packages/okam-core/src/extend/data/observable/Observer.js
function updateArrayItem(observer, idx, value) {
    observer.set(idx, value);
    this[idx] = value;
}
function updateArrayItem(observer, idx, value) {
    observer.set(idx, value);
    this[idx] = value;
}
Update array item value

@param {Observer} observer the observer
@param {number} idx the index to update
@param {*} value the value to set