Merge pull request #1194 from CruGlobal/viewModelObserveOnce

GT-1497 add observeOnce variation that is linked to the ViewModel lifecycle

Author: frett

gto-support-androidx-lifecycle/src/main/kotlin/org/ccci/gto/android/common/androidx/lifecycle/LiveData+ViewModel.kt

@@ -1,5 +1,6 @@
 package org.ccci.gto.android.common.androidx.lifecycle
 
+import androidx.annotation.MainThread
 import androidx.lifecycle.LiveData
 import androidx.lifecycle.Observer
 import androidx.lifecycle.ViewModel
@@ -47,3 +48,7 @@ fun <IN1, IN2, IN3> ViewModel.observe(
     observer(source1.value as IN1, source2.value as IN2, source3.value as IN3)
 }
 // endregion Multi-observe
+
+@MainThread
+inline fun <T> LiveData<T>.observeOnce(viewModel: ViewModel, crossinline onChanged: (T) -> Unit) =
+    observeOnceObserver(onChanged).also { observe(viewModel, it) }

gto-support-androidx-lifecycle/src/test/kotlin/org/ccci/gto/android/common/androidx/lifecycle/LiveDataViewModelObserverTest.kt

@@ -9,6 +9,9 @@ import org.junit.Before
 import org.junit.Test
 import org.mockito.kotlin.any
 import org.mockito.kotlin.argumentCaptor
+import org.mockito.kotlin.eq
+import org.mockito.kotlin.mock
+import org.mockito.kotlin.never
 import org.mockito.kotlin.times
 import org.mockito.kotlin.verify
 import org.mockito.kotlin.verifyNoMoreInteractions
@@ -84,4 +87,36 @@ class LiveDataViewModelObserverTest : BaseLiveDataTest() {
         }
     }
     // endregion Multi-observe
+
+    // region observeOnce
+    @Test
+    fun `observeOnce() - Only Called One Time`() {
+        // observe before LiveData has data
+        val observer: (Int) -> Unit = mock()
+        liveData.observeOnce(viewModel, observer)
+        verify(observer, never()).invoke(any())
+        liveData.value = 1
+        verify(observer).invoke(eq(1))
+        liveData.value = 2
+        verifyNoMoreInteractions(observer)
+
+        // observe after LiveData has data
+        val observer2: (Int) -> Unit = mock()
+        liveData.observeOnce(viewModel, observer2)
+        verify(observer2).invoke(eq(2))
+        liveData.value = 3
+        verifyNoMoreInteractions(observer2)
+    }
+
+    @Test
+    fun `observeOnce() - Respects ViewModel clear`() {
+        val observer: (Int) -> Unit = mock()
+        liveData.observeOnce(viewModel, observer)
+
+        // Lifecycle is destroyed so observer is removed before it can be called
+        viewModel.clear()
+        liveData.value = 1
+        verify(observer, never()).invoke(any())
+    }
+    // endregion observeOnce
 }