08 - Environment.md

08 - Environment

If you are familiar with SwiftUI's environment, you'll feel right at home with Meridian's.

There are two ways to store values into Meridian's environment.

Environment Objects

First, you can store objects (reference types) by their type.

For example, if you have a database:

final class Database {
    // interesting data in here
}

You can add this to your Server.

Server(errorRenderer: BasicErrorRenderer())
    .routes({
        LogInRoute()
            .on(.get("/login"))
    })
    .environmentObject(Database())
    .listen()

You can chain as many .environmentObject modifiers as you need. Objects using this approach are keyed off of their type, so there can only ever be one of each type in the environment.

To get access to this database, you can use the @EnvironmentObject property wrapper.

@EnvironmentObject var database: Database

@EnvironmentObject uses the type of the variable to find the instance of the object.

Dependent Environment Objects

Some environment objects are dependent on other environment objects. To construct one of those objects, you can use a special version of the .environmentObject modifier. For example, if your Database is dependent on the server's loopGroup, you can initialize that like so:

.environmentObject(with: { env in
    Database(loopGroup: env.loopGroup)
})

Once your database is initialized, you can use it from your responders in the same way:

@EnvironmentObject var database: Database

Keyed Environment Values

If the object you'd like to store in the environment is a value type, or you need multiple instances for a given type (like DateFormatters, for instance), you can set up keys. This works very similarly to SwiftUI.

struct ShortDateFormatterKey: EnvironmentKey {
    static var defaultValue: DateFormatter = {
        let f = DateFormatter()
        f.timeStyle = .none
        f.dateStyle = .short
        return f
    }()
}

Using an immediately-executed closure ensures that the defaultValue is only ever instantiated once.

extension EnvironmentValues {
    var shortDateFormatter: DateFormatter {
        get {
            self[ShortDateFormatterKey.self]
        }
        set {
            self[ShortDateFormatterKey.self] = newValue
        }
    }
}

This can be used with the @Environment property wrapper:

@Environment(\.shortDateFormatter) var shortDateFormatter