Flutter 方向改变后的 ListView adjustPositionForNewDimensions

Question

I am currently implementing a ListView that tries to keep the current "selected" item in the middle of the ListView.

This is achieved simply by implementing a custom ScrollPhysics that looks familiar to the PageScrollPhysics

However if the orientation of the screen changes (and thus the dimensions of the ListView) the item in the middle isn't correct anymore. I noticed there is a method that we can overwrite in the ScrollPhysics called adjustPositionForNewDimensions which works perfectly if the dimensions of the ListView changes programatically but somehow this method doesn't get triggered if the orientation changes.

This method should get called from here which in turn gets called here . But somehow on the ListView its dimensions change the variable haveDimensions is true but after an orientation change (and thus dimension change) its false resulting in skipping the correctForNewDimensions check.

It's not completly clear to me why this variable is false even though the dimensions did change. Am I missing something, or should this behavior be implemented in a different way?

Answer 1

Well if you want to change some values on orientation change then it is pretty easy with flutter, you can use an OrientationBuilder which can detect when the orientation of the current device is being changed and update your code accordingly:

body: OrientationBuilder(
  builder: (context, orientation) {
    return GridView.count(
      // Create a grid with 2 columns in portrait mode,
      // or 3 columns in landscape mode.
      crossAxisCount: orientation == Orientation.portrait ? 2 : 3,
    );
  },
),

You can find more about it on: https://docs.flutter.dev/cookbook/design/orientation