Downloadprogress Interceptor

An OkHttp interceptor which monitors download progress of HTTP response data.

Download

Downloadprogress Interceptor is available on Jitpack
add the JitPack repository in your project build.gradle (in top level dir):

repositories {
    jcenter()
    maven { url "https://jitpack.io" }
}

and the dependency in the build.gradle of the module:

dependencies {
    compile 'com.github.jobinlawrance:okhttp3-downloadprogress-interceptor:1.0.1'
}

Usage

Prerequisite

The progress monitoring won't work if the reponse doesn't have a content-length in it's header.

Creating the interceptor

We use a simple event bus ProgressEventBus to track the progress of all the download requests.
Since OkHttpClient and Retrofit is instantiated as application wise singletons, the same should be done with ProgressEventBus and it should be injected to the classes that requires the progress monitoring.

kotlin

val progressEventBus: ProgressEventBus = ProgressEventBus()
val downloadInterceptor: DownloadProgressInterceptor = DownloadProgressInterceptor(progressEventBus)

val okHttpClient = OkHttpClient.Builder()
                      .addNetworkInterceptor(downloadInterceptor)
                      .build()

val retrofit = Retrofit.Builder()
               ...                // base url, adapter factory etc
               .client(okHttpClient)
               .build();
                     

Custom Request Header

Since OkHttp client is used for all Http requests, in order to differentiate the request we are interested in, we add a custom header to it.
Our retrofit interface will look like this -

kotlin

interface PhotoService {
  @GET
  @Streaming
  fun downloadPhoto(@Url url: String, @Header(DOWNLOAD_IDENTIFIER_HEADER) identifier: String) : Observable<ResponseBody>
}

Downloading and checking progress

To receive download progress updates, we subsribe to ProgressEventBus. Whenever a download is started by retrofit all the subscibers receives a ProgressEvent which has the progress(in percent) and downloadIdentifier data among others.
Since ProgressEventBus monitors all the download requests with the custom header we set above, we check the ProgressEvent.dowloadIdentfier to filter out the ProgressEvent we are interested in.
Here's an example -

kotlin

// Inject retrofit and progressEventBus

@Inject lateinit val retrofit
@Inject lateinit val progressEventBus

val imageUrl = "https://upload.wikimedia.org/wikipedia/commons/2/24/Junonia_orithya-Thekkady-2016-12-03-001.jpg"
val imageIdentifier = "my-wiki-image"

val disposable =                                  // handle unsubscription   
                progressEventBus.observable()
                        .subscribe({
                                        if(it.downloadIdentifier == imageIdentifier) {
                                          // Display the progress here
                                          Log.d("Download Progress - ${it.progress}")
                                        }
                                   },{
                                        Log.d("ProgressEvent Error",it)
                                   })

retrofit.create(PhotoService::class.java)
                .downloadPhoto(imageUrl, imageIdentifier)
                .subscribe({   
                            // onNext(resonseBody)
                            // Download start, 
                            // code to save the responseBody to file in storage
                            
                            },{   
                                // onError()
                                // Error in dowloading
                            },{
                               // Download complete
                               if (!disposable.isDisposed) {
                                    disposable.dispose()
                                }
                            })
    

Credits

1. https://blog.playmoweb.com/view-download-progress-on-android-using-retrofit2-and-okhttp3-83ed704cb968
2. https://github.com/square/okhttp/blob/aed222454743ebe5724d6ad438fafed37956521e/samples/guide/src/main/java/okhttp3/recipes/Progress.java