SelectorObserver (#927)

* Create new SelectorObserver class

* Update DOMDataBinder to use SelectorObserver

* Update names to be more clear

* Remove attributeOldValue parameter, clarify attributes parameter

* Add documentation

1 files changed, 255 insertions, 0 deletions

diff --git a/ext/mixed/js/selector-observer.js b/ext/mixed/js/selector-observer.js
new file mode 100644
index 00000000..9f2f6c72
--- /dev/null
+++ b/ext/mixed/js/selector-observer.js
@@ -0,0 +1,255 @@
+/*
+ * Copyright (C) 2020  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Class which is used to observe elements matching a selector in specific element.
+ */
+class SelectorObserver {
+    /**
+     * Creates a new instance.
+     * @param selector A string CSS selector used to find elements.
+     * @param ignoreSelector A string CSS selector used to filter elements, or null for no filtering.
+     * @param onAdded A function which is invoked for each element that is added that matches the selector.
+     *   The signature is (element) => data.
+     * @param onRemoved A function which is invoked for each element that is removed, or null.
+     *   The signature is (element, data) => void.
+     * @param onChildrenUpdated A function which is invoked for each element which has its children updated, or null.
+     *   The signature is (element, data) => void.
+     * @param isStale A function which checks if the data is stale for a given element, or null.
+     *   If the element is stale, it will be removed and potentially re-added.
+     *   The signature is (element, data) => bool.
+     */
+    constructor({selector, ignoreSelector=null, onAdded=null, onRemoved=null, onChildrenUpdated=null, isStale=null}) {
+        this._selector = selector;
+        this._ignoreSelector = ignoreSelector;
+        this._onAdded = onAdded;
+        this._onRemoved = onRemoved;
+        this._onChildrenUpdated = onChildrenUpdated;
+        this._isStale = isStale;
+        this._observingElement = null;
+        this._mutationObserver = new MutationObserver(this._onMutation.bind(this));
+        this._elementMap = new Map(); // Map([element => observer]...)
+        this._elementAncestorMap = new Map(); // Map([element => Set([observer]...)]...)
+        this._isObserving = false;
+    }
+
+    /**
+     * Returns whether or not an element is currently being observed.
+     * @returns True if an element is being observed, false otherwise.
+     */
+    get isObserving() {
+        return this._observingElement !== null;
+    }
+
+    /**
+     * Starts DOM mutation observing the target element.
+     * @param element The element to observe changes in.
+     * @param attributes A boolean for whether or not attribute changes should be observed.
+     * @throws An error if element is null.
+     * @throws An error if an element is already being observed.
+     */
+    observe(element, attributes=false) {
+        if (element === null) {
+            throw new Error('Invalid element');
+        }
+        if (this.isObserving) {
+            throw new Error('Instance is already observing an element');
+        }
+
+        this._observingElement = element;
+        this._mutationObserver.observe(element, {
+            attributes: !!attributes,
+            childList: true,
+            subtree: true
+        });
+
+        this._onMutation([{
+            type: 'childList',
+            target: element.parentNode,
+            addedNodes: [element],
+            removedNodes: []
+        }]);
+    }
+
+    /**
+     * Stops observing the target element.
+     */
+    disconnect() {
+        if (!this.isObserving) { return; }
+
+        this._mutationObserver.disconnect();
+        this._observingElement = null;
+
+        for (const observer of this._elementMap.values()) {
+            this._removeObserver(observer);
+        }
+    }
+
+    /**
+     * Returns an iterable list of [element, data] pairs.
+     * @yields A sequence of [element, data] pairs.
+     */
+    *entries() {
+        for (const [element, {data}] of this._elementMap) {
+            yield [element, data];
+        }
+    }
+
+    /**
+     * Returns an iterable list of data for every element.
+     * @yields A sequence of data values.
+     */
+    *datas() {
+        for (const {data} of this._elementMap.values()) {
+            yield data;
+        }
+    }
+
+    // Private
+
