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Daniel Doubrovkine

aka dB., CTO at artsy.net, fun at playplay.io, NYC

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In the previous post I added and removed list items. In this post I will implement a ticking timer.

The React Native ecosystem promises a rich set of components that work seamlessly together, much like the Ruby world. My first timer implementation used react-timer-machine, a fully controllable and customizable timer component.

$ yarn add react-timer-machine
import TimerMachine from 'react-timer-machine'

Time duration math is implemented by combining moment.js and moment-duration-format.

import moment from "moment";
import momentDurationFormatSetup from "moment-duration-format";

The timer itself will start as soon as the isMeetingStarted state is toggled.

  formatTimer={(time, ms) =>
    moment.duration(ms, "milliseconds").format("h [hours], m [minutes], s [seconds]")

It works, except when it doesn’t. Expo doesn’t support background tasks and so the timer pauses when the app is backgrounded. A simple workaround is to keep track of start and end time, but we might as well get rid of the timer altogether.

We can rely on setInterval to call a method every second in the foreground and update the elapsed time by subtracting two timestamps. I find simulating a timer in this manner pretty neat, as it takes full advantage of React re-rendering components when state changes. It obviously wouldn’t be suitable for performing an action based on elapsed time since no events fire in the background.

this.setState({ elapsedTime: moment().diff(this.state.meetingStartedAt) })

You can see this code in 33-minutes-app@0f1df4. In the next post I will wire up this React Native client app to a Rails GraphQL API.