Port Undo/Redo approach with a clean API

src/types/YArray.js

@@ -68,6 +68,11 @@ export class YArray extends AbstractType {
     this.insert(0, /** @type {Array} */ (this._prelimContent))
     this._prelimContent = null
   }
+
+  _copy () {
+    return new YArray()
+  }
+
   get length () {
     return this._prelimContent === null ? this._length : this._prelimContent.length
   }

src/types/YMap.js

@@ -72,6 +72,11 @@ export class YMap extends AbstractType {
     }
     this._prelimContent = null
   }
+
+  _copy () {
+    return new YMap()
+  }
+
   /**
    * Creates YMapEvent and calls observers.
    *
@@ -106,7 +111,7 @@ export class YMap extends AbstractType {
   /**
    * Returns the keys for each element in the YMap Type.
    *
-   * @return {Iterator<string>}
+   * @return {IterableIterator<string>}
    */
   keys () {
     return iterator.iteratorMap(createMapIterator(this._map), /** @param {any} v */ v => v[0])
@@ -115,7 +120,7 @@ export class YMap extends AbstractType {
   /**
    * Returns the keys for each element in the YMap Type.
    *
-   * @return {Iterator<string>}
+   * @return {IterableIterator<string>}
    */
   values () {
     return iterator.iteratorMap(createMapIterator(this._map), /** @param {any} v */ v => v[1].content.getContent()[v[1].length - 1])
@@ -130,6 +135,24 @@ export class YMap extends AbstractType {
     return iterator.iteratorMap(createMapIterator(this._map), /** @param {any} v */ v => [v[0], v[1].content.getContent()[v[1].length - 1]])
   }
 
+  /**
+   * Executes a provided function on once on overy key-value pair.
+   *
+   * @param {function(T,string,YMap<T>):void} f A function to execute on every element of this YArray.
+   */
+  forEach (f) {
+    /**
+     * @type {Object<string,T>}
+     */
+    const map = {}
+    for (let [key, item] of this._map) {
+      if (!item.deleted) {
+        f(item.content.getContent()[item.length - 1], key, this)
+      }
+    }
+    return map
+  }
+
   /**
    * @return {IterableIterator<T>}
    */

src/types/YText.js

@@ -625,6 +625,10 @@ export class YText extends AbstractType {
     this._pending = null
   }
 
+  _copy () {
+    return new YText()
+  }
+
   /**
    * Creates YTextEvent and calls observers.
    *

src/types/YXmlElement.js

@@ -45,12 +45,10 @@ export class YXmlElement extends YXmlFragment {
    */
   _integrate (y, item) {
     super._integrate(y, item)
-    this.insert(0, /** @type {Array} */ (this._prelimContent))
-    this._prelimContent = null
     ;(/** @type {Map<string, any>} */ (this._prelimAttrs)).forEach((value, key) => {
       this.setAttribute(key, value)
     })
-    this._prelimContent = null
+    this._prelimAttrs = null
   }
 
   /**

src/types/YXmlFragment.js

@@ -14,7 +14,7 @@ import {
   YXmlFragmentRefID,
   callTypeObservers,
   transact,
-  ContentType, Transaction, Item, YXmlText, YXmlHook, Snapshot // eslint-disable-line
+  Doc, ContentType, Transaction, Item, YXmlText, YXmlHook, Snapshot // eslint-disable-line
 } from '../internals.js'
 
 import * as encoding from 'lib0/encoding.js'
@@ -130,6 +130,27 @@ export class YXmlFragment extends AbstractType {
      */
     this._prelimContent = []
   }
+  /**
+   * Integrate this type into the Yjs instance.
+   *
+   * * Save this struct in the os
+   * * This type is sent to other client
+   * * Observer functions are fired
+   *
+   * @param {Doc} y The Yjs instance
+   * @param {Item} item
+   * @private
+   */
+  _integrate (y, item) {
+    super._integrate(y, item)
+    this.insert(0, /** @type {Array} */ (this._prelimContent))
+    this._prelimContent = null
+  }
+
+  _copy () {
+    return new YXmlFragment()
+  }
+
   /**
    * Create a subtree of childNodes.
    *

src/types/YXmlText.js

@@ -9,6 +9,9 @@ import * as decoding from 'lib0/decoding.js' // eslint-disable-line
  * simple formatting information like bold and italic.
  */
 export class YXmlText extends YText {
+  _copy () {
+    return new YXmlText()
+  }
   /**
    * Creates a Dom Element that mirrors this YXmlText.
    *