# Plugins

A plugin is a function that returns an object - more specifically, the object may contain functions and components that augment and modify Swagger-UI's functionality.

### Format

A plugin return value may contain any of these keys, where `myStateKey` is a name for a piece of state:

```javascript
{
  statePlugins: {
    myStateKey: {
      actions,
      reducers,
      selectors,
      wrapActions,
      wrapSelectors
    }
  },
  components: {},
  wrapComponents: {},
  fn: {}
}
```

### System is provided to plugins

Let's assume we have a plugin, `NormalPlugin`, that exposes a `doStuff` action under the `normal` state namespace.

```javascript
const ExtendingPlugin = function(system) {
  return {
    statePlugins: {
      extending: {
        actions: {
          doExtendedThings: function(...args) {
            // you can do other things in here if you want
            return system.normalActions.doStuff(...args)
          }
        }
      }
    }
  }
}
```

As you can see, each plugin is passed a reference to the `system` being built up. As long as `NormalPlugin` is compiled before `ExtendingPlugin`, this will work without any issues.

There is no dependency management built into the plugin system, so if you create a plugin that relies on another, it is your responsibility to make sure that the dependent plugin is loaded _after_ the plugin being depended on.

### Interfaces

##### Actions

```javascript
const MyActionPlugin = () => {
  return {
    statePlugins: {
      example: {
        actions: {
          updateFavoriteColor: (str) => {
            return {
              type: "EXAMPLE_SET_FAV_COLOR",
              payload: str
            }
          }
        }
      }
    }
  }
}
```

Once an action has been defined, you can use it anywhere that you can get a system reference:

```javascript
// elsewhere
system.exampleActions.updateFavoriteColor("blue")
```

The Action interface enables the creation of new Redux action creators within a piece of state in the Swagger-UI system.

This action creator function will be exposed to container components as `exampleActions.updateFavoriteColor`. When this action creator is called, the return value (which should be a [Flux Standard Action](https://github.com/acdlite/flux-standard-action)) will be passed to the `example` reducer, which we'll define in the next section.

For more information about the concept of actions in Redux, see the [Redux Actions documentation](http://redux.js.org/docs/basics/Actions.html).

##### Reducers

Reducers take a state (which is an [Immutable.js map](https://facebook.github.io/immutable-js/docs/#/Map)) and an action, and return a new state.

Reducers must be provided to the system under the name of the action type that they handle, in this case, `EXAMPLE_SET_FAV_COLOR`.

```javascript
const MyReducerPlugin = function(system) {
  return {
    statePlugins: {
      example: {
        reducers: {
          "EXAMPLE_SET_FAV_COLOR": (state, action) => {
            // you're only working with the state under the namespace, in this case "example".
            // So you can do what you want, without worrying about /other/ namespaces
            return state.set("favColor", action.payload)
          }
        }
      }
    }
  }
}
```

##### Selectors

Selectors reach into

They're an easy way to keep logic for getting data out of state in one place, and is preferred over passing state data directly into components.


```javascript
const MySelectorPlugin = function(system) {
  return {
    statePlugins: {
      example: {
        selectors: {
          myFavoriteColor: (state) => state.get("favColor")
        }
      }
    }
  }
}
```

You can also use the Reselect library to memoize your selectors, which is recommended for any selectors that will see heavy use, since Reselect automatically memoizes selector calls for you:

```javascript
import { createSelector } from "reselect"

const MySelectorPlugin = function(system) {
  return {
    statePlugins: {
      example: {
        selectors: {
          // this selector will be memoized after it is run once for a
          // value of `state`
          myFavoriteColor: createSelector((state) => state.get("favColor"))
        }
      }
    }
  }
}
```

Once a selector has been defined, you can use it anywhere that you can get a system reference:
```javascript
system.exampleSelectors.myFavoriteColor() // gets `favColor` in state for you
```

##### Components

You can provide a map of components to be integrated into the system.

