Merge "Add optional emitFirstEvent param to setChanges()" into tm-qpr-dev am: 6451a04d78

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/19776116

Change-Id: I64cdca77a644da5ea801a1c6157947ee5d4acb9e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

packages/SystemUI/src/com/android/systemui/util/kotlin/Flow.kt

@@ -74,10 +74,19 @@ data class WithPrev<T>(val previousValue: T, val newValue: T)
 /**
  * Returns a new [Flow] that combines the [Set] changes between each emission from [this] using
  * [transform].
+ *
+ * If [emitFirstEvent] is `true`, then the first [Set] emitted from the upstream [Flow] will cause
+ * a change event to be emitted that contains no removals, and all elements from that first [Set]
+ * as additions.
+ *
+ * If [emitFirstEvent] is `false`, then the first emission is ignored and no changes are emitted
+ * until a second [Set] has been emitted from the upstream [Flow].
  */
 fun <T, R> Flow<Set<T>>.setChangesBy(
     transform: suspend (removed: Set<T>, added: Set<T>) -> R,
-): Flow<R> = onStart { emit(emptySet()) }.distinctUntilChanged()
+    emitFirstEvent: Boolean = true,
+): Flow<R> = (if (emitFirstEvent) onStart { emit(emptySet()) } else this)
+    .distinctUntilChanged()
     .pairwiseBy { old: Set<T>, new: Set<T> ->
         // If an element was present in the old set, but not the new one, then it was removed
         val removed = old - new
@@ -86,8 +95,18 @@ fun <T, R> Flow<Set<T>>.setChangesBy(
         transform(removed, added)
     }
 
-/** Returns a new [Flow] that produces the [Set] changes between each emission from [this]. */
-fun <T> Flow<Set<T>>.setChanges(): Flow<SetChanges<T>> = setChangesBy(::SetChanges)
+/**
+ * Returns a new [Flow] that produces the [Set] changes between each emission from [this].
+ *
+ * If [emitFirstEvent] is `true`, then the first [Set] emitted from the upstream [Flow] will cause
+ * a change event to be emitted that contains no removals, and all elements from that first [Set]
+ * as additions.
+ *
+ * If [emitFirstEvent] is `false`, then the first emission is ignored and no changes are emitted
+ * until a second [Set] has been emitted from the upstream [Flow].
+ */
+fun <T> Flow<Set<T>>.setChanges(emitFirstEvent: Boolean = true): Flow<SetChanges<T>> =
+    setChangesBy(::SetChanges, emitFirstEvent)
 
 /** Contains the difference in elements between two [Set]s. */
 data class SetChanges<T>(

packages/SystemUI/tests/src/com/android/systemui/util/kotlin/FlowUtilTests.kt

@@ -127,6 +127,17 @@ class SetChangesFlowTest : SysuiTestCase() {
                 )
             )
     }
+
+    @Test
+    fun dontEmitFirstEvent() = runBlocking {
+        assertThatFlow(flowOf(setOf(1, 2), setOf(2, 3)).setChanges(emitFirstEvent = false))
+            .emitsExactly(
+                SetChanges(
+                    removed = setOf(1),
+                    added = setOf(3),
+                )
+            )
+    }
 }
 
 private fun <T> assertThatFlow(flow: Flow<T>) = object {