How We Built the Add to Favorite Animation in Shop

I just want you to feel it

I use the word feeling a lot when working on animations and gestures. For example, animations or gestures sometimes feel right or wrong. I think about that word a lot because our experiences using software are based on an intuitive understanding of the real world. When you throw something in real life, it influences how you expect something on screen to behave after you drag and release it.

By putting work, love, and care into UI details and designs, we help shape the experience and feeling users have when using an app. All the technical details and work is in service of the user's experiences and feelings. The user may not consciously notice the subtle animations we create, but if we do our job well, the tiniest gesture will feel good to them.

The team working on Shop, our digital shopping assistant, recently released a feature that allows buyers to favorite products and shops. By pressing a heart button on a product, buyers can save those products for later. When they do, the product image drops into the heart icon (containing a list of favorite products) in the navigation tab at the bottom.

In this post, I’ll show you how I approached implementing the Add to Favorite animation in Shopify’s Shop app. Specifically, we can look at the animation of the product image thumbnail appearing, then moving into the favorites tab bar icon:

Together, we'll learn:

• How to sequence animations.
• How to animate multiple properties at the same time.
• What interpolation is.

Getting Started

When I start working on an animation from a video provided by a designer, I like to slow it down so I can see what's happening more clearly:

If a slowed video isn’t provided, you can record the animation using Monosnap or Quicktime. This also allows you to slowly scrub through the video. Fortunately, we also have this great motion spec to work with as well:

As you can see, the motion spec defines the sequence of animations. Based on the spec, we can determine:

• which properties are animating
• what values to animate to
• how long each animation will take
• the easing curve of the animation
• the overall order of the animations

Planning the Sequence

Firstly, we should recognize that there are two elements being animated:

• the product thumbnail
• the favorites tab bar icon

The product thumbnail is being animated first, then the Favorites tab bar icon is being animated second. Let's break it down step by step:

 

Each of the above steps is an animation that has a duration and easing curve, as specified in the motion spec provided by the motion designer. Our motion specs define these easing curves that define how a property changes over time:

Coding the Animation Sequence

Let's write code! The Shop app is a React Native application and we use the Reanimated library to implement animations.

For this animation sequence, there are multiple properties being animated at times. However, these animations happen together, driven by the same timings and curves. Therefore we can use only one shared value for the whole sequence. That shared progress value can drive animations for each step by moving from values 1 to 2 to 3 etc.

So the progress value tells us which step of the animation we are in, and we can set the animated properties accordingly. As you can see, this sequence of steps match the steps we wrote down above, along with each step's duration and easing curves, including a delay at step 3:

We can now start mapping the progress value to the animated properties!

Product Thumbnail Styles

First let's start with the product thumbnail fading in:

What does interpolate mean?

Interpolating maps a value from an input range to an output range. For example, if the input range is [0, 1] and the output range is [0, 10], then as the input increases from 0 to 1, the output increases correspondingly from 0 to 10. In this case, we're mapping the progress value from [0, 1] to [0, 1] (so no change in value).

In the first step of the animation, the progress value changes from 0 to 1 and we want the opacity to go from 0 to 1 during that time so that it fades in. “Clamping” means that when the input value is greater than 1, the output value stays at 1 (it restricts the output to the maximum and minimum of the output range). So the thumbnail will fade in during step 1, then stay at full opacity for the next steps because of the clamping.

However, we also want the thumbnail to disappear instantly at step 3. In this case, we don't use interpolate because we don't want it to animate a fade-out. Instead, we want an instant disappearance:

Now the item is fading in, but it also has to grow in scale and then shrink back a bit:

This interpolation is saying that from step 0 to 1, we want scale to go from 0 to 1.2. From step 1 to 2, we want the scale to go from 1.2 to 1. After step 2, it stays at 1 (clamping).

Let's do the final property, translating it vertically:

So we're moving from position -60 to -34 (half way behind the tab bar) between steps 2 and 3. After step 3, the opacity becomes 0 and it disappears! Let's test the above code:

Nice, it fades in while scaling up, then scales back down, then slides down halfway under the tab bar, and then disappears.

Tab Bar Icon Styles

Now we just need to write the Favorite tab bar icon styles!

First, let's handle the heart becoming filled (turning purple), then unfilled (turning white). I did this by positioning the filled heart icon over the unfilled one, then fading in the filled one over the unfilled one. Therefore, we can use a simple opacity animation where we move from 0 to 1 and back to 0 over steps 3, 4 and 5:

For the heart bouncing up and down we have:

From steps 3 to 7, this makes the icon move up and down, creating a bouncing effect. Let's see how it looks!

Nice, we now see the tab bar icon react to having a product move into it.

Match Cut

By using a single shared value, we ensured that the heart icon moves down immediately when the thumbnail disappears, creating a match cut. A “match cut” is a cinematic technique where the movement of an item immediately cuts to the movement of another item during a scene transition. The movement that the users’ eye expects as the product thumbnail moves down cuts to a matching downward movement of the heart icon. This creates an association of the item and the Favorites section in the user's mind.

In another approach, I tried using setTimeout to start the tab bar icon animation after the thumbnail one. I found that when the JS thread was busy, this would delay the second animation, which ruined the match cut transition! It felt wrong when seeing it with that delay. Therefore, I did not use this approach. Using withDelay from Reanimated would have avoided this issue by keeping the timer on the UI thread.

When I started learning React Native, the animation code was intimidating. I hope this post helps make implementing animations in React Native more fun and approachable. When done right, they can make user interactions feel great!

You can see this animation by favoriting a product in the Shop app!

Special thanks to Amber Xu for designing these animations, providing me with great specs and videos to implement them, and answering my many questions.

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