countdownTimer.dart

   return Scaffold(
      backgroundColor: Colors.white10,
      body: AnimatedBuilder(
          animation: controller,
          builder: (context, child) {
            return Padding(
              padding: EdgeInsets.all(8.0),
              child: Column(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: <Widget>[
                  Expanded(
                    child: Align(
                      alignment: FractionalOffset.center,
                      child: AspectRatio(
                        aspectRatio: 1.0,
                        child: Stack(
                          children: <Widget>[
                            Positioned.fill(
                              child:
                              AnimatedBuilder(
                                animation: controller,
                                builder:
                                    (BuildContext context, Widget child) {
                                  return CustomPaint(
                                      painter: CustomTimerPainter(
                                    animation: controller,
                                    backgroundColor: Colors.white,
                                    color: themeData.indicatorColor,
                                  ));
                                },
                              ),
                            ),
                            Align(
                              alignment: FractionalOffset.center,
                              child: Column(
                                mainAxisAlignment:
                                    MainAxisAlignment.spaceEvenly,
                                crossAxisAlignment:
                                    CrossAxisAlignment.center,
                                children: <Widget>[
                                  Text(
                                    "Count Down Timer",
                                    style: TextStyle(
                                        fontSize: 20.0,
                                        color: Colors.white),
                                  ),
                                  AnimatedBuilder(
                                      animation: controller,
                                      builder: (BuildContext context,
                                          Widget child) {
                                        return Text(
                                          timerString,
                                          style: TextStyle(
                                              fontSize: 112.0,
                                              color: Colors.white),
                                        );
                                      }),
                                ],
                              ),
                            ),
                          ],
                        ),
                      ),
                    ),
                  ),
                  AnimatedBuilder(
                      animation: controller,
                      builder: (context, child) {
                        return FloatingActionButton.extended(
                            onPressed: () {
                              if (controller.isAnimating)
                                controller.stop();
                              else {
                                controller.reverse(
                                    from: controller.value == 0.0
                                        ? 1.0
                                        : controller.value);
                              }
                            },
                            icon: Icon(controller.isAnimating
                                ? Icons.pause
                                : Icons.play_arrow),
                            label: Text(
                                controller.isAnimating ? "Pause" : "Play"));
                      }),
                ],
              ),
            );
          }),
    );