Insync context menus and overlay icons use the KDE5 API and may not work in KDE4.
Insync context menus are implemented using the KDE API KAbstractFileItemActionPlugin. The KAbstractFileItemActionPlugin:actions is called whenever a user right-clicks on a file or directory and should return a list of QActions. If this returns an empty list, no additional context menus are added. This QActions listed are always appended to the default list of options in the context menu. In our implementation, we return a KActionMenu containing the list of QActions (Add to Insync, Share, etc) so that all of our context menu options are containined in its own submenu. Right now, we only support context menu actions when a single file/directory is selected as the client doesn't support it yet. A .desktop KDE service file is needed for this to show up in the list of plugins a user can enable in the Dolphin settings.
Insync file overlay status icons are implemented using KOverlayIconPlugin. The KOverlayIconPlugin:getOverlays is called on every file/directory currently showing in Dolphin. Since it is called on every file/directory, we create a new QLocalSocket on every call as it can cause segfaults if there are too many files/directories being queried. The build library file for this is added to the KDE_PLUGIN_DIR/kf5/overlayicon folder and it should work without needing a KDE service file.
We have an additional library InsyncDolphinPluginHelper that's used in both the context menu and overlay icon libraries for doing common functions (connecting to insync.sock, sending/receiving commands to the insync.sock, etc).
mkdir build && cd build
cmake ..
cmake --build .
QJson Docs: https://doc.qt.io/qt-5/json.html