Plugins for 3.x
This is a guide for the MuseScore 3.0 plugin implementation.
Documentation
Formal, current, documentation is available in Doxygen format.
The pages here may provide supplemental information, issue work-arounds and code examples to the extent that community members working on plugins document their learnings and experiences here.
Seeking Help
If you are unable to obtain what you need from either the Doxygen documentation, or these pages, then please use the Plugins Forum Section to ask a question.
And if you are a community member it would be great if you could contribute to this documentation once you have resolved your question or issue.
Element Properties
Among the first challenges of learning how to operate on score elements within a plugin is the challenge of understanding the properties of the various elements of a score that are available to your plugin code. The starting place for that is the formal Doxygen documentation. However, that is often a bit too terse to be sufficient and does not contain any code examples to guide you. You may find the following page in this handbook to be useful: Use Case - Element Explorer
Code Examples
The currently available documentation is in great need of example code. Your best source of example code is the plugin library - this is a repository of plugins created by the community. Studying a plugin that may relate to what you are attempting to achieve is often the best pathway to success. Of course, coding styles and documentation comments in those plugins will vary; but you can seek help from the author via a comment to their plugin's library's entry and/or asking in the plugin forum.
QML
Plugins are written in 'QtQuick' / 'QML', which is a development framework maintained by the Qt Company. For MuseScore 3.x you should assume Qt version 5.9. QML is essentially javascript with a user-interface framework layered on top. The scripting portion of QML purports to be ECMA6 compliant; but you'd see in the QT forums that there are a few gaps to the standard. (For example, in Qt 5.9 the ECMA6 Arrow Function is not available.)