MMMAsyncLoadable

Use async/await with MMMLoadable.

(This is a part of MMMTemple suite of iOS libraries we use at MediaMonks.)

Installation

SPM:

.package(url: "https://github.com/mediamonks/MMMAsyncLoadable", .upToNextMajor(from: "0.1.0"))

Podfile:

source 'https://github.com/mediamonks/MMMSpecs.git'
source 'https://github.com/CocoaPods/Specs.git'
...
pod 'MMMLoadable'

Usage

❕This documentation is built with the assumption that you’re already aware of MMMLoadable. If you’re not, head over to the MMMLoadable page for more information.

AsyncLoadable

An async loadable makes it possible to fetch the content C using the async/await syntax. AsyncLoadable is a concrete implementation of AsyncLoadableProtocol, subclass AsyncLoadable to make your own loadable, so you can avoid working with generics “down the line”.

AsyncLoadable<C> requires that your MMMLoadable ‘contents’ is a single concrete type C. So if your loadable loads multiple values, either pass a tuple (recommended up to 2 values) or a wrapping struct.

It introduces 2 new methods, fetch() and fetchIfNeeded().

Simple example:

public final class UserLoadable: AsyncLoadable<UserData> {

    public override func doSync() {

        client.fetchUser { [weak self] result in

        guard let self = self else { return }

        switch result {
        case .success(let user):
            // This will set the state to `didSyncSuccessfully` and
        // populate the `content` property.
            self.setDidSyncSuccessfullyWithContent(user)
        case .failed(let error):
            // This will set the state to `didFailToSync` and
        // populate the `error` property.
            self.setFailedToSyncWithError(error)
        }
    }
    }
}

// At the call-site:
let loadable = UserLoadable()
let user = try await loadable.fetch() // This is your `content`, or it will throw the `error`.

Fetch

func fetch() async throws -> C

Fetch the content asynchronously, instead of adding a listener, this will throw upon setFailedWithError and return the content when setDidSyncSuccessfully.

Equivalent of MMMLoadable/sync().

Fetch if needed

func fetchIfNeeded() async throws -> C

Similar to fetch(), only when MMMPureLoadableProtocol/needsSync() is true.

Equivalent of MMMLoadable/syncIfNeeded().

Map

func map<T>(_ transform: @escaping (C) throws -> T) -> AsyncLoadable<T>

Map a AsyncLoadable<C> into AsyncLoadable<T> by supplying a closure that maps C into T. This is helpful if you want to quickly map a loadable from a “thin” to a “fat” model without creating unnecessary MMMLoadableProxys. E.g. AsyncLoadable<API.User> into AsyncLoadable<Models.User>.

If the original loadable is already synced / has contents available, we map it directly.

If there is an error thrown in the callback, we use that as the new AsyncLoadable/error and set it to failed.

Example

func fetchUser() -> AsyncLoadable<Models.User> {
    // apiClient.getUser() returns AsyncLoadable<API.User>
    apiClient.getUser().map { apiUser in
        return Models.User(apiModel: apiUser)
    }
}

AsyncMap

func asyncMap<T>(_ transform: @escaping (C) async throws -> T) -> AsyncLoadable<T>

Map a AsyncLoadable<C> into AsyncLoadable<T> by supplying a async closure that maps C into T. This is similar to map but allows to take a async closure, downside of this is that it won’t directly map the loadable if content is available, so you’ll have to make sure to sync it again.

If there is an error thrown in the callback, we use that as the new AsyncLoadable/error and set it to failed.

Example

func fetchUser() -> AsyncLoadable<Models.User> {
    // apiClient.getUser() returns AsyncLoadable<API.User>
    apiClient.getUser().asyncMap { apiUser in
        try await Models.FetchUser(apiModel: apiUser)
    }
}

FlatMap

func flatMap<T>(_ transform: @escaping (C) async throws -> AsyncLoadable<T>) -> AsyncLoadable<T>

FlatMap a AsyncLoadable<C> into AsyncLoadable<T> by supplying a closure that maps C into AsyncLoadable<T>. This is helpful if you want to chain loadables without having to observe each one.

For example say you have LoadableA that upon success will load LoadableB using a value in it’s contents, LoadableB will be exposed to the users, since that only contains valuable info for them. If LoadableA fails, we don’t have to try to load LoadableB.

If there is an error thrown in the callback, we use that as the new AsyncLoadable/error and set it to failed.

Please note that unlike the map function, that doesn’t take an async closure, if the content is available, we don’t map it directly, you will have to sync the loadable again. The original loadable won’t ever sync again if content is available, you will have to call sync manually to do that.

Example

func fetchLoadableB() -> AsyncLoadable<BValue> {
    loadableA().flatMap { aVal in
        return LoadableB(identifier: aVal.identifier)
    }
}

Joined

func joined<T>(_ transform: @escaping (C) async throws -> AsyncLoadable<T>) -> AsyncLoadable<(C, T)>

Join two AsyncLoadables together, from AsyncLoadable<C> and AsyncLoadable<T> to a AsyncLoadable<(C, T)>. This could come in useful when you want to grab data from C to construct your loadable T without losing C.

This behaves the same as flatMap(_:).

Example

func fetchLoadableB() -> AsyncLoadable<(AValue, BValue)> {
    loadableA().joined { aVal in
        return LoadableB(identifier: aVal.identifier)
    }
}

AsyncLoadableObserver

MMMLoadableObserver that supports an asynchronous closure as it’s callback. Or use the loadable.sink { loadable in ... } shorthand and pass an async closure.

AsyncLoadableStream

Listen to an AsyncLoadable by using an AsyncStream. This allows you to iterate over the iterator, this will stream a new AsyncLoadableStreamObject on every change in the loadable.

For example:

class MyView: UIView {

    private let loadable: AsyncLoadable<MyData>

    // It's crucial that we call `finish()` somehow, this is also called upon
    // deinit, so storing it as a property is an easy way to accomplish this.
    private var stream: AsyncLoadableStream<MyData>?

    public init(loadable: AsyncLoadable<MyData>) {

        let stream = AsyncLoadableStream(loadable)

        self.loadable = loadable
        self.stream = stream

        for await obj in stream.iterator {
            // Do something with the stream object, e.g. update UI.
            updateUI()
        }
    }

    private func updateUI() {
        loader.isHidden = loadable.loadableState != .syncing
    }
}

Please note that due to the nature of async/await in swift it’s crucial to store the stream as a local (private) property to ensure that finish() get’s called upon deinit. This stops the stream. Otherwise your Actor will get blocked indefinitely, since it will keep on waiting for new values, causing a memory leak.

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