---
id: ios
title: Building an iOS Plugin
---

<img align="right" src="/docs/assets/ios-tutorial-app.png" alt="iOS Tutorial App" width="200">

For the purpose of the tutorial, we will assume you already have an existing iOS application in
which you have a feed or list of items. As the Flipper team, we obviously concern ourselves mostly
with sea mammals, so this is what our app displays. The actual display logic is not what's interesting
here, but how we can make this data available in our Flipper desktop app.

You can find the source code of the project [on GitHub](https://github.com/facebook/flipper/tree/c55bebd1be545c63dde93e143dd5c341dc2fd20b/iOS/Tutorial/Tutorial).

## Creating a Plugin

On iOS, a Flipper plugin is a class that implements the
[`FlipperPlugin`](https://github.com/facebook/flipper/blob/master/iOS/FlipperKit/FlipperPlugin.h)
interface.

The interface is rather small and only comprises four methods:

- `(NSString *)identifier`: Specify a unique string so the JavaScript side knows where to talk to. This must match the name attribute in the `package.json` we will look into later in this tutorial.
- `(void)didConnect:(id<FlipperConnection>)connection`: This method is called when the desktop client connects and is ready to receive or send data.
- `(void)didDisconnect`: We're sure you can figure this one out.
- `(BOOL)runInBackground`: Unless this is true, only the currently selected plugin in the Flipper UI can communicate with the device. Its an optional method which you can override. Default value used is `false`.

Let's implement these methods for our sealife app:

```swift
import Foundation
import FlipperKit

class SeaMammalsPlugin: NSObject, FlipperPlugin {
  var connection: FlipperConnection? = nil
  let mammals: [MarineMammal]

  init(_ marineMammals: [MarineMammal]) {
      mammals = marineMammals
  }

  func identifier() -> String! {
    return "sea-mammals"
  }

  func didConnect(_ connection: FlipperConnection!) {
    self.connection = connection
    for (index, mammal) in mammals.enumerated() {
      connection.send("newRow", withParams: ["id": index, "title": mammal.name, "url": mammal.image.absoluteString])
    }
  }

  func didDisconnect() {
    connection = nil;
  }
}
```

The two interesting bits here are `didConnect` and `connection.send`. `connection.send` sends a message
to the desktop app and is identified with the name "newRow".

For our sample app, we're dealing with a static data source. However, in real
life, you will likely dynamically receive new data as the user interacts with
the app. So while we just send all the data we have at once in `didConnect`,
you would normally set up a listener here to instead call `connection.send("newRow", params)` as new data
arrives. `params` are nothing but a dictionary which contains the data which you want to send over the wire to the desktop app.

## Registering your Plugin

Now all you need to do is let Flipper know about your new plugin. You do this
by calling `add` on your `FlipperClient`, which is normally created
at application startup.

```swift
let client = FlipperClient.shared()
// Add all sorts of other amazing plugins here ...
client?.add(SeaMammalsPlugin(MarineMammal.defaultList))
client?.start()
```

## What next

When starting your application now, Flipper will tell the desktop application
about the plugin it supports, including "sea-mammals" and will look for a
corresponding JavaScript plugin by that name. Before building it's JavaScript counterpart, first lets build a Flipper Plugin in Android.