[TextField] Remove deprecated lifecycle method

[AndrewMusgrave]

WHY are these changes introduced?

Part of #519

WHAT is this pull request doing?

• Replaces lifecycle method
• Add test

How to 🎩

Add a console.log to textfield render to check it's current id during render. Check the console, onChange will trigger the id switch.

Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {TextField} from '@shopify/polaris';

interface State {
  id: string | undefined;
}

export default class TextFieldExample extends React.Component {
  state = {
    id: undefined,
  };

  handleChange = () => {
    this.setState(({id}) => ({
      id: id
        ? undefined
        : Math.random()
            .toString(36)
            .substring(7),
    }));
  };

  render() {
    const {id} = this.state;
    const props = {
      label: 'Store name',
      value: '',
      onChange: this.handleChange,
      id,
    };
    console.log(`Current id: ${id}`);
    return <TextField {...props} />;
  }
}

[tmlayton]

Suggested change

[AndrewMusgrave]

Prettier added this line, let me restart vscode code to check if it was an oddity.

[AndrewMusgrave]

Seems prettier wants this line

[tmlayton]

Looks good. Is it worth adding a test for the prevState.id code path, too?

[dleroux]

🎩 👍 after the conflict on unreleased of course :)