Hi again! It's about time to add some more code to the BMI Calculator app designed by Johny Vino. In this post, I will start creating bottom pacman slider. I will go through implementing the animated hint which will indicate user on how to use the slider. Let's make it happen! :)
BMI Calculator in Flutter – Part 5 – Animated Slider Hint

BMI Calculator in Flutter – Part 5 – Animated Slider Hint

Hi again! It’s about time to add some more code to the BMI Calculator app – by Johny Vino‘s design. In this post, I will start creating bottom pacman slider (see below). I will go through implementing the animated hint which will indicate user on how to use the slider. Let’s make it happen! 🙂

As always, let me show you the effect we want to achieve first. This time I am going to show you the exact result widget from this development instead of the design:


Static widgets

First, let’s create static aspects of this view, PacmanIcon and Dot are simple stateless widgets to be displayed. Since we are going to have 2 sizes of dots, the size will be parametrized:

We are going to place those widgets inside rows and paddings:

The outer Container specifies slider’s height and decoration (with shape and color) – nothing interesting here.

The main body of the widget is a Row with pacman and dots. To create the middle dots, we used List.generate – it allows us to generate any list (in our case a list of widgets) using simple generator (_generateDot) method. We can also add the bigger dot at the end of the list. Notice that we already specified Opacity for that dot, the middle ones will be animated later on. Let’s take a sneak peek on what we have:


AnimationController

Now let’s add animation controller, which will be responsible for changing dots’ opacity. For now, we can assume that this controller’s animation will take 1 second and after another 800 milliseconds, it will run again from scratch. At this point, we can also wrap middle dots with AnimatedBuilder, so that they will be rebuilt alongside animation changes. Let’s take a look at the code:

At this point it looks like this:


Intervals

As you can see, at this point every dot is animated at the same time. I guess it’s a bad hint to swipe right. Theoretically, we could create a list of AnimationControllers and have them be delayed from each other by a small amount of time, but I don’t think it would be an optimal solution. The approach I went with is the same I used for Flight Search challenge – Intervals. Intervals are special kind of animation curves that allow us to “use” only a part of AnimationController. We can specify at what point of our parent animation we want interval animation to begin and end.

Intervals concept

Let’s take a look at how it looks in code:

Notice that we also specified Tween in which we defined the minimum and maximum values for the animations. Now we can see that dots appear one after another:


Sinusoidal animation

We are very close to the final effect, now we have to deal with the following problem: After a dot is animated it stays in the maximum state but it should get back to the minimum. Unfortunately, Tween animations will always give us a curve from begin to end and we need it to go from min to max to min again. How to deal with that problem? Create our own Animation!

This animation will provide sinusoidal behavior of the value, so that every dot will nicely get from minOpacity to maxOpacity and then return back to minOpacity. The only thing left now is replacing Tween with our new animation and tweaking up the parameters to have the look we want:

And that’s it!

If there is anything unclear or there is something I did wrong, please leave a comment and tell me about it!

You can find full code for this stage in here and other posts related to this project in here.

Cheers 🙂

4 thoughts on “BMI Calculator in Flutter – Part 5 – Animated Slider Hint

  1. Hey Marcin, great article!!

    I was wondering about the implementation of the lerp() function.
    Isn’t the goal of going from min to max to min achieved with only overriding the transform function like you did?

    So my question is about the implementation of lerp(). This is the implementation in Tween class.
    T lerp(double t) {
    return begin + (end – begin) * t;
    }
    And you are doing
    return min + (max – min) * math.sin(math.pi * t);

    Would you mind explaining why is that necessary?
    Thanks!

    Also you misspelled Tween in the end of the Intervals sections, below the code sample.

    1. Hey Sebastian,
      That’s very good question. Especially that you are right!
      I didn’t notice that lerp method was only introduced in Tween, not in Animatable itself, so we could (and probably should!) just use transform method instead.

      I will update the code and the typo 😀
      Thank you 🙂

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