Flutter in Action Chapter 5 "User Interaction"

Completed taking notes on "Flutter in Action" chapter 5 "User Interaction".

https://www.manning.com/books/flutter-in-action

This chapter is named "User Interaction", but its really the user INPUT chapter as it only covers one direction of UI.  The chapter covers gestures, dismissible widgets, forms, and very briefly discusses keys.

Gestures are pretty simple in flutter, and the GestureDetector wraps its children in the widget tree providing around 30 optional callback functions (onLongPress, etc).  The more interesting gestures have an object passed back about the specific gesture (start or end coordinates, duration, etc).  Like everyone else who discovers gestures I have the immediate desire to code something up that somehow uses all possible gestures.  Its a late 2010s thing to go a little overboard with unpredictable gesture behavior in mobile apps, LOL.  Gestures are ... a little overdone in mobile apps, too confusing, no standard, no way to prompt the user to use them...  Anyway Flutter makes them easy to use... easy for the programmer anyway LOL.

Dismissibles are covered in the sense of mentioning their Flutter peculiarities.  They need  a child widget, but also need a background to show while dismissing, and a onDismissed callback that gets told which direction it was swiped away, and it needs a key.  The book glosses over it, but its easy for Flutter to translate a widget tree into a displayable element tree.  But how to push a notification backwards that an item was dismissed from the element to the appropriate widget in the widget tree, in fact how does an individual element know which widget it belongs to in general?  The Flutter-wide solution is passing a key value in the downward direction in some widgets, such as Dismissibles.  The book is a bit UI-centric working from outside in, more coverage from backend-centric working from inside the phone outwards would help understand keys in Flutter.  Maybe later in the book?

Forms occupy the majority of the chapter.  Typical Flutter style of a big Form widget wrapping many items inside it.  Forms "need" a global key of type FormState to use analogous to a controller from other widgets, it gives global access to see valid and save state of the entire form.  The book mentions forms as the only good reason to use global keys, but as per above the book focus is on the outside in rather than inside out.  The book needs more coverage of keys!  Anyway, inside the form, Flutter has three field widgets, one for wrapping any old thing (checkboxes, anything really), and two for special form fields, a TextFormfield with all the special material design features, and a DropdownButtonFormField for those scrollable dropdown lists that look so nice in a UI for postal states or similar.  An InputDecoration object gives form specific styling like those Material Design specific "optional" form tags and similar to a TestFormfield.  Aside from fields, Flutter has a FocusNode so the app can decide custom UI flow between items in a form, if you don't want default top down focus as you fill the form out.  The chapter shows some really useful standards, tricks, and ideas like not autovalidating until the item has been typed into, and grayout "hiding" the save button until validation passes for the entire form (remember the global _formKey.currentState.validate() method?)  Flutter forms can also pop a regret popup using onWillPop ("Are you sure you want to leave and lose all your unsaved data?")

The chapter does a great job of explaining multiple Flutter user input elements and explains interesting real world techniques.  I wish for more coverage of the concept of keys in Flutter.

Next up, chapter 6, "Flutter animations and using the canvas".