Mobile App Development 2022W Lecture 14

Video

Video from the lecture given on March 9, 2022 is now available:

• video
• auto-generated captions

Video is also available through Brightspace (Resources->Zoom Meetings (Recordings, etc.)->Cloud Recordings tab). Note that here you'll also see chat messages.

Notes

Lecture 14
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 - I'll be grading midterms soon, so should be another week, sorry
   - first want to release A3
 - rest of term is on Android
 
Question for the day: why does textanalyzer-5 lose state when you rotate it,
but TapDemo doesn't?
 - note that textanalyzer-5 keeps the text that you enter, it just
   forgets the mode you're in.

So what's happening is the running activity is destroyed when you rotate the screen
 - and textanalyzer-5 doesn't save the state properly

What's an activity?
 - the unit of execution in an android app
 - can have many activities
 - when you run an app in any way, you are actually running a specific activity
    - and of course there is a default activity that is run when
      you launch the app
 - when you have multiple screens, you can implement them as multiple activities
    - but really, it is about splitting up functionality, not the interface per se

So in textanalyzer-5, the state variable analysisMode is forgotten because the activity was terminated and then restarted
 - note that EditText preserves its own state after rotation
 - but MainActivity doesn't, we have to implement that functionality


onCreate in an activity is a method that is called when an activity is created
 - it has a single argument which is a Bundle
 - bundles are used to store state that can be saved and restored

Basic idea with bundles
 - when your activity will be terminated, save state into a bundle
 - when your activity is created again, restore state from the given bundle

Bundles are dictionaries that can hold different types of data
 - so key, value, with the value being able to have different types
    - and the key is a string
 - the Android runtime manages the instance state bundles,
   we just have to put data in and pull data out at the right times


You should play with TapDemo and see when the different activity lifecycle methods are called
 - when you rotate the screen, switch to another app, terminate an app, etc


So, why does Android terminate the activity when the screen is rotated?
 - because the interface has to be redrawn anyway, easist way is to
   kill the activity and start it again with new screen geometry

What about iOS apps?  Do they have a lifecycle?
 - yes definitely, much the same as Android
 - however, by default much of the state management is handled automatically
    - but you can change it

How can we fix TapDemo so that it shows the proper count after rotation?