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  3. Retrofit 2 — Reuse and Analyze Requests

Retrofit 2 — Reuse and Analyze Requests

Retrofit 2 fundamentally changes the way requests are made. Especially for developers, who are used to Retrofit 1.9 or earlier versions, the new Call class is something that takes time to get an understanding for. In this blog post, we'll show you some basic usage and features of this new mysterious class!

Before you learn more about the Call class, you might want to check out all the other Retrofit topics:

Retrofit Series Overview

  1. Getting Started and Creating an Android Client
  2. Basics of API Description
  3. Creating a Sustainable Android Client
  4. URL Handling, Resolution and Parsing
  5. How to Change API Base Url at Runtime
  6. Multiple Server Environments (Develop, Staging, Production)
  7. Share OkHttp Client and Converters between Retrofit Instances
  8. Upgrade Guide from 1.9
  9. Beyond Android: Retrofit for Java Projects
  10. How to use OkHttp 3 with Retrofit 1
  1. Synchronous and Asynchronous Requests
  2. Send Objects in Request Body
  3. Add Custom Request Header
  4. Manage Request Headers in OkHttp Interceptor
  5. Dynamic Request Headers with @HeaderMap
  6. Multiple Query Parameters of Same Name
  7. Optional Query Parameters
  8. Send Data Form-Urlencoded
  9. Send Data Form-Urlencoded Using FieldMap
  10. How to Add Query Parameters to Every Request
  11. Add Multiple Query Parameter With QueryMap
  12. How to Use Dynamic Urls for Requests
  13. Constant, Default and Logic Values for POST and PUT Requests
  14. Cancel Requests
  15. Reuse and Analyze Requests
  16. Optional Path Parameters
  17. How to Send Plain Text Request Body
  18. Customize Network Timeouts
  19. How to Trust Unsafe SSL certificates (Self-signed, Expired)
  20. Dynamic Endpoint-Dependent Interceptor Actions
  21. How to Update Objects on the Server (PUT vs. PATCH)
  22. How to Delete Objects on the Server
  1. Ignore Response Payload with Call<Void>
  2. Receive Plain-String Responses
  3. Crawl HTML Responses with jspoon (Wikipedia Example)
  4. Loading Data into RecyclerView and CardView
  1. Introduction to (Multiple) Converters
  2. Adding & Customizing the Gson Converter
  3. Implementing Custom Converters
  4. How to Integrate XML Converter
  5. Access Mapped Objects and Raw Response Payload
  6. Supporting JSON and XML Responses Concurrently
  7. Handling of Empty Server Responses with Custom Converter
  8. Send JSON Requests and Receive XML Responses (or vice versa)
  9. Unwrapping Envelope Responses with Custom Converter
  10. Wrapping Requests in Envelope with Custom Converter
  11. Define a Custom Response Converter
  1. Simple Error Handling
  2. Error Handling for Synchronous Requests
  3. Catch Server Errors Globally with Response Interceptor
  4. How to Detect Network and Conversion Errors in onFailure
  1. Log Requests and Responses
  2. Enable Logging for Development Builds Only
  3. Log Network Traffic with Stetho and Chrome Developer Tools
  4. Using the Log Level to Debug Requests
  5. Analyze Network Traffic with Android Studio Profiler
  6. Debug and Compare Requests with RequestBin
  1. Introduction to Call Adapters
  2. Custom Call Adapter to Separate OnResponse Callback
  3. How to Integrate RxJava 1.x Call Adapter
  4. How to Integrate RxJava 2.x Call Adapter
  5. How to Integrate Guava Call Adapter
  6. Custom Call Adapter to Separate Network and Gson Errors
  1. Pagination Using Query Parameter
  2. Pagination Using Link Header and Dynamic Urls (Like GitHub)
  3. Pagination Using Range Header Fields (Like Heroku)
  1. How to Upload Files to Server
  2. How to Upload Multiple Files to Server
  3. How to Upload a Dynamic Amount of Files to Server
  4. Upload Files with Progress
  5. Passing Multiple Parts Along a File with @PartMap
  6. How to Download Files from Server
  7. Download Files with Progress Updates
  8. How to Upload Files to Server
  1. Basic Authentication on Android
  2. Token Authentication on Android
  3. OAuth on Android
  4. Hawk Authentication on Android
  5. How to Refresh an Access Token
  1. Activate Response Caching (Etag, Last-Modified)
  2. Check Response Origin (Network, Cache, or Both)
  3. Force Server Cache Support with Response Interceptor
  4. Support App Offline Mode by Accessing Response Caches
  5. Analyze Cache Files
  1. Basics of Mocking Server Responses
  2. Customizing Network Behavior of Mocked Server Responses
  3. Mock APIs with JsonServer
  1. Callbacks
  2. Annotations
  3. Fluent Interface with Builders

