Add support for MountableSpan interface

Summary:
This diff introduces the `MountableSpan` interface that can be added to custom spans to receive callback with the hosting View when the component is mounted or unmounted.

This approach was also discussed in the scope of the [Composable and Extensible span RFC for Bloks](https://docs.google.com/document/d/1EKlAx9dMWsrltmmN1Ge9NfsXSycSXKwCLcmoCoxbwlE/edit?tab=t.0#bookmark=id.l3rxz5cm5xj2) but it was never implemented.

Now this interface is used for the GenAI use case implemented in the next diff.

Reviewed By: zielinskimz

Differential Revision: D64940916

fbshipit-source-id: 12b4f1647e6e7f88f54078f598d9522a0c39589c

litho-rendercore-text/src/main/java/com/facebook/rendercore/text/MountableSpan.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * 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 com.facebook.rendercore.text
+
+import android.view.View
+
+/**
+ * A span interface that allows users to receive callbacks when the hosting [View] is mounted and
+ * unmounted so that it is possible to the component to get access to it..
+ */
+interface MountableSpan {
+
+  fun onMount(view: View)
+
+  fun onUnmount(view: View)
+}

litho-rendercore-text/src/main/java/com/facebook/rendercore/text/RCTextView.java

@@ -127,7 +127,7 @@ public void draw(Canvas canvas) {
       shouldRestore = true;
     }
 
-    final OnPrePostDrawSpan[] onPrePostDrawSpans = getOnPrePostDrawableSpans();
+    final OnPrePostDrawSpan[] onPrePostDrawSpans = getOnPrePostDrawSpans();
     if (onPrePostDrawSpans.length == 0) {
       drawLayout(canvas);
     } else {
@@ -174,13 +174,20 @@ private void drawLayout(Canvas canvas) {
     mLayout.draw(canvas, getSelectionPath(), mHighlightPaint, 0);
   }
 
-  private OnPrePostDrawSpan[] getOnPrePostDrawableSpans() {
+  private OnPrePostDrawSpan[] getOnPrePostDrawSpans() {
     if (!(mText instanceof Spanned)) {
       return new OnPrePostDrawSpan[0];
     }
     return ((Spanned) mText).getSpans(0, mText.length(), OnPrePostDrawSpan.class);
   }
 
+  private MountableSpan[] getMountableSpans() {
+    if (!(mText instanceof Spanned)) {
+      return new MountableSpan[0];
+    }
+    return ((Spanned) mText).getSpans(0, mText.length(), MountableSpan.class);
+  }
+
   public void mount(TextLayout textLayout) {
     final ColorStateList colorStateList = textLayout.textStyle.textColorStateList;
     mText = textLayout.processedText;
@@ -231,6 +238,11 @@ public void mount(TextLayout textLayout) {
     if (textLayout.textStyle.accessibilityLabel != null) {
       setContentDescription(textLayout.textStyle.accessibilityLabel);
     }
+
+    final MountableSpan[] mountableSpans = getMountableSpans();
+    for (MountableSpan mountableSpan : mountableSpans) {
+      mountableSpan.onMount(this);
+    }
     invalidate();
   }
 
@@ -239,6 +251,10 @@ public boolean isTextTruncated() {
   }
 
   public void unmount() {
+    final MountableSpan[] mountableSpans = getMountableSpans();
+    for (MountableSpan mountableSpan : mountableSpans) {
+      mountableSpan.onUnmount(this);
+    }
     // NULLSAFE_FIXME[Field Not Nullable]
     mText = null;
     // NULLSAFE_FIXME[Field Not Nullable]