local-copy-with-decorator.js

import { TrackedWeakMap } from 'tracked-built-ins';

function localCopy(target, key) {
  let values = new TrackedWeakMap();
  let lastValues = new WeakMap();

  return {
    get() {
      let incoming = this.args[key];
      let lastValue = lastValues.get(this);

      if (lastValue !== incoming) {
        values.set(this, incoming);
        lastValues.set(this, incoming);
      }

      return values.get(this);
    },

    set(value) {
      values.set(this, value);
    }
  }
}

class MyComponent extends Component {
  @localCopy myValue; 
}

local-copy.js

class MyComponent extends Component {
  @tracked _myValue;

  get myValue() {
    let incoming = this.args.myValue;
    let lastValue = this._lastValue;

    if (lastValue !== incoming) {
      this._myValue = this._lastValue = incoming;
    }

    return this._myValue;
  }

  set myValue(value) {
    this._myValue = value;
  }
}