Opens the Drawer (if any).
If the scaffold has a non-null Scaffold.drawer, this function will cause the drawer to begin its entrance animation.
Normally this is not needed since the Scaffold automatically shows an appropriate IconButton, and handles the edge-swipe gesture, to show the drawer.
To close the drawer once it is open, use Navigator.pop.
See Scaffold.of for information about how to obtain the ScaffoldState.
Source
void openDrawer() {
_drawerKey.currentState?.open();
}