CircularProgressIndicator value not updated

Question

For the code shown below, I think that just calculating a new value for _progress should cause the CircularProgressIndicator to redraw. That does not appear to be happening. It never updates, although I have verified in the debugger that _progress is changed.

What do I need to do to get it to update?

      int completed = 0;
      int total = list.length;
      double _progress = 0;

      showDialog(context: this.context, builder: (BuildContext context) {
        return AlertDialog(
          title: const Text('Downloading ...'),
          content: CircularProgressIndicator(
            value: _progress));
      });
      for (FileInfo fileInfo in list) {
        if(fileInfo.publicUrl != null) {
          var filename = dir!.path + '/' + fileInfo.name!;
          await dio.download(fileInfo.publicUrl!, filename);
          completed++;
          _progress = completed/total;
        }
      }

Answer 1

The problem here is that the CircularProgressIndicator is not updating because the widget tree is not being notified of the changes in the _progress variable. Simply changing the value of a variable does not automatically cause the widgets to redraw. You need to inform the framework that a change has occurred and that it needs to rebuild the relevant widgets.

A solution is to wrap the AlertDialog in a StatefulBuilder and call setState after updating the _progress variable.

First ensure that the code you provided is in a StatefulWidget, then make changes as shown below:

int completed = 0;
int total = list.length;
double _progress = 0;

showDialog(context: this.context, builder: (BuildContext context) {
  return StatefulBuilder(
    builder: (BuildContext context, StateSetter setState) {
      return AlertDialog(
        title: const Text('Downloading ...'),
        content: CircularProgressIndicator(value: _progress),
      );
    },
  );
});

for (FileInfo fileInfo in list) {
  if(fileInfo.publicUrl != null) {
    var filename = dir!.path + '/' + fileInfo.name!;
    await dio.download(fileInfo.publicUrl!, filename);
    completed++;
    setState(() {
      _progress = completed / total;
    });
  }
}