Maximum Open Projects Sample implements a ProjectManagerListener with two methods applied to check if the current projects have been opened or closed. Each method refers to the ProjectCountingService service registered as an com.intellij.applicationService extension point. It provides methods to increase and decrease the global counter of the currently opened projects in the IDE. After opening each one, a message dialog is presented to the user with the current number.

Extension Points

Name	Implementation	Extension Point Class
`com.intellij.applicationService`	ProjectCountingService	n/a

Reference: Plugin Extension Points in IntelliJ SDK Docs

Application Listeners

Name	Implementation	Listener Class
listener	ProjectOpenCloseListener	`ProjectManagerListener`

Reference: Plugin Listeners in IntelliJ SDK Docs