Dropbox Xamarin OAuth Flow Question - Retrieving the Access Token

Question

Hi there,&nbsp;I have a mobile app on Android that uses Xamarin's tools to do some basic dropbox operations. In the past we used the Sync SDK to use dropbox quite easily. With the June v1 shut off coming up, I'm now in the process of updatng the app to use v2 Core API, namely with techniques shown in the Java SDK examples(I've read on these forums that that's the recommended place to learn from for Android).&nbsp;In any case, I'm doing the OAuth flow implementation and have a very similar question to this one:&nbsp;https://www.dropboxforum.com/t5/API-support/Cocoa-WebView-not-reacting-to-quot-Allow-quot-button-click/m-p/33714/thread-id/1025. I appear to be close to redirecting back to my app, but I must be doing something wrong.&nbsp;More info:My app uses the Implicit grant type. I've registered redirect URIs on my apps page at Dropbox developer site(for instance, I have myappname://imagegallery/ as one of them). For the OAth flow, we're assuming that the user doesn't have the official Dropbox mobile app and instead is using an external Android browser(namely Chrome). I'm able to get to the dropbox site just fine to sign in, get to the ALLOW screen but after that I get an unhandled exception in my app.&nbsp;&nbsp;Here are what my AndroidManifest.xml intent filters looks like, with important values substituted:&lt;activity android:name="ImageGallery"&nbsp; android:configChanges="orientation|keyboard"&nbsp; android:launchMode="singleTask"&gt;&nbsp; &lt;intent-filter&gt;&nbsp; &nbsp; &lt;data android:scheme="db-&lt;My_App_Key_Here&gt;"/&gt;&nbsp; &nbsp; &lt;action android:name="android.intent.action.VIEW" /&gt;&nbsp; &nbsp; &lt;category android:name="android.intent.category.BROWSABLE" /&gt;&nbsp; &nbsp; &lt;category android:name="android.intent.category.DEFAULT" /&gt;&nbsp; &lt;/intent-filter&gt;&nbsp;&nbsp;&lt;intent-filter&gt;&nbsp; &nbsp; &lt;action android:name="android.intent.action.VIEW"&gt;&lt;/action&gt;&nbsp; &nbsp; &lt;data android:scheme="myappname" android:host="imagegallery"&gt;&lt;/data&gt;&nbsp; &nbsp; &lt;category android:name="android.intent.category.DEFAULT"&gt;&lt;/category&gt;&nbsp; &nbsp; &lt;category android:name="android.intent.category.BROWSABLE"&gt;&lt;/category&gt;&nbsp; &lt;/intent-filter&gt;&lt;/activity&gt;&nbsp;I'm expecting the behavior to be as follows: The user signs in &amp; presses the allow button, then the browser switches back to the onResume() of my imagegallery activity. Unfortunately I've been stuck at this point in the auth flow for a day or two. Any advice is much appreciated, thanks.&nbsp;

Greg-DB · Answer

Can you share the full error/output for the exception you mentioned?

We can't offer much help with anything Xamarin-specific, but we'll be happy to take a look and see what's going on with the Dropbox side of things.

Thanks in advance!

NachoMurphy · Answer

Thanks for your response Greg. I'm happy to share more info.
All I'm really after is grabbing the access token from the android browser and returning to my app in order to use it. There's just a disconect between when the user hits 'Allow' and when they return to my app. I can see the access token etc in the redirect URL, but it's unclear how I'd take that for use. When I use localhost as the redirect address, after Allow an app chooser appears and I can choose my app to use to go to. However, that's when the exception occurs.
&nbsp;
The only error message I see in my output is :
&nbsp;
Thread finished: &lt;Thread Pool&gt; #4
The thread 'Unknown' (0x4) has exited with code 0 (0x0).
05-09 08:33:25.048 D/AndroidRuntime(23519): Shutting down VM
An unhandled exception occured.
&nbsp;
It's not so much an specific exception, but seemingly nothing happens when the user gets redirected.
&nbsp;
Here's what my ImageGallery activity code looks like that interacts with this:
        private void HandleUpload ()
	{
            // see if there is an existing user access token stored
            sp = GetSharedPreferences(SharedPrefsName, 
                    FileCreationMode.Private);
            AccessTokenValue = sp.GetString(AccessTokenKey, string.Empty);
            waitingOnDropboxUpload = true;

