How to show/hide password in TextFormField?

I have created a solution as per @Hemanth Raj but in a more robust way.

First declare a bool variable _passwordVisible.

Initiate _passwordVisible to false in initState()

@override
  void initState() {
    _passwordVisible = false;
  }

Following is the TextFormField widget :

TextFormField(
   keyboardType: TextInputType.text,
   controller: _userPasswordController,
   obscureText: !_passwordVisible,//This will obscure text dynamically
   decoration: InputDecoration(
       labelText: 'Password',
       hintText: 'Enter your password',
       // Here is key idea
       suffixIcon: IconButton(
            icon: Icon(
              // Based on passwordVisible state choose the icon
               _passwordVisible
               ? Icons.visibility
               : Icons.visibility_off,
               color: Theme.of(context).primaryColorDark,
               ),
            onPressed: () {
               // Update the state i.e. toogle the state of passwordVisible variable
               setState(() {
                   _passwordVisible = !_passwordVisible;
               });
             },
            ),
          ),
        );

With a credit goes to X-Wei, you can create the widget as a separate password.dart:

import 'package:flutter/material.dart';

class PasswordField extends StatefulWidget {
  const PasswordField({
    this.fieldKey,
    this.hintText,
    this.labelText,
    this.helperText,
    this.onSaved,
    this.validator,
    this.onFieldSubmitted,
  });

  final Key fieldKey;
  final String hintText;
  final String labelText;
  final String helperText;
  final FormFieldSetter<String> onSaved;
  final FormFieldValidator<String> validator;
  final ValueChanged<String> onFieldSubmitted;

  @override
  _PasswordFieldState createState() => new _PasswordFieldState();
}

class _PasswordFieldState extends State<PasswordField> {
  bool _obscureText = true;

  @override
  Widget build(BuildContext context) {
    return new TextFormField(
      key: widget.fieldKey,
      obscureText: _obscureText,
      maxLength: 8,
      onSaved: widget.onSaved,
      validator: widget.validator,
      onFieldSubmitted: widget.onFieldSubmitted,
      decoration: new InputDecoration(
        border: const UnderlineInputBorder(),
        filled: true,
        hintText: widget.hintText,
        labelText: widget.labelText,
        helperText: widget.helperText,
        suffixIcon: new GestureDetector(
          onTap: () {
            setState(() {
              _obscureText = !_obscureText;
            });
          },
          child:
          new Icon(_obscureText ? Icons.visibility : Icons.visibility_off),
        ),
      ),
    );
  }
}

Call it as:

  import 'package:my_app/password.dart';

  String _password;
  final _passwordFieldKey = GlobalKey<FormFieldState<String>>();

  PasswordField(
    fieldKey: _passwordFieldKey,
    helperText: 'No more than 8 characters.',
    labelText: 'Password *',
    onFieldSubmitted: (String value) {
      setState(() {
        this._password = value;
      });
    },
  ),

First make you widget StatefulWidget if it is a StatelessWidget.

Then have a variable bool _obscureText and pass it to your TextFormField. The toggle it with setState as required.

Example:

class _FormFieldSampleState extends State<FormFieldSample> {

  // Initially password is obscure
  bool _obscureText = true;

  String _password;

  // Toggles the password show status
  void _toggle() {
    setState(() {
      _obscureText = !_obscureText;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Sample"),
      ),
      body: new Container(
        child: new Column(
          children: <Widget>[
            new TextFormField(
              decoration: const InputDecoration(
                  labelText: 'Password',
                  icon: const Padding(
                      padding: const EdgeInsets.only(top: 15.0),
                      child: const Icon(Icons.lock))),
              validator: (val) => val.length < 6 ? 'Password too short.' : null,
              onSaved: (val) => _password = val,
              obscureText: _obscureText,
            ),
            new FlatButton(
                onPressed: _toggle,
                child: new Text(_obscureText ? "Show" : "Hide"))
          ],
        ),
      ),
    );
  }
}

Hope this helps!