Mobile App Development 2021W Lecture 21

Video

Video from the lecture given on March 31, 2021 is now available.

Notes

Lecture 21
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* Assignment 3 solutions
* app manifest
* Intents
  - explicit
  - implicit
* drawable shapes
* touch events

Tutorial 9 and A4 will come out this week, probably tomorrow.

Tutorial 10 will be optional and on iOS storyboard
 - can substitute for a tutorial that you missed
 - won't be on the final exam

Application manifest
 - tells Android how to run the app
 - this is where you define activities
   - really, classes that are associated with layouts
 - and also define intents your app will respond to
   - such as launching the app

Intents are the basic inter-process communication mechanism on android
 - think of them as messages that can be sent between programs, including a program to itself
 - used to launch activities

(By the way, intents are built on mechanisms that are not
Linux-native, rely on modifications to the Linux kernel that were only upstreamed relatively recently)

idea is to do fast message passing with minimal amount of memory copying
  - copying memory is slow
  - example, zero-copy networking

If you want to start a new class with a new screen, standard way is to use an intent
 - and your new class will have to set things up just
   as we did in MainActivity, including specifying the layout
 - be sure to add the activity to the application manifest

Launching your own class as a new activity is an "explicit" intent
 - you specified what code should run (i.e., the class)

But in many contexts, you don't know what code to run
 - you want a service provided by another application

Implicit intents let you make requests when you don't know what application will handle the request.
 - simple example: open web page in a browser
 - when you specify a default browser in android, you
   are specifying what browser will handle intents such as
   ACTION_VIEW

You have to be careful when defining intents, especially implicit intents
 - can be called by arbitrary apps
 - so, you may need to do access control
 - can be a big security hole, depending on how they are used

If you care about security or privacy at all as a developer, you need to pay attention to android intents and the permissions associated with them
 
Drawable shapes
 - in iOS SwiftUI, we'd just say Circle, Rectangle etc
   and we'd get shapes
 - in Android, it is a bit more complicated
    - with a canvas, we can do these things programmatically
      much like a canvas in a web page
 - but in a regular view, you need to specify
   drawings using drawable shapes and paths normally
   specified in XML
    - you can display using an ImageView (just like a jpeg)

Touch events in Android are a lot more complicated than in SwiftUI
 - we're not going to achieve parity with iOS in this class,
   that will have to wait until 2601


Regarding grading
 - assignments, tutorials, and participation can
   only improve your grade relative to the midterm and final