if (AccessTokenValue.Equals(string.Empty))
            {
                string authAddress = getOAuthAddress();
                Intent browserIntent = new Intent(Intent.ActionView, Android.Net.Uri.Parse(authAddress));
                StartActivityForResult(Intent.CreateChooser(browserIntent, "Open With"), 1);
            }            // continue with upload etc.
        }

public static string getOAuthAddress()
        {
            string localRedirect = "https://localhost/";

var redirect = DropboxOAuth2Helper.GetAuthorizeUri(
                OAuthResponseType.Token, 
                DropboxAppKey,
                localRedirect, 
                AuthActivity.RandomString(16), 
                true, 
                false);

string result = redirect.ToString();
            
            return result;
        }
And of course the relevant manifest snippet:
&nbsp;
&nbsp;
&lt;activity android:name="ImageGallery" android:launchMode="singleTask"&gt;
      &lt;intent-filter&gt;
	&lt;data android:scheme="db-&lt;My_App_Key&gt;" /&gt;
	&lt;action android:name="android.intent.action.VIEW" /&gt;
	&lt;category android:name="android.intent.category.BROWSABLE" /&gt;
	&lt;category android:name="android.intent.category.DEFAULT" /&gt;
      &lt;/intent-filter&gt;
      &lt;intent-filter&gt;
        &lt;action android:name="android.intent.action.VIEW"&gt;&lt;/action&gt;
        &lt;data android:scheme="https" android:host="localhost"&gt;&lt;/data&gt;
        &lt;category android:name="android.intent.category.DEFAULT"&gt;&lt;/category&gt;
        &lt;category android:name="android.intent.category.BROWSABLE"&gt;&lt;/category&gt;
      &lt;/intent-filter&gt;
&lt;/activity&gt;
&nbsp;
As I mentioned in the first post, per other forum advice I'm looking at the Java SDK and trying to emulate what that does. However it looks like as the comments in Auth.java &amp; AuthActivity.java indicate, the auth flow used there is based on the SDK's OpenWith code for use with the official DropBox mobile app.&nbsp;
&nbsp;
Now, I'm exploring using a WebView to keep everything inside my app for more control. Even then, I'll need to figure out how to grab the returned parameters from the redirect url. Thanks again for your help.

NachoMurphy · Answer

I spoke too soon Greg, here's more exception details after I each time I get out of break mode:
&nbsp;
05-09 09:01:00.418 E/AndroidRuntime(27739): FATAL EXCEPTION: main
05-09 09:01:00.418 E/AndroidRuntime(27739): Process: MyApp.App, PID: 27739
05-09 09:01:00.418 E/AndroidRuntime(27739): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{MyApp.App/MyApp.App.ImageGallery}: java.lang.ClassNotFoundException: Didn't find class "MyApp.App.ImageGallery" on path: DexPathList[[zip file "/data/app/MyApp.App-1/base.apk"],nativeLibraryDirectories=[/data/app/MyApp.App-1/lib/arm, /vendor/lib, /system/lib]]
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread.access$800(ActivityThread.java:151)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.os.Handler.dispatchMessage(Handler.java:102)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.os.Looper.loop(Looper.java:135)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread.main(ActivityThread.java:5254)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at java.lang.reflect.Method.invoke(Native Method)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at java.lang.reflect.Method.invoke(Method.java:372)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
05-09 09:01:00.418 E/AndroidRuntime(27739): Caused by: java.lang.ClassNotFoundException: Didn't find class "MyApp.App.ImageGallery" on path: DexPathList[[zip file "/data/app/MyApp.App-1/base.apk"],nativeLibraryDirectories=[/data/app/MyApp.App-1/lib/arm, /vendor/lib, /system/lib]]
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at java.lang.ClassLoader.loadClass(ClassLoader.java:511)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at java.lang.ClassLoader.loadClass(ClassLoader.java:469)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
05-09 09:01:00.418 E/AndroidRuntime(27739): 	... 10 more
05-09 09:01:00.418 E/AndroidRuntime(27739): 	Suppressed: java.lang.ClassNotFoundException: MyApp.App.ImageGallery
05-09 09:01:00.418 E/AndroidRuntime(27739): 		at java.lang.Class.classForName(Native Method)
05-09 09:01:00.418 E/AndroidRuntime(27739): 		at java.lang.BootClassLoader.findClass(ClassLoader.java:781)
05-09 09:01:00.418 E/AndroidRuntime(27739): 		at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)
05-09 09:01:00.418 E/AndroidRuntime(27739): 		at java.lang.ClassLoader.loadClass(ClassLoader.java:504)
05-09 09:01:00.418 E/AndroidRuntime(27739): 		... 13 more
05-09 09:01:00.418 E/AndroidRuntime(27739): 	Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack available
Hope this helps.
&nbsp;

