Networking Codelab

Question 1

Which 2 dependencies need to be added to the module’s build.gradle to implement Retrofit?

What is the purpose of both dependencies?

Answer 1

“com.squareup.retrofit2:retrofit:$version_retrofit”
“com.squareup.retrofit2:converter-scalars:$version_retrofit”

retrofit2:retrofit library:
1 - establishes network connection to a server
2 - communicates with the server
3 - receives and parses the JSON response into a useable format
4 - handles calls on background threads

converter-scalars: enables Retrofit to return the JSON result as a String.

Question 2

How do you add support for libraries looking to take advantage of Java 8 language features?

Answer 2

In build.gradle (:app)
android {
…

compileOptions {
sourceCompatibility JavaVersion.VERSION_1_8
targetCompatibility JavaVersion.VERSION_1_8
}

kotlinOptions {
jvmTarget = JavaVersion.VERSION_1_8.toString()
}
}

Question 3

What is required to build a Retrofit instance?

Answer 3

private val retrofit = Retrofit.Builder()
.addConverterFactory(ScalarsConverterFactory.create())
OR
.addConverterFactory(MoshiConverterFactory.create(moshi))
.baseUrl(BASE_URL)
.build()

At a minimum, Retrofit requires a base url and a converter factory in order to build a web services API.

Question 4

What does the base url point to?

What does an appended url point to?

Answer 4

Base url points to the backend server.

Appended url points to and endpoint.

Question 5

What else can a JSON object sometimes be called?

Answer 5

dictionary / hash map / associative array

Question 6

1. What does REST stand for?
2. What does SOAP stand for?
3. What is the difference between these two?

Answer 6

1. Representational State Transfer
2. Simple Object Access Protocol
3. REST - JSON or XML / HTTP - more flexible than SOAP
   SOAP - XML / usually HTTP but can be SMTP (Simple Mail Transfer Protocol)

Question 7

Which error may require you to uninstall and re run the app after setting up the first network call?

Answer 7

java.net.SocketException: socket failed: EPERM (Operation not permitted)

EPERM and EACCES are errors for processes that require root/super user

Question 8

What does API stand for?

Answer 8

Application Programming Interface

Question 9

What are the four HTTP methods for API calls?

Answer 9

GET (read)
POST (create)
PUT (update)
DELETE (delete)

CRUD = create, read, update, delete

Question 10

Define an interface for an API call

Answer 10

interface MyApiService{
@CRUD_METHOD(“endpoint”)
suspend fun myCrudFunction( ): ReturnType
}

Without coroutines, ReturnType is Call; the Call object is used to start the request.

When implementing coroutines, the call is launched from the coroutine and the ReturnType will likely be List.

suspend fun when using coroutines instead of Callbacks

Question 11

Define an implementation of an API interface

Answer 11

object MyApi {
val retrofitService : MyApiService by lazy {
retrofit.create(MyApiService::class.java) }
}

object - so each call to MyApi.retrofitService returns a singleton - only one is required in the app

lazy - because the call is computationally expensive

Question 12

Using Callbacks, not coroutines, make an API call using the implementation of your Api interface

Answer 12

MyApi.retrofitService.myCrudFunction( ).enqueue(
  object: Callback {
// implement onFailure( ) and onResponse( )
} )

enqueue starts the request on a background thread

ExpectedReturnType will likely be List unless no conversion to objects has been implemented, in which case probably String

Question 13

With Callbacks (not coroutines), how does one handle the API response?

Answer 13

Use a LiveData

onFailure(call: Call, t: Throwable) {
liveData.value = “Failure: “ + t.message
}

onResponse(call: Call, response: Response) {
liveData.value = response.body( )
}

Question 14

What needs adding to the Manifest before implementing API calls?

Answer 14

^uses-permission android:name=”android.permission.INTERNET” /^

above the ^application^ tag

Question 15

To implement Moshi, what needs to be done with dependencies?

Answer 15

In the module’s build.gradle:

Add
implementation “com.squareup.moshi:moshi-kotlin:$version_moshi”

Replace converter-scalars implementation with
implementation “com.squareup.retrofit2:converter-moshi:$version_retrofit”

Question 16

What does Moshi do?

Provide syntax as an example of how.

Answer 16

Moshi converts/maps a JSON into custom Kotlin objects.

data class MyObject(
@Json(name="endpoint_key") val myObjectAttribute: Type
)

NB - if the endpoint_key also makes a good attribute name, you can just declare: val endpoint_key: Type

Question 17

How do you build a Moshi instance?

Answer 17

private val moshi = Moshi.Builder( )
.add(KotlinJsonAdapterFactory( ) )
.build( )

Question 18

With coroutines (not Callbacks), how does one handle the API response?

Answer 18

viewModelScope.launch {
  try {
    val listResult = MyApi.retrofitService.myCrudFunction()
    // use LiveData and manipulate function's return value here
  } catch (e: Exception) {
  // manipulate $e here
  }
}