Now that we know how the API works, let's use Retrofit to interact with it. 0:00 Retrofit uses interfaces to represent the API we are connecting to. 0:05 Let's start by creating a new Interface. 0:09 Here in my API package, I'm just going to create a new Java Class and 0:11 the type is going to be Interface. 0:16 I'm just gonna call it Api. 0:19 Next, we're going to add a method for each API call. 0:21 The API we're using returns back active listings from Etsy, so 0:24 I'm gonna call mine activeListings. 0:28 Now, Retrofit can be a little overwhelming when you're first learning how to use it, 0:32 but it has an elegant syntax that helps us understand what it's doing. 0:36 And, it does a lot of work for us, with only a little bit of code. 0:40 Retro fit uses annotations to tell it where the URI is and 0:44 what HTTP protocol to use. 0:48 From the documentation earlier, we know our protocol is get and 0:51 the URI is slash listing slash active. 0:55 Let's go ahead and annotate our method using the available retro fit annotations. 0:58 It's gonna ask us to import those as we would any other class or object type. 1:03 And, we also need to include a string that represents the URI, 1:08 which is /listings/active. 1:12 To include URL parameters, 1:14 we can include those as optional parameters in the active listings method. 1:18 I'm gonna annotate mine as Query, and I'm gonna give it a name. 1:23 The name is includes. 1:27 So, the includes was the image and 1:29 shop data that was coming through the API with our listings call. 1:31 We need to make sure we import those as well. 1:35 And, we need to give this a name as a variable. 1:40 Just gonna call it includes. 1:44 Finally, we need to include a callback. 1:46 Notice that the type of this method is void. 1:48 That means it's going to callback immediately once you call it. 1:50 But, we need to include a callback which when the response happens and 1:53 completes, we'll get a callback on the main UI thread. 1:57 Retrofit includes a class called Callback. 2:00 The callback mechanism is just like what we used in the weather app using OkHttp in 2:03 an earlier course. 2:07 This callback is gonna be typed to the Java class that the network response will 2:09 be converted into by Retrofit. 2:13 This means that Retrofit automatically parses the network response, and 2:15 creates an object of the type you want, with all the data set on it. 2:18 All you have to do is give it the class that corresponds to 2:22 the response from the API. 2:25 In this case, we wanna go ahead and use the ActiveListings class. 2:27 Again, we wanna go ahead and import this. 2:31 Great. 2:34 Our API is all set up now. 2:35 Next, we're going to add to our Etsy class. 2:37 We're gonna add a method to retrieve back an instance of the API interface we 2:40 just built. 2:43 We're gonna open the Etsy class, let's go ahead and add a method in here. 2:45 Private, static. 2:49 And, it's gonna return back the API we just built. 2:50 And, I'm gonna call it getApi. 2:54 We're going to use the included RestAdapter class 2:55 that's a part of Retrofit. 2:59 RestAdapter has a builder that's a part of it, 3:01 so we're gonna say new RestAdapter.Builder, and 3:04 with the builder pattern we can easily configure it with any options we need. 3:07 The first we're gonna do is set the end point. 3:11 So, from the Etsy documentation, 3:14 we know the endpoint is https://openapi.etsy.com/v2. 3:17 Once we set the end point, we can just call .build and then .create. 3:23 And, when you call .create what you're gonna do 3:30 is pass in the interface that you wanted to create an object from. 3:33 So we're gonna pass in the Api class. 3:37 Now, this going to return back an instance of Api with the end point set to it. 3:40 One thing we might've noticed is that we haven't set up the API 3:46 key anywhere in the request. 3:49 We can do this by adding in a request interceptor which allows us to 3:50 append a parameter at the end of the request after it's initially created, so 3:55 let's go ahead and make a method that will do this. 3:59 I'm going to create another method and 4:01 it's going to return back a request interceptor. 4:03 Just gonna call it getInterceptor. 4:07 And, it's gonna return back a new request interceptor. 4:13 What I'm gonna do is just add in the parameter that I need, 4:17 my API key with every request. 4:20 So, I'm gonna take the request and just say addEncodedQueryParameter. 4:23 And, the name of it is api key. 4:28 And, we started here in this value of capital API KEY. 4:31 We just append it there. 4:35 Finally, back in our builder, 4:38 we can call into setRequestInterceptor and call getInterceptor. 4:40