draggable2.dart

DragTarget(
    builder: (context, List<FlutterLogoColor> candidateData, rejectedData) {
        return Container(
            height: 50.0,
            width: 50.0,
            decoration: BoxDecoration(
                color: Colors.yellow[600],
                shape: BoxShape.circle
            ),
            child: Icon(Icons.delete, size: 40, color: Colors.white70,),
        );
    },
    onWillAccept: (data) {
        if (data == FlutterLogoColor.red) {
            return true;
        } else {
            _scaffoldKey.currentState.showSnackBar(
                SnackBar(
                    content: Text("Not Allowed!"),
                    duration: Duration(seconds: 1),
                ));
            return false;
        }
    },
    onAccept: (data) {
        _scaffoldKey.currentState.showSnackBar(
            SnackBar(
                content: Text("Deleted!"),
                duration: Duration(seconds: 1),
                backgroundColor: Colors.red,
            ));
    },
)