Add LiveData.skip()

Author: shiki

WordPress/src/main/java/org/wordpress/android/util/LiveDataUtils.kt

@@ -2,6 +2,7 @@ package org.wordpress.android.util
 
 import android.arch.lifecycle.LiveData
 import android.arch.lifecycle.MediatorLiveData
+import android.arch.lifecycle.Observer
 import android.arch.lifecycle.Transformations
 import kotlinx.coroutines.CoroutineScope
 import org.wordpress.android.viewmodel.SingleMediatorLiveEvent
@@ -224,3 +225,35 @@ fun <T> LiveData<T>.filter(predicate: (T) -> Boolean): LiveData<T> {
     }
     return mediator
 }
+
+/**
+ * Suppresses the first n items by this [LiveData].
+ *
+ * Consider this for example:
+ *
+ * ```
+ * val connectionStatusLiveData = getConnectionStatusLiveData()
+ * connectionStatusLiveData.skip(1).observe(this, Observer {
+ *     refresh()
+ * })
+ * ```
+ *
+ * The first value emitted by `connectionStatusLiveData` would be ignored and [Observer] will nto be called.
+ */
+fun <T> LiveData<T>.skip(times: Int): LiveData<T> {
+    check(times > 0) { "The number of times to skip must be greater than 0" }
+
+    var skipped = 0
+    val mediator = MediatorLiveData<T>()
+    mediator.addSource(this) { value ->
+        skipped += 1
+
+        if (skipped <= times) {
+            return@addSource
+        } else {
+            mediator.postValue(value)
+        }
+    }
+
+    return mediator
+}

WordPress/src/test/java/org/wordpress/android/util/LiveDataUtilsTest.kt

@@ -66,4 +66,82 @@ class LiveDataUtilsTest : BaseUnitTest() {
         sourceB.value = null
         assertThat(combineMap.value).isEqualTo(mapOf(keyA to valueA))
     }
+
+    @Test
+    fun `when skipping 1 on a LiveData emitting nothing, nothing is emitted`() {
+        // Given
+        val source = MutableLiveData<String>()
+        check(source.value == null)
+
+        val skip = source.skip(1)
+
+        // When
+        var emitCount = 0
+        skip.observeForever {
+            emitCount += 1
+        }
+
+        // Then
+        assertThat(emitCount).isZero()
+        assertThat(skip.value).isNull()
+    }
+
+    @Test
+    fun `when skipping 1 on a LiveData emitting a single value, nothing is emitted`() {
+        // Given
+        val source = MutableLiveData<String>().apply { value = "Alpha" }
+        val skip = source.skip(1)
+
+        // When
+        var emitCount = 0
+        skip.observeForever {
+            emitCount += 1
+        }
+
+        // Then
+        assertThat(emitCount).isZero()
+        assertThat(skip.value).isNull()
+    }
+
+    @Test
+    fun `when skipping 1 on a LiveData emitting multiple values, the first value is not emitted`() {
+        // Given
+        val source = MutableLiveData<String>().apply { value = "Alpha" }
+        val skip = source.skip(1)
+
+        // When
+        var emitCount = 0
+        skip.observeForever {
+            emitCount += 1
+        }
+
+        source.postValue("Bravo")
+
+        // Then
+        assertThat(emitCount).isOne()
+        assertThat(skip.value).isEqualTo("Bravo")
+    }
+
+    @Test
+    fun `when skipping 3 on a LiveData emitting multiple values, the first three are not emitted`() {
+        // Given
+        val source = MutableLiveData<String>()
+        val skip = source.skip(3)
+
+        // When
+        var emitCount = 0
+        val emittedValues = mutableListOf<String>()
+        skip.observeForever { value ->
+            emitCount += 1
+
+            value?.let { emittedValues.add(it) }
+        }
+
+        listOf("Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot", "Golf", "Hotel").forEach(source::postValue)
+
+        // Then
+        assertThat(emitCount).isEqualTo(5)
+        assertThat(emittedValues).isEqualTo(listOf("Delta", "Echo", "Foxtrot", "Golf", "Hotel"))
+        assertThat(skip.value).isEqualTo("Hotel")
+    }
 }