The Interpolator Interface

In the Android development framework, Interpolator is defined as an interface. This allows methods to accept an interpolator that can bring its own configuration and not tie developers to a specific implementation. As of this writing, there are 11 indirect subclasses of the Interpolator interface. They are:

• Linear Interpolator: The generated values between the two fixed points are evenly distributed. For example, consider a = 1 and b = 5 to be the fixed points. Linear interpolation between a and b would look like: 1 -> 2 -> 3 -> 4 -> 5, where the numbers between 1 and 5 have been generated.
• Accelerate Interpolator: This interpolator generates values that initially have a small difference between them and then ramps up the difference gradually until it reaches the endpoint. For example, the generated values between 1 -> 5 with accelerated interpolation could be 1 -> 1.2 -> 1.5 -> 1.9 -> 2.4 -> 3.0 -> 3.6 -> 4.3 -> 5. Notice how the difference between consecutive values grows consistently.
• Decelerate Interpolator: In the sense that the accelerate interpolator generates values in an accelerating fashion, a decelerate interpolator generates values that are “slowing down” as you move forward in the list of generated values. So, the values generated initially have a greater difference between them and the difference gradually reduces until the endpoint is reached. Therefore, the generated values between 1 -> 5 could look like 1 -> 1.8 -> 2.5 -> 3.1 -> 3.6 -> 4.0 -> 4.3 -> 4.5 -> 4.6 -> 4.7 -> 4.8 -> 4.9 -> 5. Again, pay attention to the difference between consecutive values growing smaller.
• Accelerate Decelerate Interpolator: This interpolator starts out with a slow rate of change and accelerates towards the middle. As it approaches the end, it starts decelerating, i.e. reducing the rate of change.
• Anticipate Interpolator: This interpolation starts by first moving backward, then “flings” forward, and then proceeds gradually to the end. This gives it an effect similar to cartoons where the characters pull back before shooting off running. For example, generated values between 1 -> 3 could look like: 1 -> 0.5 -> 2 -> 2.5 -> 3. Notice how the first generated value is “behind” the starting value and how it jumps forward to a value ahead of the starting value. It then proceeds uniformly to the endpoint.
• Bounce Interpolator: To understand this interpolator, consider a meter scale that’s standing vertically on a solid surface. The starting value is at the top and the end value is at the bottom, touching the surface. Consider now, a ball that is dropped next to the meter scale. The ball on hitting the surface bounces up and down a few times until finally coming to rest on the surface. With the bounce interpolator, the generated values are similar to the list of values the ball passes by alongside the meter scale. For example, the generated values between 1 -> 5 could be 1 -> 2 -> 3 -> 4 -> 5 -> 4.2 -> 5 -> 4.5 -> 5. Notice how the generated values bounce.
• Overshoot Interpolator: This interpolator generates values uniformly from the start to end. However, after hitting the end, it overshoots or goes beyond the last value by a small amount and then comes back to the endpoint. For example, the generated values between 1 -> 5 could look like: 1 -> 2 -> 3 -> 4 -> 5 -> 5.5 -> 5.
• Anticipate Overshoot Interpolator: This interpolator is a combination of the anticipate and overshoot interpolators. That is, it first goes backward from the starting value, flings forward and uniformly moves to the endpoint, overshoots it, and then returns to the endpoint.

An interpolator is a function (in the mathematical sense) that outputs values “interpolated” between a range of values that are given to it as input. Interpolation is simply a method to generate new data points between two fixed data points. The exact values of these generated data points are determined by the kind of interpolation is performed. For example, in linear interpolation, all the generated values are evenly distributed between the fixed points. While an understanding of interpolation is helpful, it isn’t necessary to get started animating your views in your apps. In fact, the animation perspective of interpolation might prove helpful in understanding it! So, let’s get started. In this example, we’ll create a simple app with a list of buttons. Each of these buttons is for a specific type of interpolated animation which kicks off when you press it. The animation is a simple horizontal translation that moves the button to the right.