Recipe: LiveData.observe with NonNull

Problem

  viewModel.order.observe(viewLifecycleOwner, Observe {  
      it?.let { applyCurrentOrder(it) }
  })

toooooo many redundant code for just observe NonNull object with LiveData.

Ingredients

1. lifecycle-livedata-ktx version 2.1.0 or higher
2. Kotlin

Metod

First.

Add a dependency lifecycle-livedata-ktx in app/build.gradle.

    dependencies {
        def lifecycle_version = "2.1.0" // or higher
        ...
        implementation "androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version"
        ...
    }

Second.(Optional)

If your project not apply Java8 yet.
add below in app/build.gradle.

android {
      ..
      compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
      }
      // For Kotlin projects
      kotlinOptions {
        jvmTarget = "1.8"
      }
    }

Third.

Implements your ViewModel(AAC) class as code below.

class MyOrderViewModel: ViewModel() {
  private val _order: MutableLiveData<Order> = MutableLiveData()
  val order: LiveData<Order> get() = _order
  ..
  // Important: don't update _order directly in other class. just call function.
  fun updateCurrentOrder(order: Order) {
    _order.value = order
  }
}

Fourth.

Observe data your Activity/Fragment class with ktx(important!!!).

// You must add LivedData.observe Kotlin extension function.
import androidx.lifecycle.observe

class MyOrderFragment: Fragment(R.layout.my_fragment) {
  private val viewModel: MyOrderViewModel by viewModels()

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    // No longer need to write just like viewModel.order.observe(viewLifecycleOwner, Obseve{})
    viewModel.order.observe(viewLifecycleOwner) {
        // No need null check
        applyCurrentOrder(it)
    }
  }

  fun applyCurrentOrder(order: Order) {
    ..
  }
}

Additional Technique (Alternately)

If you already wrote a lot of MutableLiveData instead of LiveData, you can also do the following.

interface MyOrderViewModel {
  val order: LiveData<Order>
}

class MyOrderViewModelImpl: ViewModel(), MyOrderViewModel {
  override val order: MutableLiveData<Order> = MutableLiveData()
  ..
  fun updateCurrentOrder(order: Order) {
    this.order.value = order
  }
}

import androidx.lifecycle.observe

class MyOrderFragment: Fragment(R.layout.my_fragment) {
  private val viewModel: MyOrderViewModel by viewModels<MyOrderViewModelImpl>()
  ..
}

More Information

about lifecycle: https://developer.android.com/jetpack/androidx/releases/lifecycle
about java8: https://developer.android.com/studio/write/java8-support