Reuse of Call Objects

In this blog post, we'll make use of the code from our how to download files blog post. If you're interested in the details, give it a quick read. Otherwise, keep going. What kind of request it is, doesn't really matter. All the features we'll show you apply for all Retrofit requests.

One Request for Each Call Object

One of the first things you've to know about the Call class and its instances: each instance is special made for one and only one request. You cannot simply use the same object and call execute() or enqueue() again:

FileDownloadService downloadService = ServiceGenerator.create(FileDownloadService.class);

Call<ResponseBody> originalCall = downloadService.downloadFileWithDynamicUrlSync(fileUrl);  
Callback<ResponseBody> downloadCallback = new Callback<ResponseBody>() {...};

// correct usage:
originalCall.enqueue(downloadCallback);

// some other actions in between
// ...

// incorrect reuse:
// if you need to make the same request again, don't use the same originalCall again!
// it'll crash the app with a java.lang.IllegalStateException: Already executed.
originalCall.enqueue(downloadCallback); // <-- would crash the app

Duplicated Call Objects

Instead, if you want to make the exact same request again, you can use the clone() method on the Call instance to create a copy of it. It'll contain the exact same settings and configuration. You can use the new copy to make an identical request to the server:

FileDownloadService downloadService =  
    ServiceGenerator.create(FileDownloadService.class);

Call<ResponseBody> originalCall =  
    downloadService.downloadFileWithDynamicUrlSync(fileUrl);
Callback<ResponseBody> downloadCallback = new Callback<ResponseBody>() {...};

// correct reuse:
Call<ResponseBody> newCall = originalCall.clone();  
newCall.enqueue(downloadCallback);

This can be very useful if you need to send the same request multiple times.

Analyzing Requests With the Call Object

As you've already seen, Retrofit 2 includes the Call instance to every callback method. Thus, if the request was successful or even if it failed, you still can access the original Call object. But the reason to include it is not so that you can reuse the call (hint: use clone()), it's that you can analyze the request that was made using call.request(). This will return the complete request data. Let's look at an example:

FileDownloadService downloadService =  
    ServiceGenerator.create(FileDownloadService.class);

Call<ResponseBody> originalCall =  
    downloadService.downloadFileWithDynamicUrlSync(fileUrl);

originalCall.enqueue(new Callback<ResponseBody>() {  
    @Override
    public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
        checkRequestContent(call.request());
    }

    @Override
    public void onFailure(Call<ResponseBody> call, Throwable t) {
        checkRequestContent(call.request());
    }
});

Both callback methods are passing the request to a new helper method checkRequestContent(), which accesses the request headers, body and url:

private void checkRequestContent(Request request) {  
    Headers requestHeaders = request.headers();
    RequestBody requestBody = request.body();
    HttpUrl requestUrl = request.url();

    // todo make decision depending on request content
}

This might be useful to you if you want to review the request, which was send to the server.

Preview Requests

It's also good to know that you can utilize call.request() at any time, even if the request wasn't scheduled or made yet! It'll generate the request just like it would for a regular network call.

Important: the call.request() method does some significant computing, if the request wasn't executed yet. It's not recommended to get a preview of the data via .request() on Android's UI/Main thread!

Summary

In this blog post, we've made you a little more familiar with the Call class and how to use it. We've shown that you can use each instance only once. Furthermore, we've also shown you a way how to duplicate requests. Finally, you've learned how you can analyze the request content at any time!

Do you have a firm grasp of the Call class yet? Let us know in the comments!

Norman Peitek

android & node.js dev

https://twitter.com/peitek

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