A. Appendix A: Chapter 5 Solution 1

Written by Vincent Ngo

First, you need to make ExploreScreen a StatefulWidget because you need to preserve the state of the scroll controller.

Next, add a ScrollController property in _ExploreScreenState:

ScrollController _controller;

Then, add a function called scrollListener(), which is the function callback that will listen to the scroll offsets.

void _scrollListener() {
  // 1
  if (_controller.offset >= _controller.position.maxScrollExtent &&
      !_controller.position.outOfRange) {
    print('i am at the bottom!');
  }
  // 2
  if (_controller.offset <= _controller.position.minScrollExtent &&
      !_controller.position.outOfRange) {
    print('i am at the top!');
  }
}

Here’s how the code works:

1. Check the scroll offset to see if the position is greater than or equal to the maxScrollExtent. If so, the user has scrolled to the very bottom.
2. Check if the scroll offset is less than or equal to minScrollExtend. If so, the user has scrolled to the very top.

Within _ExploreScreenState, override initState(), as shown below:

@override
void initState() {
  // 1
  _controller = ScrollController();
  // 2
  _controller.addListener(_scrollListener);
  super.initState();
}

Here’s how the code works:

1. You initialize the scroll controller.
2. You add a listener to the controller. Every time the user scrolls, scrollListener() will get called.

Within the ExploreScreen’s parent ListView, all you have to do is set the scroll controller, as shown below:

return ListView(
        controller: _controller,
        ...

That will tell the scroll controller to listen to this particular list view’s scroll events.

Finally, add a function called dispose().

@override
void dispose() {
  _controller.removeListener(_scrollListener);
  super.dispose();
}

The framework calls dispose() when you permanently remove the object and its state from the tree. It’s important to remember to handle any memory cleanup, such as unsubscribing from streams and disposing of animations or controllers. In this case, you’re removing the scroll listener.

Here are some examples of when you might need a scroll controller:

• Detect if you’re at a certain offset.
• Control the scroll movement by animating to a specific index.
• Check to see if the scroll view has started, stopped or ended.