Adding Qt Designer Plugins
You can use Qt APIs to create plugins that extend Qt applications. This enables you to add your own widgets to Qt Designer. The most flexible way to include a plugin with an application is to compile it into a dynamic library that is shipped separately, and detected and loaded at runtime.
The applications can detect plugins that are stored in the standard plugin subdirectories. For more information on how to create and locate plugins and to change the default plugin path, see How to Create Qt Plugins.
For more information about how to create plugins for Qt Designer, see Using Custom Widgets with Qt Designer.
Locating Qt Designer Plugins
Qt Designer fetches plugins from the standard locations and loads the plugins that match its build key. Qt Designer is delivered both as a standalone application and integrated into Qt Creator. The correct folder to place the plugins depends on whether you use the standalone Qt Designer or the integrated Qt Designer.
The integrated Qt Designer fetches plugins from the Qt Creator installation directory. Designer plugins are loaded from <QtCreator>\bin\plugins\designer on Windows, <QtCreator>/lib/Qt/plugins/designer on Linux, and Qt Creator.app/Contents/PlugIns/designer on macOS.
To check which plugins were loaded successfully and which failed, choose Tools > Form Editor > About Qt Designer Plugins.
The standalone Qt Designer is part of the Qt library used for building projects, located in <Qt_version>\<compiler>\bin in the Qt installation directory. It fetches plugins from the \plugins\designer subdirectory of bin. To check which plugins were loaded successfully and which failed, choose Help > About Plugins on Linux and Windows (or Qt Creator > About Plugins on macOS).
Matching Build Keys
The Qt Creator that is included in pre-built Qt packages on Windows is built with the Microsoft Visual Studio compiler. If you install a Qt version that was built with the MinGW/g++ compiler, plugins built with this version of Qt cannot be loaded by Qt Creator because the build-keys do not match. The same is true if you use a Qt version that is newer than the Qt version that Qt Creator was built with, or that is otherwise incompatible. The plugins can then only be used in the standalone version of Qt Designer. Choose Help > About Qt Creator to check the Qt version and compiler that Qt Creator was built with.
To use Qt Designer plugins that were built for the shipped Qt version, make sure that Qt Creator is built with the same compiler by either recompiling Qt Creator using MinGW or installing a Qt version that was built with Microsoft Visual Studio, depending on which configuration you want to use for your applications.