Friendlier construction API for BackStackScreen

* Deprecates the confusing `bottom: T, rest: List` constructor
* Adds `fromList` and `fromListOrNull` factory functions to `BackStackScreen.Companion`, since no one ever finds the `List` extensions

workflow-ui/compose/src/androidTest/java/com/squareup/workflow1/ui/compose/ComposeViewTreeIntegrationTest.kt

@@ -582,7 +582,9 @@ internal class ComposeViewTreeIntegrationTest {
   }
 
   private fun WorkflowUiTestActivity.setBackstack(vararg backstack: TestComposeRendering) {
-    setRendering(BackStackScreen(EmptyRendering, backstack.asList()))
+    setRendering(
+      BackStackScreen.fromList(listOf<AndroidScreen<*>>(EmptyRendering) + backstack.asList())
+    )
   }
 
   data class TestOverlay(

workflow-ui/core-android/src/androidTest/java/com/squareup/workflow1/ui/container/fixtures/BackStackContainerLifecycleActivity.kt

@@ -145,7 +145,7 @@ internal class BackStackContainerLifecycleActivity : AbstractLifecycleTestActivi
     setRendering(backstack.asList().toBackstackWithBase())
 
   private fun List<TestRendering>.toBackstackWithBase() =
-    BackStackScreen(BaseRendering, this)
+    BackStackScreen.fromList(listOf<Screen>(BaseRendering) + this)
 }
 
 internal fun ActivityScenario<BackStackContainerLifecycleActivity>.viewForScreen(

workflow-ui/core-common/api/core-common.api

@@ -213,8 +213,10 @@ public final class com/squareup/workflow1/ui/container/BackStackConfigKt {
 }
 
 public final class com/squareup/workflow1/ui/container/BackStackScreen : com/squareup/workflow1/ui/Container, com/squareup/workflow1/ui/Screen {
+	public static final field Companion Lcom/squareup/workflow1/ui/container/BackStackScreen$Companion;
 	public fun <init> (Lcom/squareup/workflow1/ui/Screen;Ljava/util/List;)V
 	public fun <init> (Lcom/squareup/workflow1/ui/Screen;[Lcom/squareup/workflow1/ui/Screen;)V
+	public synthetic fun <init> (Ljava/util/List;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public fun asSequence ()Lkotlin/sequences/Sequence;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun get (I)Lcom/squareup/workflow1/ui/Screen;
@@ -229,6 +231,11 @@ public final class com/squareup/workflow1/ui/container/BackStackScreen : com/squ
 	public fun toString ()Ljava/lang/String;
 }
 
+public final class com/squareup/workflow1/ui/container/BackStackScreen$Companion {
+	public final fun fromList (Ljava/util/List;)Lcom/squareup/workflow1/ui/container/BackStackScreen;
+	public final fun fromListOrNull (Ljava/util/List;)Lcom/squareup/workflow1/ui/container/BackStackScreen;
+}
+
 public final class com/squareup/workflow1/ui/container/BackStackScreenKt {
 	public static final fun toBackStackScreen (Ljava/util/List;)Lcom/squareup/workflow1/ui/container/BackStackScreen;
 	public static final fun toBackStackScreenOrNull (Ljava/util/List;)Lcom/squareup/workflow1/ui/container/BackStackScreen;

workflow-ui/core-common/src/main/java/com/squareup/workflow1/ui/container/BackStackScreen.kt

@@ -3,6 +3,9 @@ package com.squareup.workflow1.ui.container
 import com.squareup.workflow1.ui.Container
 import com.squareup.workflow1.ui.Screen
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
+import com.squareup.workflow1.ui.container.BackStackScreen.Companion
+import com.squareup.workflow1.ui.container.BackStackScreen.Companion.fromList
+import com.squareup.workflow1.ui.container.BackStackScreen.Companion.fromListOrNull
 
