When we're working with APIs, we care a lot about the interface itself.

It's an understanding between the user, us, and the service, Dark Sky.

It's much like a contract.

It tells us what we need to provide to the service.

In exchange, it tells us what the service will return.

The rest of the details, like where they get the weather information,

how they manage it,etc, is hidden from us, which is fine.

Let's take a look at the Dark Sky API, and see how to use their latest version.

We care about this one here, the Forecast Request.

The Forecast Request returns current weather data, along with forecast date for

the next hour, 48 hours, and the next week.

This is more or less the interface we will be using for our Stormy app.

We'll need to do a bit of work, of course, but don't worry,

it isn't going to stretch us too much, and I'll walk you through it step by step.

Essentially, we are going to make a request at this URL and

then handle the response Dark Sky provides.

We'll dive into that in more detail soon.

The Dark Sky API is a web API that follows the common RESTful programming pattern.

REST stands for

Representational State Transfer, which in it of itself may not mean much.

In our context, it means that when an API adheres to the principles of REST,

it doesn't require us, the client, to know anything about the specific

structure of the API, it's another black box, if you will.

A RESTful API provides whatever information

the client needs to interact with by means of an identifier.

In most cases, this is a URL.

A URL is a reference to a resource like

http://www.google.com, or

in our case here, the special URL that will give us a specific weather forecast.

RESTful programming isn'tt necessarily restricted to the web, but

most web-based APIs are built following the RESTful pattern.

In our case, our client, the Stormy app,

doesn't know anything at all about the Dark Sky API.

We simply request the data we need through an exposed URL

with certain keywords that specify what data we want back.

The API then returns this data to our client in a specified format, XML,

JSON, or some other data format.

In the case of Dark Sky, we see JSON data,

which is part of the contract the API establishes with the client.

With a RESTful API, that's the only real restriction.

The client and server need to agree on the data format being used.

JSON data is very common for web APIs to provide.

And that's what we'll use in this course.

Returning to the Dark Sky API documentation, if we look at this URL

under Forecast Request, we see how we need to construct the request to the API.

The URL has this base component of api.darksky.net/forecast,

and then needs specific identifiers, such as a private API key,

and the longitude and latitude coordinates, for our location.

But wait, a private API key?

What's that all about?

We'll need to register for a Dark Sky developer account so

we can get an API key and use this service.

We click on the Sign Up button up at the top of the page, we can register.

It's super simple, and all you need to provide is an email address and password.

I'll put the link in the Teacher's Notes, if you need it as well.

Once you're registered, you'll see a request to provide billing information.

We can ignore that because we'll utilize their free service tier.

The free tier allows 1,000 requests a day.

This will be plenty for our purposes.

If you decide to publish your app on the Play Store,

you'll probably want to upgrade to the paid tier.

Great, once you've register with Dark Sky,

join me in the next video where we'll start our project.