Function reaction

Example

import { observable } from 'mobx';
import { component, observer, reaction } from 'web-cell';

@component({ tagName: 'my-tag' })
@observer
export class MyTag extends HTMLElement {
    @observable
    accessor count = 0;

    @reaction(({ count }) => count)
    handleCountChange(newValue: number, oldValue: number) {
        console.log(`Count changed from ${oldValue} to ${newValue}`);
    }

    render() {
       return (
           <button onClick={() => this.count++}>
               Up count {this.count}
           </button>
       );
   }
}
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