Hide FAB when onscreen keyboard appear

I hope this is what you are looking for

Scaffold(resizeToAvoidBottomInset: false)

For future reference, this is an implementation following the basic idea from https://github.com/flutter/flutter/issues/26100 but using already existing mixins to reduce quirks and code:

class FixedCenterDockedFabLocation extends StandardFabLocation
    with FabCenterOffsetX, FabDockedOffsetY {
  const FixedCenterDockedFabLocation();

  @override
  String toString() => 'FloatingActionButtonLocation.fixedCenterDocked';

  @override
  double getOffsetY(
      ScaffoldPrelayoutGeometry scaffoldGeometry, double adjustment) {
    final double contentBottom = scaffoldGeometry.contentBottom;
    final double bottomMinInset = scaffoldGeometry.minInsets.bottom;
    if (bottomMinInset > 0) {
      // Hide if there's a keyboard
      return contentBottom;
    }
    return super.getOffsetY(scaffoldGeometry, adjustment);
  }
}

You can then use it as simple as floatingActionButtonLocation: const FixedCenterDockedFabLocation(), (don't forget the const ;)).

Feel free to use this code without limitations or attribution.

You can achieve it by checking for keyboard visibility using viewInsets and hide fab based on it.

Example:

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(
    title: "Example",
    home: new FabHideOnKeyboard(),
  ));
}

class FabHideOnKeyboard extends StatefulWidget {
  @override
  _FabHideOnKeyboardState createState() => new _FabHideOnKeyboardState();
}

class _FabHideOnKeyboardState extends State<FabHideOnKeyboard> {
  @override
  Widget build(BuildContext context) {
    final bool showFab = MediaQuery.of(context).viewInsets.bottom==0.0;
    return Scaffold(
      resizeToAvoidBottomPadding: true,
      body:Container(
        alignment: Alignment.center,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text("TextField:"),
            TextField()
          ],
        ),
      ),
      floatingActionButton: showFab?Icon(Icons.add):null,
    );
  }
}

Wrap your FloatingActionButton with a Visibility widget and control the visibility with a bool value.

Snippet:

Widget build(context) {
  bool keyboardIsOpen = MediaQuery.of(context).viewInsets.bottom != 0;
  return Scaffold(
    body: // ...
    floatingActionButton: Visibility(
      visible: !keyboardIsOpen,
      child: // your FloatingActionButton
    ),
  );
}

This solution also gets rid of the animation when the button disappears and appears.

Be sure that the class extends StatefulWidget and you have created a state for it