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Nodes are defined by creating a class, e.g. jsoneditor_myplugin/json_editor/node/marquee.php.
Plugin example
To see how this all fits together in a working example, take a look at the example
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weka_marquee
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plugin:
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https://git.totaralearning.com/projects/EXAMPLE/repos/weka_marquee/browse
This plugin implements support for the <marquee> tag in the Weka editor. This could serve as a starting point for your own plugin, or just as a reference.
You can also look at how the core Weka extensions are implemented in
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client/component/editor_weka/src/js/extensions/.