start mecab only after enabling the setting

ext/bg/js/backend.js

@@ -98,6 +98,10 @@ class Backend {
         }
 
         this.anki = options.anki.enable ? new AnkiConnect(options.anki.server) : new AnkiNull();
+
+        if (options.parsing.enableMecabParser) {
+            this.mecab.startListener();
+        }
     }
 
     async getFullOptions() {

ext/bg/js/mecab.js

@@ -19,16 +19,20 @@
 
 class Mecab {
     constructor() {
+        this.port = null;
         this.listeners = {};
         this.sequence = 0;
-        this.startListener();
     }
 
     async parseText(text) {
+        if (this.port === null) {
+            return {};
+        }
         return await this.invoke('parse_text', {text});
     }
 
     startListener() {
+        if (this.port !== null) { return; }
         this.port = chrome.runtime.connectNative('yomichan_mecab');
         this.port.onMessage.addListener((message) => {
             const {sequence, data} = message;
@@ -41,6 +45,14 @@ class Mecab {
         });
     }
 
+    stopListener() {
+        if (this.port === null) { return; }
+        this.port.disconnect();
+        this.port = null;
+        this.listeners = {};
+        this.sequence = 0;
+    }
+
     invoke(action, params) {
         return new Promise((resolve, reject) => {
             const sequence = this.sequence++;

ext/bg/js/settings.js

@@ -154,6 +154,7 @@ async function formWrite(options) {
 
 function formSetupEventListeners() {
     $('input, select, textarea').not('.anki-model').not('.ignore-form-changes *').change(utilAsync(onFormOptionsChanged));
+    $('#parsing-mecab-enable').change(onParseMecabChanged);
     $('.anki-model').change(utilAsync(onAnkiModelChanged));
 }
 
@@ -440,6 +441,20 @@ function onMessage({action, params}, sender, callback) {
 }
 
 
+/*
+ * Text parsing
+ */
+
+function onParseMecabChanged(e) {
+    const mecab = utilBackend().mecab;
+    if (e.target.checked) {
+        mecab.startListener();
+    } else {
+        mecab.stopListener();
+    }
+}
+
+
 /*
  * Anki
  */

ext/bg/settings.html

@@ -410,6 +410,35 @@
                 </div>
             </div>
 
+            <div id="text-parsing">
+                <h3>Text Parsing Options</h3>
+
+                <p class="help-block">
+                    Yomichan can attempt to parse entire sentences or longer text blocks on the search page,
+                    adding furigana above words and a small space between words.
+                </p>
+
+                <p class="help-block">
+                    Two types of parsers are supported. The first one, enabled by default, works using the built-in
+                    scanning functionality by automatically advancing in the sentence after a matching word.
+                </p>
+
+                <p class="help-block">
+                    The second type is an external program called <a href="https://en.wikipedia.org/wiki/MeCab" target="_blank" rel="noopener">MeCab</a>
+                    that uses its own dictionaries and a special parsing algorithm. To get it working, you must first
+                    install it and <a href="https://github.com/siikamiika/yomichan-mecab-installer" target="_blank" rel="noopener">a native messaging component</a>
+                    that acts as a bridge between the program and Yomichan.
+                </p>
+
+                <div class="checkbox">
+                    <label><input type="checkbox" id="parsing-scan-enable"> Enable text parsing using installed dictionaries</label>
+                </div>
+
+                <div class="checkbox">
+                    <label><input type="checkbox" id="parsing-mecab-enable"> Enable text parsing using MeCab</label>
+                </div>
+            </div>
+
             <div class="ignore-form-changes">
                 <div>
                     <img src="/mixed/img/spinner.gif" class="pull-right" id="dict-spinner" alt>
@@ -587,35 +616,6 @@
                 </div></div>
             </div>
 
-            <div id="text-parsing">
-                <h3>Text Parsing Options</h3>
-
-                <p class="help-block">
-                    Yomichan can attempt to parse entire sentences or longer text blocks on the search page,
-                    adding furigana above words and a small space between words.
-                </p>
-
-                <p class="help-block">
-                    Two types of parsers are supported. The first one, enabled by default, works using the built-in
-                    scanning functionality by automatically advancing in the sentence after a matching word.
-                </p>
-
-                <p class="help-block">
-                    The second type is an external program called <a href="https://en.wikipedia.org/wiki/MeCab" target="_blank" rel="noopener">MeCab</a>
-                    that uses its own dictionaries and a special parsing algorithm. To get it working, you must first
-                    install it and <a href="https://github.com/siikamiika/yomichan-mecab-installer" target="_blank" rel="noopener">a native messaging component</a>
-                    that acts as a bridge between the program and Yomichan.
-                </p>
-
-                <div class="checkbox">
-                    <label><input type="checkbox" id="parsing-scan-enable"> Enable text parsing using installed dictionaries</label>
-                </div>
-
-                <div class="checkbox">
-                    <label><input type="checkbox" id="parsing-mecab-enable"> Enable text parsing using MeCab</label>
-                </div>
-            </div>
-
             <div>
                 <div>
                     <img src="/mixed/img/spinner.gif" class="pull-right" id="anki-spinner" alt>