire Wizard
    }
    return _map.GetNextScreenFactoryInstance(state);
}

但现在我一直在制作GetNextScreenFactoryInstance方法：

public IWizardScreen GetNextScreenFactoryInstance(IWizardScreen screenThatClosed)
{
    var state = screenThatClosed.WizardAggregateState;
    // This is where I'm stuck. How do I get the instance using the Factory, when I only know the previous ViewModel
    // ** Half-Pseudocode
    var nextType = GetNextScreenType(screenThatClosed);
    var viewModelFactory = get delegate factory based on type?;
    var invokedInstance = viewModelFactory.Invoke(state);
    // **
    return invokedInstance as IWizardScreen;
}

您可以随意更改GetNextScreenFactoryInstance。只要我能在地图中的上一个步骤的基础上获得下一个步骤ViewModel。

注意：其他相关代码，可以在链接中找到，但我也会把它发布在这里，把它放在一起：

WizardTransitionMap（唯一的变化是它不再是Singleton，所以我们可以实例化自己的地图）：

public class WizardTransitionMap : Dictionary<Type, Dictionary<WizardTransition, Type>>
{
    public void Add<TIdentity, TResponse>(WizardTransition transition) 
        where TIdentity : IScreen 
        where TResponse : IScreen
    {
        if (!ContainsKey(typeof(TIdentity)))
        {
            Add(typeof(TIdentity), new Dictionary<WizardTransition, Type> { { transition, typeof(TResponse) } });
        }
        else
        {
            this[typeof(TIdentity)].Add(transition, typeof(TResponse));
        }
    }
    public Type GetNextScreenType(IWizardScreen screenThatClosed)
    {
        var identity = screenThatClosed.GetType();
        var transition = screenThatClosed.NextTransition;
        if (!transition.HasValue) return null;
        if (!ContainsKey(identity))
        {
            throw new InvalidOperationException(String.Format("There are no states transitions defined for state {0}", identity));
        }
        if (!this[identity].ContainsKey(transition.Value))
        {
            throw new InvalidOperationException(String.Format("There is no response setup for transition {0} from screen {1}", transition, identity));
        }
        return this[identity][transition.Value];
    }
}

我们的InitializeMap方法：

protected override void InitializeMap()
{
    _map = new WizardTransitionMap();
    _map.Add<ScreenOneViewModel, ScreenTwoViewModel>(WizardTransition.Next);
    _map.Add<ScreenTwoViewModel, ScreenOneViewModel>(WizardTransition.Previous);
    _map.Add<ScreenTwoViewModel, ScreenThreeViewModel>(WizardTransition.Next);
    _map.Add<ScreenThreeViewModel, ScreenTwoViewModel>(WizardTransition.Previous);
    _map.Add<ScreenThreeViewModel, ScreenThreeViewModel>(WizardTransition.Done);
}