Memory limitations and responding to warnings with combined cache?

[dreymonde]

Memory limitations and responding to warnings with combined cache?

[dreymonde]

Hi @serendipityapps! Thanks for reaching out.
You’re right, out-of-the-box MemoryStorage does not react to any system warnings. Shallows tries to be a very unopinionated about the logic of your app, so it doesn’t do anything behind your back. Instead, Shallows was designed to be able to achieve any kind of behavior easily.

First: technically, you can manually clear a MemoryStorage if you have a direct reference to it. There’s a mutable storage property which you can use:

let memoryCache = MemoryStorage<Filename, Data>()
let combined = memoryCache.combined(with: diskStorage) // Storage<Filename, Data>
// ...
memoryCache.storage = [:]

Second: here’s how I would make a «memory warnings aware» storage in Shallows:

public final class MemoryWarningsAwareMemoryStorage<Key : Hashable, Value> : StorageProtocol {
    
    public let memoryStorage: MemoryStorage<Key, Value>
    
    public init(memoryStorage: MemoryStorage<Key, Value>) {
        self.memoryStorage = memoryStorage
        self.startListening()
    }
    
    private func startListening() {
        NotificationCenter.default.addObserver(forName: .UIApplicationDidReceiveMemoryWarning,
                                               object: self,
                                               queue: nil) { (_) in
            self.memoryStorage.storage = [:]
        }
    }
    
    deinit {
        NotificationCenter.default.removeObserver(self)
    }
    
    public func retrieve(forKey key: Key, completion: @escaping (Result<Value>) -> ()) {
        memoryStorage.retrieve(forKey: key, completion: completion)
    }
    
    public func set(_ value: Value, forKey key: Key, completion: @escaping (Result<Void>) -> ()) {
        memoryStorage.set(value, forKey: key, completion: completion)
    }
    
}

extension MemoryStorage {
    
    public func lowMemoryAware() -> Storage<Key, Value> {
        return MemoryWarningsAwareMemoryStorage(memoryStorage: self).asStorage()
    }
    
}

And then:

let memoryStorage = MemoryStorage<Filename, Data>().lowMemoryAware()
let combined = memoryStorage.combined(with: diskStorage)

Hope this helps. Let me know! 🙂

[serendipityapps]

Hey Oleg,

Thanks for quick reply and great to see your method of clearing storage was so simple 😂.

The solution provided for memory aware memory storage also looks very useful. So much so, I wonder if this or a variant should be included in source anyway. I would think that this behaviour would probably be the default behaviour (expected?) and maybe include a flag to NOT use auto clearance if required by client?

[dreymonde]

This is a very valid point. Asking for such widely-needed feature to be included by default is, of course, reasonable. But there was a design decision when making Shallows to keep it very small and to the point. I don’t want to make assumptions about how developers will structure their logic. For example, if someone does not have a disk-backed solution (and the MemoryStorage is a single source of truth), auto clearance can be an almost impossible-to-detect bug.

[serendipityapps]

Fair enough, thanks again. 👍

[dreymonde]

You’re welcome! Ping me anytime if you'll have any questions