Using Text Field widget an app can interact with user input. So how do we get the value from the widget ?

There are two ways, one is use change event to store values to a local final variable using SetState of the widget and the second is try using Editing-Controller, which can also offer service like clearing box etc. I go with controller.

Controller

Create a text editing controller and attach it to the widget

final mycontroller = TextEditingController();

attach the controller to the widget and it will look like the following

  child: TextField(
                decoration: InputDecoration(hintText: 'Enter todo here'),
 controller: mycontroller,
                onChanged: (value) {},
              ),

Using the controller

You can use the controller to access the data and clear the data as follows

OutlineButton(
              onPressed: () {                
                var newtodo = new Todo(mycontroller.text);               
                text.clear();
})