Developing mobile applications involves two primary concerns:

• Logic (state, business rules, data, API integration)
• UI

The MVVM (Model-View-ViewModel) approach is a way of separating the above concerns into distinct areas of development so that:

• The two can be developed and tested independently
• They can each be developed by specialised developers where
  • UI section needs Zebble UI skills
  • Logic needs C# skills only
• Better testability of the Logic
• Better performance, where the UI view lifecycle is managed by the framework

Development lifecycle

In the Zebble MVVM approach, the lifecycle of a typical project is the following:

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1. The application wireframes are designed
2. In parallel
   • The UI graphical design will start
   • The tech lead, will design an abstract view model in plain C#, also known as ViewModel Skeleton
3. The ViewModel Skeleton will be used as a foundation of the project by the team:
   • C# developers​ will complete the ViewModel logic implementation and integrate it with the backend APIs
   • QA engineers will design and write the test scripts
4. Once the UI Design is ready, the Zebble UI developers should extract reusable Components from the design.
   • Each Component will represent one unique shape used once or multiple times throughout the application. It will be implemented using a C# class that inherits from a base Zebble view, or another component. dor a Zbl component (if it's a composite). Also it has its CSS definitions defined directly on the component type (rather than CssClass or ID)
5. Once the Components and the view model skeleton are both completed, the Zebble UI developers will create ZBL markup templates to visualise the view model on the screen. 

ViewModel Skeleton

This is a vital step in the project lifecycle and provides a foundation for the whole team to collaborate effectively. The purpose of this process is to convert the structure of the UI screens and states, previously represented as visual wireframes, into their C# equivalent models.  

For each app page in the wireframes, an abstract Page class is defined.

For every dynamic (data-based) visible element of the page, a property is defined as:public readonly Bindable<PropertyType> PropertyName = new ("Default value");

All such state objects (bindables) must have a default value (ideally not null) to provide a solid zero state. This is important to allow the UI developers to create and test the template ZBL files against those without facing null errors. 

For every interactable action on the page, a method is defined as:

public void TapLogin()
{
}The view model classes defined at this stage are just skeleton. The methods are left empty. Also no page lifecycle methods or events are implemented. 

Defining the ViewModel skeleton in this way will provide a strongly typed object model, to orchestrate all UI development efforts. It's a technical design excercise and is normally done by the tech lead in the project, with the rest of the team participating to understand it. It should not take more than a few minutes per page to create this. So this whole excercise should not take more than a few hours for an average sized project. This should be done as the first step in the technical implementation phase, on the first day of the project.

Execution model

In a typical Zebble solution in Visual Studio, the Domain project (shared source) contains the business logic, and the App.UI project contains the UI code. There are also 3 runtime projects for each platform (UWP, Android and iOS) all under the Run folder. In MVVM model, 2 new projecs are added to the solution:

• App.ViewModel (shared project) which contains the abstract application UI structure and logic
• Run\VM (Console app) used for instant running and testing of the application logic and abstract UI.

All 4 runtime projects (UWP, Android, iOS and now VM) will reference the Domain and the new App.ViewModel projects.
The App.UI project will be referenced only in the platform specific ones, but not the Run\VM console app.

Run\VM (console app)

The Run\VM project plays a key role in the MVVM development model. It is a pure (c# only) runtime environment to run the application's logic.

This allows non Zebble developers, i.e. people with just C# skills, to develop the vast majority of the mobile apps' code. It also makes it much faster to run and test the app and speeds up the development iterations. In addition, it makes it easy to write end-to-end unit and integration code to test the logic and workflow of the apps based on an abstract page model, without worrying about the real UI rendering.