View Model Extensions

This is a small collection of helper classes/methods to simplify the usage of ViewModel in kotlin projects.

Usage

When getting a ViewModel from ViewModelProviders you generally have to write:

ViewModelProviders.of(fragment)[T::class.java]

and when you want to have a custom ViewModelProvider.Factory it gets even more convoluted:

ViewModelProviders.of(fragment, object: ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        //create ViewModel based on the supplied modelClass
    }
})[T::class.java]

instead you can use easy-to-use property delegate inside a ComponentActivity or Fragment:

val model: MyViewModel by viewModels()

or when you want to use a custom ViewModel with a non-empty constructor:

val model: MyViewModel by viewModelsCustom { MyViewModel("a string", 12) }

Migrating from version 1.7.0 to 1.8.0

Some functions have been removed, because they are available in the androidx ktx packages. Those functions are:

• LiveData.observe
• LiveData.observeRequired
• LiveData.map
• LiveData.switchMap

All those functions are available by using

implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.2.0"

Events

When wanting to publish events as live data, there is a problem when multiple observers register to the same LiveData, because normal LiveData does not regulate consuming of events.

There is a solution presenting by Jose Alcérreca on Medium (find it here) , where a new Event class is introduced to regulate consuming of events.

This Event class is now available as part of this library, simply define a LiveData of an Event:

private val _warning = mutableLiveDataOf<Event<String>?>()
val warning = _warning.asLiveData()

and then you can consume the event in the activity or fragment of you choice:

model.warning.observe(this) { event ->
    event?.getContentIfNotHandled()?.let {
        Toast.makeText(this, it, Toast.LENGTH_SHORT).show()
    }
}

this will guarantee that the event is only consumed once.

Utility Methods

To create a MutableLiveData object you can use the mutableLiveDataOf function. You can turn any MutableLiveData into a LiveData using the asLiveData function:

private val _number = mutableLiveDataOf(5)
val number = _number.asLiveData()

When using a AndroidViewModel there is now an extension property to access the application without using the getter.

When wanting to transform LiveData with Transformations you can now use an extension method like:

private val _number = liveDataOf(5)
val number = _number.asLiveData()

val numberMultitude = number.map { it * 25 }

Installation

add jitpack to repositories.

top-level build.gradle

allprojects {
    repositories {
        google()
        jcenter()
        maven { url 'https://jitpack.io' }
    }
}

add ViewModelExtensions to application build.gradle

implementation 'com.github.mueller-wulff:ViewModelExtensions:1.8.0'

License

Copyright 2018 Hans Markwart

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.