Be mindful of the key names for the components you provide, as you'll need to use those names to refer to the components elsewhere.

```javascript
class HelloWorldClass extends React.Component {
  render() {
    return <h1>Hello World!</h1>
  }
}

const MyComponentPlugin = function(system) {
  return {
    components: {
      HelloWorldClass: HelloWorldClass
      // components can just be functions, these are called "stateless components"
      HelloWorldStateless: () => <h1>Hello World!</h1>,
    }
  }
}
```

```javascript
// elsewhere
const HelloWorldStateless = system.getComponent("HelloWorldStateless")
const HelloWorldClass = system.getComponent("HelloWorldClass")
```

You can also "cancel out" any components that you don't want by creating a stateless component that always returns `null`:

```javascript
const NeverShowInfoPlugin = function(system) {
  return {
    components: {
      info: () => null
    }
  }
}
```

##### Wrap-Actions

Wrap Actions allow you to override the behavior of an action in the system.

They are function factories with the signature `(oriAction, system) => (...args) => result`.

A Wrap Action's first argument is `oriAction`, which is the action being wrapped. It is your responsibility to call the `oriAction` - if you don't, the original action will not fire!

This mechanism is useful for conditionally overriding built-in behaviors, or listening to actions.

```javascript
// FYI: in an actual Swagger-UI, `updateSpec` is already defined in the core code
// it's just here for clarity on what's behind the scenes
const MySpecPlugin = function(system) {
  return {
    statePlugins: {
      spec: {
        actions: {
          updateSpec: (str) => {
            return {
              type: "SPEC_UPDATE_SPEC",
              payload: str
            }
          }
        }
      }
    }
  }
}

// this plugin allows you to watch changes to the spec that is in memory
const MyWrapActionPlugin = function(system) {
  return {
    statePlugins: {
      spec: {
        wrapActions: {
          updateSpec: (oriAction, system) => (str) => {
            // here, you can hand the value to some function that exists outside of Swagger-UI
            console.log("Here is my API definition", str)
            return oriAction(str) // don't forget! otherwise, Swagger-UI won't update
          }
        }
      }
    }
  }
}
```

##### Wrap-Selectors

Wrap Selectors allow you to override the behavior of a selector in the system.

They are function factories with the signature `(oriSelector, system) => (state, ...args) => result`.

This interface is useful for controlling what data flows into components. We use this in the core code to disable selectors based on the API definition's version.

```javascript
import { createSelector } from 'reselect'

// FYI: in an actual Swagger-UI, the `url` spec selector is already defined
// it's just here for clarity on what's behind the scenes
const MySpecPlugin = function(system) {
  return {
    statePlugins: {
      spec: {
        selectors: {
          url: createSelector(
            state => state.get("url")
          )
        }
      }
    }
  }
}

const MyWrapSelectorsPlugin = function(system) {
  return {
    statePlugins: {
      spec: {
        wrapSelectors: {
          url: (oriSelector, system) => (state, ...args) => {
            console.log('someone asked for the spec url!!! it is', state.get('url'))
            // you can return other values here...
            // but let's just enable the default behavior
            return oriSelector(state, ...args)
          }
        }
      }
    }
  }
}
```

##### Wrap-Components

Wrap Components allow you to override a component registered within the system.

Wrap Components are function factories with the signature `(OriginalComponent, system) => props => ReactElement`.

```javascript
const MyWrapBuiltinComponentPlugin = function(system) {
  return {
    wrapComponents: {
      info: (Original, system) => (props) => {
        return <div>
          <h3>Hello world! I am above the Info component.</h3>
          <Original {...props} />
        </div>
      }
    }
  }
}
```

```javascript
// Overriding a component from a plugin

const MyNumberDisplayPlugin = function(system) {
  return {
    components: {
      NumberDisplay: ({ number }) => <span>{number}</span>
    }
  }
}

const MyWrapComponentPlugin = function(system) {
  return {
    wrapComponents: {
      NumberDisplay: (Original, system) => (props) => {
        if(props.number > 10) {
          return <div>
            <h3>Warning! Big number ahead.</h3>
          </div>
        } else {
          return <Original {...props} />
        }
      }
    }
  }
}
```

##### fn

The fn interface allows you to add helper functions to the system for use elsewhere.

```javascript
import leftPad from "left-pad"

const MyFnPlugin = function(system) {
  return {
    fn: {
      leftPad: leftPad
    }
  }
}
```