Day 3 Services

Day 3 - Services, Dependency Injection and async development


fun-with-di	Angular Dependency Injection
fun-with-promises	Creation and usage of Promises, `async` - `await`
movies-browser-ex2	The sulution for Exercise 2
fun-with-rxjs	Introduction to Observables, Subjects, And Reactive X

We talked about the motivation to use a dependency injection infrastructure
We saw how to create a service - an object encapsulating a bit of logic and perhaps data
We defined the terms:
- Injection request - A consumer that asks for a dependency (service) in the constructor
- Injector - An object responsible for resolving (providing, injecting) the injection request
- Provider - The algorithm, or logic, used to resolve the injected object
We saw how to define injecter and provider using the providers property of a Module, or Component
We understood the effect of defining a component as injector, and how to use the component hierarchy as injector hierarchy
We saw that we can not use an interface as injection request and used an abstract class instead
We saw how to define a provider that used another class instead of the requested class
Finally, we saw how to define a service as Injectable so it can also consume dependencies

We defined a promise as an object that tells us if an action that takes time has completed, and what is the result
We saw that we can not "pull" that information, but rather have to use the .then and .catch methods and provide callbacks that will be called when the promise completes
We saw how to create a promise using the Promise Constructor.
We understood what async and await keywords do and how do they affect the compilation
We understood that the .then method, and the async keywords, also create promises, out of other promises
We saw how to ue Promise.resolve to create a promise that is already completed when it is created
We saw how to use Promise.all to group promises together and wrap them in a single promise that returns all the results from all the promises

We understood the meaning of a Stream
We defined what an Observer<T> is and understood that it has 3 methods
- next(T)
- complete()
- error(err)
We understood that Observable<T> is an object that allows observers to subscribe
- subscribe(Observer<T>)
We saw how to define an observer explicitly by supplying the 3 methods and their implementation
We saw how to create an observable using the interval operator
We saw that 2 observers that subscribe on different times, get different sets of events that are not synchronized
We understoof the difference between cold observables and warm observables
We saw how to create a custom observable using the observable constructor
We got familiar with the Subject object and understood that it is basically an event...
We learned about BehaviorSubject and understood that it is just a subject that sends the latest event to a new observer on the moment of subscription
We learned how to develop stateful services using BehaviorSubject and expose it as an observable using the .asObservable() method
We understood that if we subscribe to an observable we must also unsubscribe from it when we are done with it to avoid memory leaks
We saw that we can bind directly to an observable using the async pipe in the html

Repeat the Observable, Subject and BehaviorSubject example, becuase it requires a bit of repeating :-)
Continue the movie-browser example using reactive X and understand how to develop a fully reactive application
Understand how to use reactive X operators - which are the heart of reactiveX
become familiar with the HttpClientModule to communicate with servers over http