Demo of difference between accessing views from OkHttp 2 Callback and MainThreadCallback.

CallError.java

public class CallError extends RuntimeException {

    public static CallError networkError(Request response, IOException exception) {
        return new CallError(response, Kind.NETWORK, exception);
    }

    public static CallError httpError(Request response) {
        return new CallError(response, Kind.HTTP, null);
    }

    public enum Kind {
        /**
         * An {@link java.io.IOException} occurred while communicating to the server.
         */
        NETWORK,
        /**
         * A non-200 HTTP status code was received from the server.
         */
        HTTP
    }

    private final Request request;
    private final Kind kind;

    CallError(Request request, Kind kind, Throwable throwable) {
        super(throwable);
        this.request = request;
        this.kind = kind;
    }

    public Request getRequest() {
        return request;
    }

    public Kind getKind() {
        return kind;
    }
}

MainThreadCallback.java

public abstract class MainThreadCallback implements Callback {

    private final Handler mHandler = new Handler(Looper.getMainLooper());

    @Override
    public final void onFailure(final Request request, final IOException e) {
        callFailure(CallError.networkError(request, e));
    }

    @Override
    public final void onResponse(final Response response) throws IOException {
        if (response.isSuccessful() && response.body() != null) {
            try {
                final String body = response.body().string();
                mHandler.post(new Runnable() {

                    @Override
                    public void run() {
                        success(body);
                    }
                });
            } catch (final IOException e) {
                callFailure(CallError.networkError(response.request(), e));
            }
        } else {
            callFailure(CallError.httpError(response.request()));
        }
    }

    private void callFailure(final CallError error) {
        mHandler.post(new Runnable() {

            @Override
            public void run() {
                failure(error);
            }
        });
    }

    public abstract void success(String responseBody);

    public abstract void failure(CallError error);

}

OkHttpActivity.java

public class OkHttpActivity extends AppCompatActivity {

    private OkHttpClient mClient = new OkHttpClient();

    private TextView mResponseText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_ok_http);

        mResponseText = (TextView) findViewById(R.id.response_text);
    }

    //onClick listener for a Button, crashes with "android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views."
    public void normalCallback(View view) {
        mClient.newCall(new Request.Builder().url("http://drive.google.com/uc?export=view&id=0B2ZerSqwiAA-dGdWMk1RaFRlQTQ").build()).enqueue(new Callback() {
            @Override
            public void onFailure(Request request, IOException e) {
                mResponseText.setText("failure");
            }

            @Override
            public void onResponse(Response response) throws IOException {
                mResponseText.setText("success");
            }
        });
    }

    //onClick listener for a Button
    public void mainThreadCallback(View view) {
        mClient.newCall(new Request.Builder().url("http://drive.google.com/uc?export=view&id=0B2ZerSqwiAA-dGdWMk1RaFRlQTQ").build()).enqueue(new MainThreadCallback() {
            @Override
            public void success(String responseBody) {
                mResponseText.setText("success");
            }

            @Override
            public void failure(CallError error) {
                mResponseText.setText("failure");
            }
        });
    }
}