ReasonML Quick Start Guide

Velocity

To get the velocity of a user's swipe, we'll need to also store the current time along with the touch event's clientX. Let's bind to the browser's performance.now() method:

[@bs.val] [@bs.scope "performance"] external now: unit => float = "";

And we'll make some room for the touch's current time in the touches type:

type touches = {  first: option((float, float)),  last: option((float, float)),};

In the reducer, we then change Some(clientX) to Some((clientX, now())).

Now, we can calculate the velocity of a user's swipe in the TouchEnd case:

| TouchEnd =>  if (state.isOpen) {    let (x, t) =      Belt.Option.getWithDefault(state.touches.first, (0.0, 0.0));    let (x', t') =      Belt.Option.getWithDefault(state.touches.last, (0.0, 0.0)); let velocity ...