+    _onMutation(mutationList) {
+        for (const mutation of mutationList) {
+            switch (mutation.type) {
+                case 'childList':
+                    this._onChildListMutation(mutation);
+                    break;
+                case 'attributes':
+                    this._onAttributeMutation(mutation);
+                    break;
+            }
+        }
+    }
+
+    _onChildListMutation({addedNodes, removedNodes, target}) {
+        const selector = this._selector;
+        const ELEMENT_NODE = Node.ELEMENT_NODE;
+
+        for (const node of removedNodes) {
+            const observers = this._elementAncestorMap.get(node);
+            if (typeof observers === 'undefined') { continue; }
+            for (const observer of observers) {
+                this._removeObserver(observer);
+            }
+        }
+
+        for (const node of addedNodes) {
+            if (node.nodeType !== ELEMENT_NODE) { continue; }
+            if (node.matches(selector)) {
+                this._createObserver(node);
+            }
+            for (const childNode of node.querySelectorAll(selector)) {
+                this._createObserver(childNode);
+            }
+        }
+
+        if (
+            this._onChildrenUpdated !== null &&
+            (addedNodes.length !== 0 || addedNodes.length !== 0)
+        ) {
+            for (let node = target; node !== null; node = node.parentNode) {
+                const observer = this._elementMap.get(node);
+                if (typeof observer !== 'undefined') {
+                    this._onObserverChildrenUpdated(observer);
+                }
+            }
+        }
+    }
+
+    _onAttributeMutation({target}) {
+        const selector = this._selector;
+        const observers = this._elementAncestorMap.get(target);
+        if (typeof observers !== 'undefined') {
+            for (const observer of observers) {
+                const element = observer.element;
+                if (
+                    !element.matches(selector) ||
+                    this._shouldIgnoreElement(element) ||
+                    this._isObserverStale(observer)
+                ) {
+                    this._removeObserver(observer);
+                }
+            }
+        }
+
+        if (target.matches(selector)) {
+            this._createObserver(target);
+        }
+    }
+
+    _createObserver(element) {
+        if (this._elementMap.has(element) || this._shouldIgnoreElement(element) || this._onAdded === null) { return; }
+
+        const data = this._onAdded(element);
+        const ancestors = this._getAncestors(element);
+        const observer = {element, ancestors, data};
+
+        this._elementMap.set(element, observer);
+
+        for (const ancestor of ancestors) {
+            let observers = this._elementAncestorMap.get(ancestor);
+            if (typeof observers === 'undefined') {
+                observers = new Set();
+                this._elementAncestorMap.set(ancestor, observers);
+            }
+            observers.add(observer);
+        }
+    }
+
+    _removeObserver(observer) {
+        const {element, ancestors, data} = observer;
+
+        this._elementMap.delete(element);
+
+        for (const ancestor of ancestors) {
+            const observers = this._elementAncestorMap.get(ancestor);
+            if (typeof observers === 'undefined') { continue; }
+
+            observers.delete(observer);
+            if (observers.size === 0) {
+                this._elementAncestorMap.delete(ancestor);
+            }
+        }
+
+        if (this._onRemoved !== null) {
+            this._onRemoved(element, data);
+        }
+    }
+
+    _onObserverChildrenUpdated(observer) {
+        this._onChildrenUpdated(observer.element, observer.data);
+    }
+
+    _isObserverStale(observer) {
+        return (this._isStale !== null && this._isStale(observer.element, observer.data));
+    }
+
+    _shouldIgnoreElement(element) {
+        return (this._ignoreSelector !== null && element.matches(this._ignoreSelector));
+    }
+
+    _getAncestors(node) {
+        const root = this._observingElement;
+        const results = [];
+        while (true) {
+            results.push(node);
+            if (node === root) { break; }
+            node = node.parentNode;
+            if (node === null) { break; }
+        }
+        return results;
+    }
+}