Interpolators are very useful to model movement for your UI elements. In this article, we’ll focus on how to create aesthetic motion by using two simple tools: animator and interpolator instances.

# Some Animation Theory

How do interpolators work? Well, when it comes to changing the property of an object over time, they take an initial and a final value and create a series of ‘middle values’ between them, according to a specific rule. This rule is a mathematical function. As an example, consider a simple geometric shape as an object of which properties can be animated. Let’s have a look at changing a property of this shape, namely, its horizontal position.

## Linear change

The simplest interpolator is the linear interpolator. Give this interpolator an initial and final position, and the interpolator will calculate a series of middle points, equally separated from each other, along which the shape will pass in order to get to the final position. Take a look at how the square moves. From the graph of position over time, you can see that the position value follows a linear path through the graph as the position of the square changes. This illustrates that, per unit of time, the position value changes in equal parts, with no variation in speed. In other words, the linear interpolator creates constant speed.

## Easing

Easing refers to a value’s speed changing over time. In the case of an object’s position, for example, you’ll see that its rate of motion changes, by moving slow and then fast or vice versa. Look at an example of the same shape as before, but now the shape changes its speed during its motion:

As you can see from the graph of the shape’s position over time, the value now plots a curved path. It illustrates quite clearly what’s happening:

• At first, as time passes, the value of position doesn’t change much; the shape has a slow movement.
• As we reach the middle of our animation, the shape accelerates steeply. The shape now moves much faster because the change in position per unit of time is greater.
• Finally, it decelerates until it reaches zero speed; its motion finishes.

To create this kind of motion, we use easing interpolators, which create variable speed. This is the most appealing interpolator because of the characteristics it gives to objects that change. Usually, the kind of change that is more appealing to the eye is what is called an ease in-out, in which it accelerates at the start and decelerates at the end. It starts slow, becomes faster in the middle, and ends slow. Easing interpolators are perfect to animate objects in this way. So now, let’s look at how we can create them.

#development #motion-design #android #ui

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