1. Reactive Android Development
CS T & CS T
Summer 2016
RxAndroid Continued

2. Invariants
Reactive programming should be thought of in terms of invariants
This is particularly true for Sodium
But also applies to the Reactive Extensions
The simplest example is 'map'
sum = list.map(l -> add_up_the_elements(l))
Once this is established, no matter what happens to the size or content of list, sum will always be equal to the sum of its elements.

3. Invariants Aid Correctness
This isn't anything new
Remember the inductive proof for insertion sort
List A = <random list of elements>;
List Sorted = <empty list>;
for(element a : A) {
Sorted = insert_element(a,Sorted); }

4. DRY (Don't Repeat Yourself)
This is a standard software engineering principle
Enforcing the same invariant in multiple places leads to brittle code
If the invariant changes, you will have to hunt for all of the places that might be impacted by the change.

5. The Challenge
Find a way to state your requirements in terms of invariants that can be enforced in only one statement.

6. "Beware the man with one gun"

7. Beware the man with one gun
Or, if you prefer
"No man can serve two masters"

8. Beware the man with one gun
Or, if you prefer
"No man can serve two masters"
Each of your stages should enforce only one invariant

9. Observables
The basic element of Rx
Observable
Hot
Cold

10. Observers
OnNext
OnCompleted
OnError

11. observeOn & subscribeOn

12. Cleaning Up References
If your activity subscribes to an observable, then the observable maintains a reference to your activity
Even if the Android OS tries to delete your activity, it won't be garbage-collected.

13. Cleaning Up References
The subscribe method returns an Rx.Subscription object that can be used to unsubscribe
In the OnDestroy method, for example.