Flutter prevent close dialog on back pressed

Question

I am using the showDialog function to build a dialog box, but I need to avoid that when the user presses the back button, the dialog box does not close, this is my code:

showDialog(
      barrierDismissible: false,
      context: context,

      builder: (BuildContext context) {
        // return object of type Dialog
        return AlertDialog(
          title: new Text("Hello"),
          content: new SingleChildScrollView(
            child: Container(),
          actions: <Widget>[
            // usually buttons at the bottom of the dialog

            new FlatButton(
              child: new Text("Close"),
              onPressed: () {
              },
            ),
          ],
        );
      },
    );

How can I get it to not close?

Answer 1

You can also do it by setting onWillPop: () => Future.value(false) inside WillPopScope. Right now onWillPop:(){} as mentioned in Accepted answer gives warning.

Warning Message: This function has a return type of 'Future', but doesn't end with a return statement. Try adding a return statement, or changing the return type to 'void'.

So, use onWillPop: () => Future.value(false) or onWillPop: () async {return false;}, instead.

showDialog(
  barrierDismissible: false,
  context: context,
  builder: (BuildContext context) {
    return WillPopScope(
      child: AlertDialog(
        ................
      ),
      onWillPop: () => Future.value(false),
    );
  },
);

Answer 2

You need to use a WillPopScope. It will use the function on onWillPop to determine if the dialog closes or not. In this case always false, so the user can't use the back button to close the dialog.

showDialog(
  barrierDismissible: false,
  context: context,
  builder: (BuildContext context) {
    // return object of type Dialog
    return WillPopScope(
      child: AlertDialog(
        title: new Text("Hello"),
        content: new SingleChildScrollView(
          child: Container(),
        ),
        actions: <Widget>[
          // usually buttons at the bottom of the dialog

          new FlatButton(
            child: new Text("Close"),
            onPressed: () {
              Navigator.of(context).pop();
            },
          ),
        ],
      ),
      onWillPop: () async {
        return false;
      },
    );
  },
);

Answer 3

You need to enclose your AlertDialon on a WillPopScope like this:

showDialog(
      barrierDismissible: false,
      context: context,

      builder: (BuildContext context) {
        // return object of type Dialog
        return WillPopScope(
            onWillPop: (){},
            child:AlertDialog(
            title: new Text("Hello"),
            content: new SingleChildScrollView(
              child: Container(),),
            actions: <Widget>[
              // usually buttons at the bottom of the dialog
              new FlatButton(
                child: new Text("Close"),
                onPressed: () {
                },
              ),
            ],
          )
        )
      },
    );

The WillPopScope provides you a onWillPop parameter, where you can pass a function of what you want when the child pop. In this case, the parameter receive an empty function so it wont pop.