Greg-DB · Answer

Thanks for the additional information!&nbsp;
&nbsp;
If I understand correctly, you're using the API v2 .NET SDK, but you're trying to implement the app authorization flow the way the API v2 Java SDK&nbsp;does it. These two SDKs work differently though, so there will be a disconnect between the two. You won't be able to easily reconcile the two, so I&nbsp;recommend picking one or the other (whatever makes the most sense for your app/platform) and following the documentation/sample for the one you pick.
&nbsp;
For reference, the Android app sample that uses the API v2 Java SDK&nbsp;uses a custom URL scheme as defined in the Manifest&nbsp;to directly&nbsp;receive the redirect after sending the user through the app authorization flow.&nbsp;Dropbox does some special handling for you to make use of that "db-&lt;app_key&gt;" URL scheme.
&nbsp;
The API v2 .NET SDK on the other hand uses either the standard OAuth 2 'token' or&nbsp;'code' flow. You can see how the code flow is initiated here and received here. There's a sample of initiating the token flow here (which your code so far uses) and receiving it here. I.e., you can use ParseTokenFragment&nbsp;to parse the token from the redirect URI.
&nbsp;
(Also, I should note that you should no longer use web view for the OAuth app authorization flow, in order to support the embedded Google Sign In flow, as Google is no longer allowing their sign in flow to be processed in web views.)
&nbsp;
In any case, the actual exception you're getting seems to be a&nbsp;ClassNotFoundException for class "MyApp.App.ImageGallery". That's not part of the&nbsp;Dropbox SDK though, and seems to just be part of your app, so I'm afraid I can't offer help with that in particular.

NachoMurphy · Answer

Thanks for the response.
In light of the Web View news, I'm going to opt for only using the .NET SDK after all. It's true that I had been using parts of both SDKs before. It's frustrating because in the time between my posts I had implemented a WebView soution that was partially working. Oh well, so much for that.
&nbsp;
Back to the issue then: I'm using the .NET approach to the OAuth flow. I use the android browser to navigate to the authorize URL. Then the user would sign in if needed and hit Allow. My newest issue then becomes: I get how the .NET SDK example uses BrowserNavigating to&nbsp;capture the event and can then parse the access token from the URI, but how would I do that for android? Is there a way to attach a listener to the browser intent? This is the immediate obstacle I'm facing&nbsp;at present.
&nbsp;
Here's some revised code:
        private void HandleDropboxOperation()
        {
            waitingOnDropboxUpload = true;
            taskCode = AuthActivity.RandomString(16);
            Toast.MakeText(this, "Uploading...", ToastLength.Short).Show();

//see if there is an existing user access token stored
            sp = GetSharedPreferences(SharedPrefsName, FileCreationMode.Private);
            AccessTokenValue = sp.GetString(AccessTokenKey, string.Empty);
            
            if (AccessTokenValue.Equals(string.Empty))
            {
                this.OAth2State = Guid.NewGuid().ToString("N");
                var authorizeUri = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Token, DropboxAppKey, new Uri(RedirectUri), state: Guid.NewGuid().ToString("N"));
                //no browser object available - use intent instead
                Android.Net.Uri.Builder b = new Android.Net.Uri.Builder();
                b.AppendPath(authorizeUri.ToString());
                Intent browserIntent = new Intent(Intent.ActionView, b.Build());
                StartActivityForResult(Intent.CreateChooser(browserIntent, "Open With"), 1);
                //below is from the .NET SDK code - how to implement in Android?
                //OAuth2Response result = DropboxOAuth2Helper.ParseTokenFragment(e.Uri);                //proceed with upload after token is obtained
            }
            else
            {
                InitiateUpload();
            }
        }

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