Simple, Lightweight MVVM Framework for Unity3D

For a background on MVVM checkout the Wikipedia page

The goal of this project is to implement some of the concepts I knew and loved from developing WPF applications, mainly that they be very quick to build, and easy to maintain. It also attempts to solve some of the Unity-specific pain points I've uncovered over the last few years working with Unity.

MVVM stands for Model-View-ViewModel. It is a software application architechture designed to decouple view logic from business logic when building software. This is good practice for a number of reasons including reusability, maintainability, and speed of development.

A model is simply a structure of data. Sometimes a model will reflect real world types (i.e. car, desk) and sometimes it will reflect types from web server, etc...

A view is the presentation layer of an application. In Unity-MVVM, this can be a Unity UI element like button or text field. In addition, you can use the same logic for 3D gameobjects. Views should handle their own transitions (animations etc...) based on state changes in the ViewModel.

A ViewModel (or VM) is what holds the data that will be presented on a view. It contains all the properties that can be bound to view elements. All ViewModels inherit from INotifyPropertyChanged which alerts the system when data changes and a UI element needs to be updated.

Generally speaking, with MVVM, Models don't know anything, Views don't care about anything, and ViewModels hold everything together.

What this means is that ViewModels subscribe to data providers (HTTP Requests, Services, etc...) and notify Views when data has been changed. This is called a DataBinding. By default, this is a One-Way relationshop (i.e. ViewModel can update data on a View, but not the other way around.

Sometimes, Views will need to change data (i.e. an input field.) This can be done with a Two-Way Databinding relationship. When you update a field in a UI, the ViewModel will handle that change and forward it to the relevant service or provider.

You can use as many or as fiew ViewModels as you want in your application. Typically you would create one for each screen on an app: Screen1ViewModel or SettingsPageViewModel for example. A ViewModel should inherit from ViewModelBase which implements INotifyPropertyChanged and provides a method alerting subscribers of changes.

When writing a ViewModel, you want to think about what sort of data is needed to be provided to your view and create bindable properties for each element or collection of elements.

public string Text
{
  get { return _text; } // Getter returns backing field
  set
  {
    if (value != _text) // Only update field if value is changed
    {
      _text = value;    // Set backing field
      NotifyPropertyChanged(nameof(Text)); // Alert subscribers to change in Property with name of property
    }
  }
}

[SerializeField]
string _text;

Included in Unity-MVVM is a Visual Studio code snippet which is helpful for writing bindable properties in your ViewModel. To install on Windows, open the .snippet file with Visual Studio.

Once this is installed type bindprop + (Tabx2) to create a bindable property snippet

Use CollectionView to create, edit, and delete Prefabs based on the contents of an ObservableCollection or ObservableRangeCollection. Inherit from CollectionViewBase to set custom data as needed.

Custom Views can be created by inheriting from ViewBase. ViewBase types all have a Visibility Enum property to hold ViewStates (Visible, Hidden, Collapsed.) ViewBase implements a simple fade animation by default. Create a custom view to define behaviours for Visibility state transitions.

To bind a ViewModel to a View, there are a series of MonoBehaviours you can add directly on the GameObject. They have custom editor scripts which allow you to bind to ViewModel Properties without writing any code

This is the most basic binding between a ViewModel and a View. Dropdowns allow you to select the VM and the property which you want to subscribe to changes. Component is the destination component which you will be updating. DstProp is a dropdown which allows you to pick the Property which will be updated on the View.

This is the same as a OneWayDataBinding except that it also allows you to select a UnityEvent to subscribe to which will update the Data from the View to the VM

Important: Defualt DataBindings disconnect when GameObjects are Disabled. Do not use OneWayDataBinding Or TwoWayDataBinding to turn GameObjects on and off, use the specialized types below

This is a OneWayDataBinding which automatically targets the IsActive Property on the GameObject. It also does not disable when the GameObject is disabled. Use this to turn GameObjects on and off.

This is a OneWayDataBinding which automatically targets the Visibility Property on a Componenty inheriting from ViewBase. It also does not disable when the GameObject is disabled. Use this to trigger In/Out transitions in your custom Views

This is a OneWayDataBinding which binds to an ObservableCollection or ObservableRangeCollection. Use this with a CollectionView to create arrays of elements on the scene.

Sometimes you want to bind to a property in the ViewModel, but you want that to be altered before it is set on the View Property. You can use a Converter to make a string all lowecase/uppercase with TextTransformationConverter or check if an enum equals a certain value with EnumToBoolConverter. Some Converters have parameters so you can tailor the output to a specific Property.

You can write your own converters by inheriting from ValueConverterBase. You then need to implement abstract methods Convert and ConvertBack (ConvertBack is only used by TwoWayDataBindings)

Sometimes you want to bind an event on the View to a method or property on your ViewModel. Unity-MVVM provides two ways to do this

This allows you to bind a UnityEvent from a view to a method on your ViewModel. The binding expects a method with the same signature as the UnityEvent. i.e. if the event is a UnityEvent<string> you need to bind to a void MethodName(sting argName) in your VM.

This allows you to bind a UnityEvent from a view to a property on your ViewModel. In the Value field, you can specify a value to set the property to OR the name of a property on your view to set from. In the latter case, check the box marked IsProperty.

• Currently there is no way to bind to nested properties
• No way to filter with CollectionViewSource
• No Code Snippet template for OSX
• No way to access indexed properties on CollectionViewSource
• Need cleaner way to bind property with EventPropertyBinding

Open to feedback and PRs

MIT License - do with it what you will, just don't blame me if it doesn't work :)

Copyright (c) 2018 Nate Turley turley.nate@gmail.com