Lifecycle methods

This is a repo for a lecture-workshop I gave on React 16 lifecycle methods in February 2020. Here is a video from that lecture.

I am using a codebase already known to students, namely Hogwarts lab.

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Goals:

• you will understand what lifecycle methods are;
• you will understand when lifecycle methods fire up;
• you will understand why we should never put setState() in render();
• you will know how to prevent unnecessary rerendering of a component;
• you will understand a memory leak symptoms, causes and treatment;
• you will remember forever not to put props in state;

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Getting started:

To start your adventure, clone this repo, then cd into a folder of choice, run npm install in your terminal to install all the dependencies and then npm start to start react server.

This repo contains three folders:

• starter_kit for those who want to code-along;
• lecture_kit which is the code I will use in the lecture (visible in the video);
• done_kit in case we don't have time to cover all bonuses and you'd like to see how certain features/methods can be implemented.

Component tree and wireframe

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Agenda

This lecture is meant as a space for exploring the superpowers of React lifecycle methods. The lecture is in a form of quick-pace fun riddles and will include practice exercices.

NOTE: There are no awesomeness points and mistakes are viewed as a part of learning.

1. Intro + rules:
   • readme overview
   • notifications
   • time and break
   • speaking and questions
   • parking lot
2. Builder analogy:
   • what are lifecycle methods?
   • explore the analogy here
3. Appetizers:
   • infinite loop of rerendering!
4. First hurdle: logging spree
   • constructor
   • render
   • shouldComponentUpdate
   • componentDidMount
   • componentDidUpdate
   • componentWillUnmount
5. Break (5 mins)
6. Second hurdle: SEO and accessibility
   • componentDidMount
7. Third hurdle: timer
   • componentDidMount
8. Fourth Hurdle: preventing memory leakage
   • componentWillnmount
   • or componentDidUpdate
9. Fifth Hurdle: controlling rerendering
   • shouldComponentUpdate
10. Sixth Hurdle: forbidden knowledge
    • getDerivedStateFromProps
11. Not covered + Bonus
    • error handling
12. SWABTs review + Q&A

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Resources:

• Lecture video
• Lifecycle methods - React docs
• Lifecycle methods graph
• Generating random color
• What is binding functions?

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Appetizers: stack overflow by rerendering

Have you ever tried setState in render()? Let's try it and see what happens!

Error: Maximum update depth exceeded. This can happen when a component repeatedly calls setState inside componentWillUpdate or componentDidUpdate. React limits the number of nested updates to prevent infinite loops.

Oho. React doesn't like that. That's because render() is supposed to be a pure function, which means that every single time it should give the same return value. Also, changes in state trigger render and so you reach stack overflow, or infinite loop. Instead, put the initial one in componentDidMount or as a callback function to one of the DOM nodes.

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First hurdle: logging spree

In this hurdle, your task is to put a console.log in constructor, render, componentDidMount, shouldComponentUpdate, componentDidUpdate and componentWillUnmount methods in: App.js, HogContainer.js and PigTile.js. Once you've done that, run your app and see what order the components are being rendered.

styling logs

As you see, that's a lot of logs. At the top of each component, I have provided a color unique for this component. Perhaps you want to color-code your messages? Here's how to do it:

console.log(`c%Hello`, `color: ${nameOfTheVariable}`)

Of course, you can provide your own color ideas or change styling as you wish (e.g. backgroundColor or fontSize)

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Second hurdle: SEO and accessibility

Have you noticed that majority of react pages have the same title (visible on the tab)? That's because in index.html it has been declared this way and never updated. Wouldn't it be nice if you could see the subpage title at any given time? That will also boost the SEO and the screen reader experience. Here's pseudo-code version of what you want to do:

• in componentDidMount you want to overwrite the title, or add to the current one with this syntax:

document.title = "some name"

Add document title in App, HogContainer and PigTile.

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Third hurdle: timer

Imagine you want to have a countdown for each hog for some reason -- let's say it's a game where a user can click on a given hog only for the first five seconds. In this hurdle, you will build a timer that counts down. Here's pseudo-code version of what you want to do:

• in componentDidMount set an interval that will update the state every second;
• comment out previous console.logs in render and componentDidUpdate;
• let's put a conditional that will console.log only one pig's lifecycle methods so we don't get overwhelmed. In PigTile put this in shouldComponentUpdate

if (this.props.hog.name === "Piggy smalls"){
    console.log(`🐽 🐽 🐽 🐽`)
    console.log(`%cShould I update?`, `color: ${generalTileColor}`)
}

in componentDidUpdate:

if (this.props.hog.name === "Piggy smalls"){
    console.log(`%cOkay, I've updated!`, `color: ${generalTileColor}`)
}

and in render:

if (this.props.hog.name === "Piggy smalls"){
    console.log(`%c${this.props.hog.name}:`, `color: ${generalTileColor}`, this.state.timeToGo)
}

setInterval syntax

The setInterval function accepts two arguments: a callback function that describes the desired action, and the number of miliseconds. The below syntax will update state every second.

this.interval = setInterval(
            () => this.setState({
              timeToGo: this.state.timeToGo - 1
            }),
            1000
          )

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Fourth hurdle: memory leakage

With the timer running, click on "greased". You will see this error:

proxyConsole.js:56 Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.

That's because you have an async method of setInterval running in the background -- you need to terminate it!

• go to componentWillUnmount();
• clear interval:

clearInterval(this.interval)

And how can we disable the interval in componentDidUpdate?

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Fifth Hurdle: controlling rerendering

Let's say you want the HogContainer to only update if the number of hogs is not smaller than 14. You could use shouldComponentUpdate(nextProps, nextState) method and write an expression that will return a boolean, for instance:

return nextProps.hogs.length > 14

Now, click "greased". Nothing changes. You have disabled updates on this component.

For fun, you can go now to your PigTile and use shouldComponentUpdate again. Say you want the countdown stop at 0. Put this expression in:

return nextState.timeToGo > -1

Now, run the page and observe console.logs. After a second click and unclick "greased". That restarts some tiles countdown. However, as you see, the updates stop for all tiles as soon as the first one hit 0.

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Sixth Hurdle: forbidden knowledge

React newbies tend to put props in state. Whoa, there's even a method that allows for that! Let's explore it.

• go to HogContainer;
• paste this method:

    static getDerivedStateFromProps(nextProps) {
         const code = nextProps.code
         return {code}
    }

Feel judged by Dan Abramov.

NOTE: in all seriousness, you should never put your props in your state. React won't yell at you but your instructors and employers will. Do not do that. This method is for a very specific edge case: while dealing with a transition animation library, you need to keep track of the window size.

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NOT COVERED: getSnapshotBeforeUpdate

This method is only used in cases when you're creating an infinite scroll and want to always have the pointer at the same place without jumping to the top.

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BONUS: error handling

Code along