Unable to get Android App Link to work in an Android app written in Flutter

Question

I am trying to implement Moneris hosted pay page from a flutter mobile app but this question is relevant to any app that wants to implement Android App Links. In order to redirect back from the hosted pay page, I need to implement Android App Link (haven't gotten to do iOS version yet). Based on this article, in order for App Links to work, one needs:

1. Identify a redirect URL on the Moneris hosted pay page (or on the page where a redirect might take place, in case of a generic example);
2. Modify AndroidManifest.xml to associate redirects with activities;
3. Implement a way in Flutter to intercept App Links;
4. Place the Digital Asset Links JSON file on the web site (same domain as redirect URL) in the .well-known folder.

For #1, I have identified 2 URLs, one for approved payments (https://example.com/moneris_approved) and one for declined payments (https://example.com/moneris_declined). URLs have to be in https for the whole thing to work. Moneris can reply with a POST with XML, POST with parms or GET with parms. Tried all three.

For #2, I have modified AndroidManifest.xml both by hand and with help of Android Studio. Relevant pieces look like this:

<application
    android:name="io.flutter.app.FlutterApplication"
    android:label="<label>"
    android:icon="@mipmap/ic_launcher">
    <activity
        android:name=".MainActivity"
        android:launchMode="singleTop"
        android:theme="@style/LaunchTheme"
        ...              
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

        <intent-filter android:autoVerify="true">
            <action android:name="android.intent.action.VIEW" />    
            <category android:name="android.intent.category.DEFAULT" />
            <category android:name="android.intent.category.BROWSABLE" />    
            <data
                android:scheme="https"
                android:host="example.com"
                android:pathPattern="/moneris_approved" />
        </intent-filter>    
        <intent-filter android:autoVerify="true">
            <action android:name="android.intent.action.VIEW" />   
            <category android:name="android.intent.category.DEFAULT" />
            <category android:name="android.intent.category.BROWSABLE" />    
            <data
                android:scheme="https"
                android:host="example.com"
                android:pathPattern="/moneris_declined" />
        </intent-filter>    
    </activity>
</application>

For #3, I am using uni_links package which takes care of channeling incoming App Links to my app.

For #4, I have created and uploaded the assetlinks.json file and placed in the right folder. Included both debug and release SHA256 fingerprints. The file looks like this:

[
    {
        "relation": ["delegate_permission/common.handle_all_urls"],
        "target": {
            "namespace": "android_app",
            "package_name": "com.example.<name>",
            "sha256_cert_fingerprints":
            ["........"]
        }
    }
]

Now, after doing all that, when Moneris redirects to provided URLs, I am seeing a 404 page coming from my hosting site.

To verify the setup, I have:

• Successful tested statement file with the Statement List Generator and Tester;
• Successfully tested digital asset link file with Google APIs (https://digitalassetlinks.googleapis.com/v1/statements:list?source.web.site=https://example.com&relation=delegate_permission/common.handle_all_urls)
• Successfully tested the URL intent using ADB shell (am start -W -a android.intent.action.VIEW -c android.intent.category.BROWSABLE -d "https://example.com/moneris_approved")
• Successfully checked link policies using ADB (adb shell dumpsys package d)

At this point, I don't know how else to test it or what the problem is. I don't think the problem is with my app but rather soemthing is not closing the loop between Moneris redirect and it coming back to my app.

Update 1

I devised a testing approach like this:

1. Created a link-test page on my site with links mimicking redirects (i.e. same host/path as in the app's AndroidManifest.xml;
2. Created a simple Java Android App using Android Studio. Using App Link Assistant I updated its AndroidManifest, created intents and reacted to those coming in. Also updated the assetlinks.json file on my website. Tested it with the link-test page running on my emulator and everything worked as expected. App opened up ok.
3. Repeated #2 but this time with a Flutter app using the uni_links package. Went throught he same steps of creating slightly different links on my link-test page, updating AndroidManifest and assetlinks.json file. Tested it with the link-test page running on my emulator and everything worked as expected. App opened up ok.
4. Run the app I am working on on the emulator and put the links I am expecting to get from Moneris on my link-test page and tsted on the emulator. Everything worked as expected. App opened up ok.

So now my question is: what is the difference between me clicking on a link on my page (HTTP GET) vs. Moneris redirecting to the same link using HTTP GET? Me clicking on the link works. Moneris redirect does not.

Solution

I can see that you want to handle deep link right in the flutter code. That is possible but you have to use one of the libraries for that - for example this.

If you want to do it by yourself you have to create a platform specific Method Channels for that. This answer shows how to do it.

Also you can find a more detailed explanation here

Hope it helps.

Comments

[Ovidiu]

Have you tried testing your implementation using the App Links Assistant? In Android Studio, Ctrl + Shift + A -> App Links Assistant -> Test App Links. It might give you some relevant errors.

[Igor]

thanks for suggesting an answer but I think you might have misunderstood the intent of what I am trying to do here. This has nothing to do with authentication or authorization - I looked up apps.googleusercontent.com and it took me here (developers.google.com/identity/sign-in/web/server-side-flow‌​). Consequently, no Client Secret is passed on the call.

[Mohamed Elrashid]

@Igor I use this URL Sechma for paypal redirect, it's not specific for authentication or authorization , also good 👍to know that there is no client secret stroed in the app

[Igor]

thanks for clarification. Unfortunately, Moneris only supports http and https schemas so using com.googleusercontent.apps as schema will not work for me. And while I get that it works for you, your implementation contradicts my understanding of how Android App Links should work as per this: developer.android.com/training/app-links. i am puzzled.

[Mohamed Elrashid]

@Igor 😲 thanks for the link , your are right , I used the url to skip the verification step for local devlopment , try to change android:pathPatter to android:pathPrefix

[Igor]

Glad I could help...mind you, I'm back to square one.

[Igor]

I will review the last link (to the Medium article) but as for the rest, I am using the uni_link flutter package. And I have it tested with all tests provided by Google yet it still does not work. Hence my question.