Learn Flutter from UI challenges

As a continuation of my first article, I am going to take a new challenge. This one will be a bit more complex than the previous (the shimmer). I call it a flip animation:

It’s good enough for a challenge, isn’t it? Yep, we are going to play with an animation running a likely 3-D effect.

How it works

At a first glance, the idea is simple: we have a stack of panels, each is broken into 2 halves, each half can rotate around X-axis and reveal a next one.

How to do it in code? I am going to divide it into 2 tasks:

Cut a panel into 2 halves

Rotate a half of panel around X-axis

So how does Flutter support us? Looking into Flutter document, I found two Widgets fit for our tasks: ClipRect and Transform.

Implementation

Cut a panel into 2 halves:

ClipRect widget has a clipper parameter to define the size and location of the clip rect but its document also suggests another way to use ClipRect, by combining it with an Align:

Try it out:

That’s it. Additionally, the child lets us freely design the content of our animation (a text, an image, no matter).

Rotate a half of panel around X-axis:

Transform widget has a transform parameter of type Matrix4 to define the kind of transform applied and Matrix4 exposes a factory constructor called rotationX(), it looks like what we need, let try it for the upper half of panel:

Try it out:

What!!!! It looks like a scale effect, doesn’t it?

What’s wrong? Answer this question is the hardest point of this challenge. I turn to Flutter document, example codes, articles… and end up with this one. It points out that changing value at row 3 and column 2 of Matrix4 will change its perspective and bring up 3-D effect into our transformation:

Try it out:

Cool. But what about the magic number of 0.006? To be honest, I don’t know how to calculate it exactly, just try some values and pick one I feel good.

The rest is to add animation for our widget. It needs a bit tricky here. In fact, each panel has content on both sides (front and back) but it’s silly to make it in code because there’s only one side visible at a moment. I assume we are going to create an animation that panels flip upward, so our animation turns out to be two phases (sequentially), the first is to flip the lower half upward to make the animation of revealing the lower half of a next panel while hiding the lower half of current panel, the second is to flip the upper half in the same direction to reveal the upper half of the next while hiding the upper half of the current:

The implementation code of this animation is quite long, it’s not a good idea to embed it here. You can find it in the link I put at the bottom of this article. Here’s our final result:

Awesome. We have just finished another UI challenge with Flutter. The more you practice, the better you become. I will keep looking new challenges, solve it with Flutter and share the results with you. Thanks for reading.

P/S: There’s something wrong in the perspective transform (causes the transformed image skewed), I use a very small value instead of zero for rotateX() to temporarily get around this matter.