I'm making an qt application named onscreenapp to help showing onscreen.
The mainly design ideas are as below:

• There are four type onscreen.

	critical
	exclamation
	information
	question

• onscreen may have 0~3 buttons(no button onscreen only show 3 seconds).
• when application want to show onscreen,it will call showWindow to request
  onscreenapp to show its onscreen.The arguments of showWindow is as below:

{
	"application_id": "onscreenapp",
	"parameter": {
		"title": "onscreen title",
		"type": "critical/exclamation/question/information",
		"contents": "message contents",
		"buttons": ["button_name1", "button_name2", "button_name3"]
	}
}

• In onscreenapp only can show one onscreen at the same time,so the last request onscreen will show.

About the sequence of show/hide onscreen, please look at showOnscreen.svg.

Sourcecode