Today, a client asked us a very simple question: Why, when clicking on a link in an email sent by a web app, iPhone opens the mobile site through Safari instead of the app, which is installed on that very same iPhone? Good question. In fact, many of us have asked this question before. On your iPhone, try clicking on that new follower’s profile link that Twitter emails you when you get a new Twitter follower. The mobile site will open instead of the Twitter app you have installed. Or when you get a Facebook email: clicking on any link will open the Facebook mobile site through Safari, even though you have the Facebook mobile application installed.

This is a usability failure. There should be a way for developers or users themselves to instruct certain links to open in certain apps. For example, I wish all the Twitter links I click on my iPhone would open TweetBot, or that all the Facebook links I click on would open the Facebook native app. If these really big apps are not doing this already, it might just not be possible.

I went ahead and asked one of our iOS developers. He said he never tried to do this himself, but he saw that there was a way to bind an app to a custom URL scheme so when that URL is evoked, the app is launched. Great, but when we started going through the use cases, we realized that this might not be the best solutions. Here are some scenarios where this solution might not help:

Scenario 1: Clicking on a custom URL scheme on a non-iOS device. Obviously, this will not work. So you’d have to somehow make sure that these custom links are only sent to iOS devices.

Scenario 2: Clicking on a custom URL scheme on an iOS device that doesn’t have the application installed. We have not tried this, but saw somewhere in the comments on the posting referenced above that Safari will alert the user that it cannot open the link. This could be fixed by offering two links: If you have the app, click here. If not, click there. In our opinion, the usability cost doesn’t justify the benefit.

Scenario 3: Clicking on a custom URL scheme on an iOS device that has the app installed. This will work just fine. Except in the case where you use an independent Twitter client and Twitter decides to launch their own app from the links generated from their site.

In summary, there seems to be a good reason why this is still a bit of an annoying usability problem in iOS. It is one of those things that one thinks should be easy enough to accomplish until the implications are considered.

What is your take on this? Have you experience this before? Does it bother you? Or do you use a different platform other than iOS where this isn’t a problem?