refactor text model to memoize decorations

lib/models/text.js

@@ -103,7 +103,7 @@ class Text extends new Record(DEFAULTS) {
     let { characters, cache } = this
     if (characters == cache) return this
 
-    const decorations = decorator(this)
+    const decorations = this.getDecoratedCharacters(decorator)
     if (decorations == characters) return this
 
     return this.merge({
@@ -112,10 +112,20 @@ class Text extends new Record(DEFAULTS) {
     })
   }
 
+  /**
+   * Get the decorated characters.
+   *
+   * @return {List} characters
+   */
+
+  getDecoratedCharacters(decorator) {
+    return decorator(this)
+  }
+
   /**
    * Get the decorated characters grouped by marks.
    *
-   * @return {List}
+   * @return {List} ranges
    */
 
   getDecoratedRanges() {
@@ -125,7 +135,7 @@ class Text extends new Record(DEFAULTS) {
   /**
    * Get the characters grouped by marks.
    *
-   * @return {List}
+   * @return {List} ranges
    */
 
   getRanges() {
@@ -234,6 +244,7 @@ class Text extends new Record(DEFAULTS) {
  */
 
 memoize(Text.prototype, [
+  'getDecoratedCharacters',
   'getDecoratedRanges',
   'getRanges',
   'getRangesForCharacters'