getSnapshotBeforeUpdate method

dynamic getSnapshotBeforeUpdate(Map prevProps, Map prevState)

ReactJS lifecycle method that is invoked immediately after re-rendering when new props and/or state values are committed.

This method is not called for the initial render.

Use this as an opportunity to perform preparation before an update occurs.

Example:

@override
getSnapshotBeforeUpdate(Map prevProps, Map prevState) {
  // Previous props / state can be analyzed and compared to this.props or
  // this.state, allowing the opportunity perform decision logic.
  // Are we adding new items to the list?
  // Capture the scroll position so we can adjust scroll later.

  if (prevProps.list.length < props.list.length) {
    // The return value from getSnapshotBeforeUpdate is passed into
    // componentDidUpdate's third parameter (which is new to React 16).

    final list = _listRef;
    return list.scrollHeight - list.scrollTop;
  }

  return null;
}

@override
componentDidUpdate(Map prevProps, Map prevState, dynamic snapshot) {
  // If we have a snapshot value, we've just added new items.
  // Adjust scroll so these new items don't push the old ones out of view.
  //
  // (snapshot here is the value returned from getSnapshotBeforeUpdate)
  if (snapshot !== null) {
    final list = _listRef;
    list.scrollTop = list.scrollHeight - snapshot;
  }
}

See: reactjs.org/docs/react-component.html#getsnapshotbeforeupdate

Implementation

dynamic getSnapshotBeforeUpdate(Map prevProps, Map prevState) {}