onTap property

VoidCallback onTap

The handler for SemanticsAction.tap.

This is the semantic equivalent of a user briefly tapping the screen with the finger without moving it. For example, a button should implement this action.

VoiceOver users on iOS and TalkBack users on Android can trigger this action by double-tapping the screen while an element is focused.

On Android prior to Android Oreo a double-tap on the screen while an element with an onTap handler is focused will not call the registered handler. Instead, Android will simulate a pointer down and up event at the center of the focused element. Those pointer events will get dispatched just like a regular tap with TalkBack disabled would: The events will get processed by any GestureDetector listening for gestures in the center of the focused element. Therefore, to ensure that onTap handlers work properly on Android versions prior to Oreo, a GestureDetector with an onTap handler should always be wrapping an element that defines a semantic onTap handler. By default a GestureDetector will register its own semantic onTap handler that follows this principle.

Implementation 

VoidCallback get onTap => _onTap;
void onTap= (VoidCallback value)

Implementation 

set onTap(VoidCallback value) {
  _addArgumentlessAction(SemanticsAction.tap, value);
  _onTap = value;
}