Q:

CKEditor - Widget - set a toolbar for the button

In a CKEditor plugin, you can specify a toolbar for a button with this :
    init:function(editor){
        editor.ui.addButton('myplug',{
            label:'my plug',
            command:'myplug',
            toolbar:'mytoolbar'

With widget, I don't find this possibility. Is there a way to do that without move the node in JS, that is a bit complicated ?

A:

You can do the same with widgets. Here's a widget's plugin.js file with a button and toolbar declaration under the init function:
CKEDITOR.plugins.add( 'mywidget', {
    requires: 'widget',
    icons: 'mywidget',
    init: function( editor ) {
        CKEDITOR.dialog.add('mywidget', this.path + 'dialogs/mywidget.js')
        editor.widgets.add( 'mywidget' , {
            //
            // Your widget logic is here ...
            //
        });

        editor.ui.addButton('mywidget', {
            label: 'My Widget'
            command: 'mywidget'
            toolbar: 'mytoolbar, 1'
        });
    }
} );

You'll need to add the "mytoolbar" toolbar in your config.js file, but I suppose you already have because you mentioned being able to add a button for a plug-in.

