session-ios/Session/Utilities/BackgroundPoller.swift

import PromiseKit
import SessionSnodeKit

@objc(LKBackgroundPoller)
public final class BackgroundPoller : NSObject {
    private static var closedGroupPoller: ClosedGroupPoller!
    private static var promises: [Promise<Void>] = []

    private override init() { }

    @objc(pollWithCompletionHandler:)
    public static func poll(completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        promises = []
        promises.append(pollForMessages())
        promises.append(contentsOf: pollForClosedGroupMessages())
        let openGroups: [String:OpenGroup] = Storage.shared.getAllUserOpenGroups()
        openGroups.values.forEach { openGroup in
            let poller = OpenGroupPoller(for: openGroup)
            poller.stop()
            promises.append(poller.pollForNewMessages(isBackgroundPoll: true))
        }
        when(resolved: promises).done { _ in
            completionHandler(.newData)
        }.catch { _ in
            completionHandler(.failed)
        }
    }
    
    private static func pollForMessages() -> Promise<Void> {
        let userPublicKey = getUserHexEncodedPublicKey()
        return getMessages(for: userPublicKey)
    }
    
    private static func pollForClosedGroupMessages() -> [Promise<Void>] {
        let publicKeys = Storage.shared.getUserClosedGroupPublicKeys()
        return publicKeys.map { getMessages(for: $0) }
    }
    
    private static func getMessages(for publicKey: String) -> Promise<Void> {
        return SnodeAPI.getSwarm(for: publicKey).then2 { swarm -> Promise<Void> in
            guard let snode = swarm.randomElement() else { throw SnodeAPI.Error.generic }
            return SnodeAPI.getRawMessages(from: snode, associatedWith: publicKey).then(on: DispatchQueue.main) { rawResponse -> Promise<Void> in
                let messages = SnodeAPI.parseRawMessagesResponse(rawResponse, from: snode, associatedWith: publicKey)
                let promises = messages.compactMap { json -> Promise<Void>? in
                    // Use a best attempt approach here; we don't want to fail the entire process if one of the
                    // messages failed to parse.
                    guard let envelope = SNProtoEnvelope.from(json),
                        let data = try? envelope.serializedData() else { return nil }
                    let job = MessageReceiveJob(data: data, isBackgroundPoll: true)
                    return job.execute()
                }
                return when(fulfilled: promises) // The promise returned by MessageReceiveJob never rejects
            }
        }
    }
}