 /**
  * Represents an active screen ([top]), and a set of previously visited screens to which we may
@@ -13,25 +16,31 @@ import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
  *
  * UI kits are expected to provide handling for this class by default.
  *
- * @param bottom the bottom-most entry in the stack
- * @param rest the rest of the stack, empty by default
+ * @see fromList
+ * @see fromListOrNull
  */
 @WorkflowUiExperimentalApi
-public class BackStackScreen<StackedT : Screen>(
-  bottom: StackedT,
-  rest: List<StackedT>
+public class BackStackScreen<StackedT : Screen> private constructor(
+  public val frames: List<StackedT>
 ) : Screen, Container<Screen, StackedT> {
   /**
    * Creates a screen with elements listed from the [bottom] to the top.
    */
   public constructor(
     bottom: StackedT,
     vararg rest: StackedT
-  ) : this(bottom, rest.toList())
+  ) : this(listOf(bottom) + rest)
 
-  override fun asSequence(): Sequence<StackedT> = frames.asSequence()
+  @Deprecated(
+    "Use fromList",
+    ReplaceWith("BackStackScreen.fromList(listOf(bottom) + rest)")
+  )
+  public constructor(
+    bottom: StackedT,
+    rest: List<StackedT>
+  ) : this(listOf(bottom) + rest)
 
-  public val frames: List<StackedT> = listOf(bottom) + rest
+  override fun asSequence(): Sequence<StackedT> = frames.asSequence()
 
   /**
    * The active screen.
@@ -49,7 +58,7 @@ public class BackStackScreen<StackedT : Screen>(
     return if (other == null) {
       this
     } else {
-      BackStackScreen(frames[0], frames.subList(1, frames.size) + other.frames)
+      BackStackScreen(frames + other.frames)
     }
   }
 
@@ -75,16 +84,37 @@ public class BackStackScreen<StackedT : Screen>(
   override fun toString(): String {
     return "${this::class.java.simpleName}($frames)"
   }
+
+  public companion object {
+    /**
+     * Builds a [BackStackScreen] from a non-empty list of [frames].
+     *
+     * @throws IllegalArgumentException is [frames] is empty
+     */
+    public fun <T : Screen> fromList(frames: List<T>): BackStackScreen<T> {
+      require(frames.isNotEmpty()) {
+        "A BackStackScreen must have at least one frame."
+      }
+      return BackStackScreen(frames)
+    }
+
+    /**
+     * Builds a [BackStackScreen] from a list of [frames], or returns `null`
+     * if [frames] is empty.
+     */
+    public fun <T : Screen> fromListOrNull(frames: List<T>): BackStackScreen<T>? {
+      return when {
+        frames.isEmpty() -> null
+        else -> BackStackScreen(frames)
+      }
+    }
+  }
 }
 
 @WorkflowUiExperimentalApi
-public fun <T : Screen> List<T>.toBackStackScreenOrNull(): BackStackScreen<T>? = when {
-  isEmpty() -> null
-  else -> toBackStackScreen()
-}
+public fun <T : Screen> List<T>.toBackStackScreenOrNull(): BackStackScreen<T>? =
+  fromListOrNull(this)
 
 @WorkflowUiExperimentalApi
-public fun <T : Screen> List<T>.toBackStackScreen(): BackStackScreen<T> {
-  require(isNotEmpty())
-  return BackStackScreen(first(), subList(1, size))
-}
+public fun <T : Screen> List<T>.toBackStackScreen(): BackStackScreen<T> =
+  Companion.fromList(this)

workflow-ui/core-common/src/test/java/com/squareup/workflow1/ui/container/BackStackScreenTest.kt

@@ -45,9 +45,8 @@ internal class BackStackScreenTest {
 
   @Test fun `bottom and rest`() {
     assertThat(
-      BackStackScreen(
-        bottom = S(1),
-        rest = listOf(S(2), S(3), S(4))
+      BackStackScreen.fromList(
+        listOf(element = S(1)) + listOf(S(2), S(3), S(4))
       )
     ).isEqualTo(BackStackScreen(S(1), S(2), S(3), S(4)))
   }