Remove exotic Modifier APIs, fix normalization (#1548)

* Remove exotic Modifier APIs, fix normalization

As we consider removing/reimagining Modifier as a concept in our composables, remove more of their exotic API which is focused on usage as interceptor-like structures (from Compose UI) rather than as key/value structures (from Redwood).

Added tests for remaining API behavior, including that of an inconsistency in how combined modifiers behave. The CombinedModifier type retains association on how it was assembled despite being otherwise a list-like structure. So ((a then b) then c) did not equal (a then (b then c)) in old code. Now, if an CombinedModifier is being added to another, its elements are unpacked forming a normal form. Additionally, since nothing references this type or needs to use it directly, remove it from the public API.

* Ensure capturing lambda instance is reused in toString

* Include comment

* Invoke constructor directly instead of polymorphic function

redwood-runtime/src/commonMain/kotlin/app/cash/redwood/Modifier.kt

@@ -25,68 +25,33 @@ import kotlin.native.ObjCName
 @Stable
 @ObjCName("Modifier", exact = true)
 public interface Modifier {
-
-  /**
-   * Accumulates a value starting with [initial] and applying [operation] to the current value
-   * and each element from outside in.
-   *
-   * Elements wrap one another in a chain from left to right; an [Element] that appears to the
-   * left of another in a `+` expression or in [operation]'s parameter order affects all
-   * of the elements that appear after it. [foldIn] may be used to accumulate a value starting
-   * from the parent or head of the modifier chain to the final wrapped child.
-   */
-  public fun <R> foldIn(initial: R, operation: (R, Element) -> R): R
-
-  /**
-   * Accumulates a value starting with [initial] and applying [operation] to the current value
-   * and each element from inside out.
-   *
-   * Elements wrap one another in a chain from left to right; an [Element] that appears to the
-   * left of another in a `+` expression or in [operation]'s parameter order affects all
-   * of the elements that appear after it. [foldOut] may be used to accumulate a value starting
-   * from the child or tail of the modifier chain up to the parent or head of the chain.
-   */
-  public fun <R> foldOut(initial: R, operation: (Element, R) -> R): R
-
   /**
    * Iterates over all [Element]s in this [Modifier].
    */
   public fun forEach(block: (Element) -> Unit)
 
-  /**
-   * Returns `true` if [predicate] returns true for any [Element] in this [Modifier].
-   */
-  public fun any(predicate: (Element) -> Boolean): Boolean
-
-  /**
-   * Returns `true` if [predicate] returns true for all [Element]s in this [Modifier] or if
-   * this [Modifier] contains no [Element]s.
-   */
-  public fun all(predicate: (Element) -> Boolean): Boolean
-
   /**
    * Concatenates this modifier with another.
    *
    * Returns a [Modifier] representing this modifier followed by [other] in sequence.
    */
   public infix fun then(other: Modifier): Modifier =
-    if (other === Modifier) this else CombinedModifier(this, other)
+    if (other === Modifier) {
+      this
+    } else if (other is CombinedModifier) {
+      // Normalize the element chain to be left-associative when multiple are added on the right.
+      var result: Modifier = this
+      other.forEach { element -> result = CombinedModifier(result, element) }
+      result
+    } else {
+      CombinedModifier(this, other)
+    }
 
   /**
    * A single element contained within a [Modifier] chain.
    */
   public interface Element : Modifier {
-    override fun <R> foldIn(initial: R, operation: (R, Element) -> R): R =
-      operation(initial, this)
-
-    override fun <R> foldOut(initial: R, operation: (Element, R) -> R): R =
-      operation(this, initial)
-
     override fun forEach(block: (Element) -> Unit): Unit = block(this)
-
-    override fun any(predicate: (Element) -> Boolean): Boolean = predicate(this)
-
-    override fun all(predicate: (Element) -> Boolean): Boolean = predicate(this)
   }
 
   /**
@@ -98,48 +63,38 @@ public interface Modifier {
   // modifier extension factory expression.
   @ObjCName("EmptyModifier", exact = true)
   public companion object : Modifier {
-    override fun <R> foldIn(initial: R, operation: (R, Element) -> R): R = initial
-    override fun <R> foldOut(initial: R, operation: (Element, R) -> R): R = initial
     override fun forEach(block: (Element) -> Unit) {}
-    override fun any(predicate: (Element) -> Boolean): Boolean = false
-    override fun all(predicate: (Element) -> Boolean): Boolean = true
     override infix fun then(other: Modifier): Modifier = other
     override fun toString(): String = "Modifier"
   }
 }
 
