Downloading multiple files using AWS Amplify for Android

Question

I'm building an Android app which uses AWS Amplify to list and download files from S3.

The sample code shows downloading is asynchronous:

Amplify.Storage.downloadFile()
    "ExampleKey",
    new File(getApplicationContext().getFilesDir() + "/download.txt"),
    result -> Log.i("MyAmplifyApp", "Successfully downloaded: " + result.getFile().getName()),
    error -> Log.e("MyAmplifyApp",  "Download Failure", error)
);

I wish to download the (possibly many) files in a background thread, and notify the main thread after all files have been downloaded (or an error occurred). Questions:

What is the best way to achieve this functionality?

P.S. I've tried the RxAmplify, which exposes RxJava Observables on which I can call blockingSubscribe(). However, the bindings are very new, and I've encountered some app-crashing uncaught exceptions using it.

Jameson · Accepted Answer · 2020-09-29T07:09:26.270

0

With Vanilla Amplify

downloadFile() will perform its work on a background thread. Just use one of the standard approaches to go back onto the main thread, from the callback:

Handler handler = new Handler(context.getMainLooper());
File file = new File(context.getFilesDir() + "/download.txt");

Amplify.Storage.downloadFile(
    "ExampleKey", file,
    result -> {
        handler.post(() -> {
            Log.i("MyAmplifyApp", "Successfully downloaded: " + result.getFile().getName());
        });
    },
    error -> Log.e("MyAmplifyApp",  "Download Failure", error)
);

With Rx Bindings

But personally, I would use the Rx Bindings. The official documentation includes snippets for the Rx API. Here's a more tailored example:

File file = new File(context.getFilesDir() + "/download.txt");
RxAmplify.Storage.downloadFile("ExampleKey", file)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(result -> {
        Log.i("RxExample", "Download OK.");
    }, failure -> {
        Log.e("RxExample", "Failed.", failure);
    });

Running Many Downloads In Parallel

Build a collection of Singles by calling RxAmplify.Storage.downloadFile("key", local). Then, use Single.mergeArray(...) to combine them all. Subscribe to that, in the same way as above.

RxStorageCategoryBehavior storage = RxAmplify.Storage;
Single
    .mergeArray(
        storage.downloadFile("one", localOne)
            .observeResult(),
        storage.downloadFile("two", localTwo)
            .observeResult()
    )
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(/* args ... */);

Reporting Bugs

You mention that you encountered an unexpected exception. If so, please file a bug here, and I will fix it.

edited Sep 29 '20 at 07:09

answered Sep 22 '20 at 05:01

Jameson

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Thanks. Using your example as-is, I got and error for `mergeArray()`: "reason: no instance(s) of type variable(s) T exist so that RxProgressAwareSingleOperation conforms to SingleSource extends T>". I guess I should add `observeResult()` to each `downloadFile()`? – bavaza Sep 29 '20 at 07:04
+ I wonder how I can observe both result and progress in this case... – bavaza Sep 29 '20 at 07:06
Ah true, had to add `.observeResult()` on the calls inside the `mergeArray(...)`. Updated the post. To observe the progress as well, create a collection of the operations. Then, form an `Observable` by merging the various `.observeProgress()`. – Jameson Sep 29 '20 at 07:10
Thanks @Jameson. How can I merge an unknown number of singles? Namely, the result of a previous `list()` operation of the bucket? – bavaza Sep 29 '20 at 07:13
@bavaza Here's a starting place for that. You may want to create your own `Download` model that keeps the key, file, and operation all in one place. Otherwise, you can't tell which download the progress is for. https://gist.github.com/jamesonwilliams/bf66c47e46a1588daed4efa980acf7e6 – Jameson Sep 29 '20 at 07:39
Thanks. I get a build error for the `flatMapSingle`: "error: incompatible types: cannot infer type-variable(s) R.flatMapSingle(item -> RxAmplify.Storage.downloadFile(item, new File(path, item))); (argument mismatch; bad return type in lambda expression RxProgressAwareSingleOperation cannot be converted to SingleSource extends R>) where R,T are type-variables: R extends Object declared in method flatMapSingle(Function super T,? extends SingleSource extends R>>) T extends Object declared in class Observable". – bavaza Sep 29 '20 at 08:00
@bavaza oh yea use flatMap instead of flatMapSingle. – Jameson Sep 29 '20 at 08:02

Downloading multiple files using AWS Amplify for Android

1 Answers1

With Vanilla Amplify

With Rx Bindings

Running Many Downloads In Parallel

Reporting Bugs