todo-mvvm-live-kotlin

This version of the app is called todo-mvvm-live-kotlin, and it is the kotlin version of todo-mvvm-live. It uses some Architecture Components like ViewModel, LiveData, and other lifecycle-aware classes.

This sample is not final, as the Architecture Components are in alpha stage at the time of writing this document.

What you need

Before exploring this sample, you should familiarize yourself with the following topics:

• The project README
• The todo-mvp sample
• The todo-mvvm-databinding sample
• The MVVM architecture
• The Kotlin programming language

Implementing the app

Although the parent sample already used ViewModels, as it was following an MVVM architecture, the Architecture Components have different restrictions by design.

In the MVVM architecture, Views react to changes in the ViewModel without being explicitly called. However, the MVVM architecture presents some challenges when working with some Android components.

Live events

A new SingleLiveEvent class is created, which extends MutableLiveData so it's lifecycle-aware. It's used for communication between ViewModels and UI views (activities and fragments).

Instead of holding data, it dispatches data once. This is important to prevent events being fired after a rotation, for example.

A convenient use for this is navigation. There is no reference to the View from a ViewModel so the communication between them must happen via a subscription. ViewModels expose events like openTaskEvent and views subscribe to them. For example:

    private fun subscribeToNavigationChanges(viewModel: TaskDetailViewModel) {
        // The activity observes the navigation commands in the ViewModel
        viewModel.run {
            editTaskCommand.observe(this@TaskDetailActivity,
                    Observer { this@TaskDetailActivity.onStartEditTask() })
            deleteTaskCommand.observe(this@TaskDetailActivity,
                    Observer { this@TaskDetailActivity.onTaskDeleted() })
        }
    }

Snackbar

To show a Snackbar, you must use a static call to pass a view object. In this kotlin example, we wrap it in an extension function for View defined as follows:

fun View.showSnackbar(snackbarText: String) {
    Snackbar.make(this, snackbarText, Snackbar.LENGTH_LONG).show()
}

A ViewModel, however, doesn't have the necessary reference to the view hierarchy to call this function. Instead, you can manually subscribe the snackbar to a Snackbar event. In this case the subscription is made to a SingleLiveEvent<Int> and takes a string resource ID (hence the Int parameter type). There's only one snackbar and there should only be one active observer at a time. Messages are only shown once.

TasksAdapter

There is no TaskItemViewModel in this branch for each particular item in the list, so tasks in the list only communicate with the list's ViewModel.

Using ViewModels in bindings with the Data Binding Library

ViewModels are used to show data of a particular screen, but they don't handle user actions. For that it's much more convenient to create user actions listeners or even presenters that hold no state during configuration changes and hence are easy to recreate. See TaskItemUserActionsListener for an example.

Repository does not use LiveData

For simplicity and similarity with the parent branch, the repository does not use LiveData to expose its data.

Code Metrics

-------------------------------------------------------------------------------
Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Kotlin                          58            898           1755           3228 (3060 in MVP-kotlin)
XML                             22            127              0            974
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SUM:                            80           1015           1755           4192
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