-/**
- * A node in a [Modifier] chain. A CombinedModifier always contains at least two elements;
- * a Modifier [outer] that wraps around the Modifier [inner].
- */
-@ObjCName("CombinedModifier", exact = true)
-public class CombinedModifier(
+private class CombinedModifier(
   private val outer: Modifier,
   private val inner: Modifier,
 ) : Modifier {
-  override fun <R> foldIn(initial: R, operation: (R, Modifier.Element) -> R): R =
-    inner.foldIn(outer.foldIn(initial, operation), operation)
-
-  override fun <R> foldOut(initial: R, operation: (Modifier.Element, R) -> R): R =
-    outer.foldOut(inner.foldOut(initial, operation), operation)
-
   override fun forEach(block: (Modifier.Element) -> Unit) {
     outer.forEach(block)
     inner.forEach(block)
   }
 
-  override fun any(predicate: (Modifier.Element) -> Boolean): Boolean =
-    outer.any(predicate) || inner.any(predicate)
-
-  override fun all(predicate: (Modifier.Element) -> Boolean): Boolean =
-    outer.all(predicate) && inner.all(predicate)
-
   override fun equals(other: Any?): Boolean =
     other is CombinedModifier && outer == other.outer && inner == other.inner
 
   override fun hashCode(): Int = outer.hashCode() + 31 * inner.hashCode()
 
-  override fun toString(): String = "[" + foldIn("") { acc, element ->
-    if (acc.isEmpty()) element.toString() else "$acc, $element"
-  } + "]"
+  override fun toString(): String = buildString {
+    append('[')
+
+    var first = true
+    val appendElement: (Modifier.Element) -> Unit = { element ->
+      if (!first) append(", ")
+      first = false
+      append(element)
+    }
+    outer.forEach(appendElement)
+    inner.forEach(appendElement)
+
+    append(']')
+  }
 }

redwood-runtime/src/commonTest/kotlin/app/cash/redwood/ui/ModifierTest.kt

@@ -0,0 +1,114 @@
+/*
+ * Copyright (C) 2023 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package app.cash.redwood.ui
+
+import app.cash.redwood.Modifier
+import assertk.assertThat
+import assertk.assertions.hashCodeFun
+import assertk.assertions.isEqualTo
+import assertk.assertions.isSameAs
+import assertk.assertions.toStringFun
+import kotlin.test.Test
+import kotlin.test.fail
+
+class ModifierTest {
+  @Test fun forEachUnitModifierNeverInvoked() {
+    Modifier.forEach {
+      fail()
+    }
+  }
+
+  @Test fun forEachOneElement() {
+    val a = NamedModifier("A")
+    var called = 0
+    a.forEach { element ->
+      assertThat(element).isSameAs(a)
+      called++
+    }
+    assertThat(called).isEqualTo(1)
+  }
+
+  @Test fun forEachManyElements() {
+    val a = NamedModifier("A")
+    val b = NamedModifier("B")
+    val c = NamedModifier("C")
+    val expected = listOf(a, b, c)
+    var called = 0
+    (a then b then c).forEach { element ->
+      assertThat(element).isSameAs(expected[called])
+      called++
+    }
+    assertThat(called).isEqualTo(3)
+  }
+
+  @Test fun thenIgnoresUnitModifier() {
+    assertThat(Modifier then Modifier).isSameAs(Modifier)
+
+    val a = NamedModifier("A")
+    assertThat(a then Modifier).isSameAs(a)
+    assertThat(Modifier then a).isSameAs(a)
+  }
+
+  @Test fun thenElementsToElements() {
+    val a = NamedModifier("A")
+    val b = NamedModifier("B")
+    val c = NamedModifier("C")
+    val d = NamedModifier("D")
+    val expected = listOf(a, b, c, d)
+    var called = 0
+    ((a then b) then (c then d)).forEach { element ->
+      assertThat(element).isSameAs(expected[called])
+      called++
+    }
+    assertThat(called).isEqualTo(4)
+  }
+
+  @Test fun toStringEmpty() {
+    assertThat(Modifier).toStringFun().isEqualTo("Modifier")
+  }
+
+  @Test fun toStringOne() {
+    val a = NamedModifier("A")
+    assertThat(a).toStringFun().isEqualTo("A")
+  }
+
+  @Test fun toStringMany() {
+    val a = NamedModifier("A")
+    val b = NamedModifier("B")
+    val c = NamedModifier("C")
+    assertThat((a then b) then c).toStringFun().isEqualTo("[A, B, C]")
+    assertThat(a then (b then c)).toStringFun().isEqualTo("[A, B, C]")
+  }
+
+  @Test fun hashCodeMany() {
+    val a = NamedModifier("A")
+    val b = NamedModifier("B")
+    val c = NamedModifier("C")
+    assertThat((a then b) then c).hashCodeFun().isEqualTo((a then (b then c)).hashCode())
+  }
+
+  @Test fun equalsMany() {
+    val a = NamedModifier("A")
+    val b = NamedModifier("B")
+    val c = NamedModifier("C")
+    assertThat((a then b) then c).isEqualTo(a then (b then c))
+    assertThat(a then (b then c)).isEqualTo((a then b) then c)
+  }
+}
+
+private class NamedModifier(private val name: String) : Modifier.Element {
+  override fun toString() = name
+}