I don’t think it’s a good idea to build an API and then build an app; when you first work on an API and an app, I think it’s important to build the API with the app.
Earlier this year I wrote a blog post that mentioned this[1], but in short: great apps have great user experiences, so working on the app’s user experience should be one of the first things you do. If you’re able to figure out exactly what you’ll need in an API to build the best user experience, then building the API might work for you; however, if you’re still trying to nail down down how your app is going to work, solidifying an API before building your app will probably result in an inefficient API or an API that needs to change.
Earlier this year I wrote a blog post that mentioned this[1], but in short: great apps have great user experiences, so working on the app’s user experience should be one of the first things you do. If you’re able to figure out exactly what you’ll need in an API to build the best user experience, then building the API might work for you; however, if you’re still trying to nail down down how your app is going to work, solidifying an API before building your app will probably result in an inefficient API or an API that needs to change.
[1] http://chasenlehara.com/blog/api-first-and-only-features/