title: Flutter Execute Method so long the button pressed
body: <p>I want to execute a method while a user is pressing down on a button. In pseudocode:</p>
  <pre><code>while (button.isPressed) {
    executeCallback();
  }
  </code></pre>
  <p>In other words, the <code>executeCallback</code> method should fire repeatedly as long as the user is pressing down on the button, and stop firing when the button is released. How can I achieve this in Flutter?</p>