Extract Viewer Commands to ViewerCommand class

[david-allison]

Non-functional prep work for #6052

Also removes code from AbstractFlashCardViewer, which is a side-goal of mine.

Don't mind if this is squashed, commits are small to ensure they're readable in review.

Checklist

• You have not changed whitespace unnecessarily (it makes diffs hard to read)
• You have a descriptive commit message with a short title (first line, max 50 chars).
• Your code follows the style of the project (e.g. never omit braces in if statements)
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code

[timrae]

Here is an overview of what got changed by this pull request:

Issues
======
+ Solved 2
- Added 3
           

Complexity decreasing per file
==============================
+ AnkiDroid/src/main/java/com/ichi2/anki/AbstractFlashcardViewer.java  -3
         

See the complete overview on Codacy

[mikehardy]

one tiny question but otherwise LGTM

[Arthur-Milchior]

Why do you want to remove code from abstrractflashcardviewer ?

[david-allison]

Why do you want to remove code from abstrractflashcardviewer ?

Primarily: It's so large that it's become hard to reason about the state of the object. There's a lot of responsibilities which are combined, and some state is implicitly modelled as the state of the UI elements, rather than explicitly. "Single responsibility principle" is the typical term to Google, but everyone has different opinions on what it means.

If you want to add new code, then it's still fairly easy, but it increases the class size. If you want to modify existing code, then it takes much longer as you'll struggle to mentally manage the context required to make the change without introducing regressions. Testing also becomes harder for this reason, as it's hard to exhaustively test all code paths.

Finally, there's a lot of code in AbstractFlashcardViewer that is review-only (ease calculation was one that I moved recently, gestures is up for discussion) which would fit much better in the Reviewer class.

[mikehardy]

This went out with 2.10alpha72 - available as soon as google is done processing it