ViewModels.md

ViewModels

In a MVVM architecture, the view-models turns into the main core of the app.

Views depend on view-models to present the information to the users and execute user actions, but view-models don’t need to know anything about the views. Applying this principle permits reusing the same view-model in different view implementations for different platforms.

In general, many of the properties and methods are common for all the different view-models so we use inheritance to reuse functionality.

ViewModelBase

This view-model is the base class for all view-models in the application and offers the following funcionality:

• Resolves and exposes the common services that every view-model in the app uses, which are: IContextService, INavigationService, IMessageService, IDialogService and the ILogService.
• It handles the Log common operations for all view-models in the app.
• It's responsable of communicate the status of the view-model using the Message Service.

The following code is a simplified implementation of the ViewModelBase.

public class ViewModelBase : ObservableObject
{
    public ViewModelBase(ICommonServices commonServices)
    {
        ContextService = commonServices.ContextService;
        NavigationService = commonServices.NavigationService;
        MessageService = commonServices.MessageService;
        DialogService = commonServices.DialogService;
        LogService = commonServices.LogService;
    }

    public IContextService ContextService { get; }
    public INavigationService NavigationService { get; }
    public IMessageService MessageService { get; }
    public IDialogService DialogService { get; }
    public ILogService LogService { get; }

    virtual public string Title => String.Empty;

    public async void LogInformation(string source, string action, string message, string description)
    {
        await LogService.WriteAsync(LogType.Information, source, action, message, description);
    }

    public async void LogWarning(string source, string action, string message, string description)
    {
        await LogService.WriteAsync(LogType.Warning, source, action, message, description);
    }

    public async void LogError(string source, string action, string message, string description)
    {
        await LogService.WriteAsync(LogType.Error, source, action, message, description);
    }

    public void StatusReady()
    {
        MessageService.Send(this, "StatusMessage", "Ready");
    }
    public void StatusMessage(string message)
    {
        MessageService.Send(this, "StatusMessage", message);
    }
    public void StatusError(string message)
    {
        MessageService.Send(this, "StatusError", message);
    }
}

Domain Base ViewModels

We have defined 2 base view-models representing a common data structure to be displayed in then Inventory Sample app: List and Detail generic view-models.

GenericListViewModel

class GenericListViewModel<TModel> : ViewModelBase where TModel : ObservableObject

This base view-model handles the common operations of a List, like:

• Filter the list of items.
• Single and multiple item selection handling.
• Refresh, New and Delete operations over the list of items.

This class contains abstract methods to be implemented by the concrete clasess like OnRefresh, OnNew or OnDeleteSelection.

GenericDetailsViewModel

class GenericDetailsViewModel<TModel> : ViewModelBase where TModel : ObservableObject, new()

This generic view-model puts focus on:

• Display the detail of the model as ReadOnly or Edit mode.
• Validates user input before saving changes.
• Delete current item.

This class contains abstract methods to be implemented by the concrete clasess like SaveItem, DeleteItem or GetValidationConstraints.