Prevent memory leak(s) from QuizChecker.

[dbrant]

Here's a general guideline for designing singleton classes:

• Do not ever store instances of Context as a member variable in your class. Even if you're absolutely sure that you will clear out the Context when you're done, you will forget, and it will cause a memory leak because you will give it an instance of Activity by mistake.

Anyway, in this case it wasn't even necessary to keep an instance of Context. Also, this class maintains an instance of CompoundDisposable that keeps the results of Rx observables, which is great, but it never gets disposed! I hooked up the correct disposal mechanism.

[codecov-io]

Codecov Report

Merging #2656 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2656      +/-   ##
=========================================
- Coverage    2.75%   2.75%   -0.01%     
=========================================
  Files         260     260              
  Lines       12430   12432       +2     
  Branches     1125    1125              
=========================================
  Hits          343     343              
- Misses      12060   12062       +2     
  Partials       27      27

Impacted Files	Coverage Δ
...ain/java/fr/free/nrw/commons/quiz/QuizChecker.java	0% <0%> (ø)	⬆️
...r/free/nrw/commons/contributions/MainActivity.java	0% <0%> (ø)	⬆️

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[maskaravivek]

Here application context is being injected by dagger. @dbrant Is having an instance of application context also considered dangerous?

In this particular class, I agree that storing the context isn't required and activity context can be used everywhere.

[dbrant]

Here application context is being injected by dagger.

LeakCanary was showing a leak that pointed to the QuizChecker class, and when I looked at it, my eyes automatically jumped to the instance of Context.
But in fact the leak was caused by failing to dispose your CompoundDisposable, which this PR also fixes.

Technically, storing a reference to the application context isn't so bad. However, since Android (in its wisdom) doesn't differentiate between application and activity contexts, it's simply too dangerous to do this. There are almost always ways of accomplishing what you want without keeping a reference to a Context.

[maskaravivek]

Okay, thanks for explaining. Will avoid storing a reference of the context as much as possible. :)

[misaochan]

Whoops, yes, I remember this advice being mentioned at the 2017 hackathon, lol. Seems like we missed this issue when we code reviewed the PR.

I'll add a note to our wiki so hopefully it doesn't happen again.