android - Okhttp response callbacks on the main thread

Question

I have created a helper class to handle all of my http calls in my app. It is a simple singleton wrapper for okhttp that looks like this (I have omitted some unimportant parts):

public class HttpUtil {

    private OkHttpClient client;
    private Request.Builder builder;

    ...

    public void get(String url, HttpCallback cb) {
        call("GET", url, cb);
    }

    public void post(String url, HttpCallback cb) {
        call("POST", url, cb);
    }

    private void call(String method, String url, final HttpCallback cb) {
        Request request = builder.url(url).method(method, method.equals("GET") ? null : new RequestBody() {
            // don't care much about request body
            @Override
            public MediaType contentType() {
                return null;
            }

            @Override
            public void writeTo(BufferedSink sink) throws IOException {

            }
        }).build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Request request, Throwable throwable) {
                cb.onFailure(null, throwable);
            }

            @Override
            public void onResponse(Response response) throws IOException {
                if (!response.isSuccessful()) {
                    cb.onFailure(response, null);
                    return;
                }
                cb.onSuccess(response);
            }
        });
    }


    public interface HttpCallback  {

        /**
         * called when the server response was not 2xx or when an exception was thrown in the process
         * @param response - in case of server error (4xx, 5xx) this contains the server response
         *                 in case of IO exception this is null
         * @param throwable - contains the exception. in case of server error (4xx, 5xx) this is null
         */
        public void onFailure(Response response, Throwable throwable);

        /**
         * contains the server response
         * @param response
         */
        public void onSuccess(Response response);
    }

}

Then, in my main activity, I use this helper class :

HttpUtil.get(url, new HttpUtil.HttpCallback() {
            @Override
            public void onFailure(Response response, Throwable throwable) {
                // handle failure
            }

            @Override
            public void onSuccess(Response response) {
                // <-------- Do some view manipulation here
            }
        });

onSuccess throws an exception when the code runs :

android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

From my understanding, Okhttp callbacks run on the main thread so why do I get this error ?

** Just as a side note, I have created HttpCallback interface to wrap Okhttp's Callback class because I wanted to change the behaviour of onResponse and onFailure so I could unite the logic of handling failed responses due to i/o exception and failed responses due to server problems.

Thanks.

Answer 1

From my understanding, Okhttp callbacks run on the main thread so why do I get this error ?

This is not true. Callbacks run on a background thread. If you want to immediately process something in the UI you will need to post to the main thread.

Since you already have a wrapper around the callback you can do this internally in your helper so that all HttpCallback methods are invoked on the main thread for convenience.