Callback — คืออะไร ฟังก์ชันเรียกกลับและทำงานอย่างไร

func fetchData(completion: @escaping (Result<Data, Error>) -> Void) {
    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        if let error = error {
            completion(.failure(error))
            return
        }
        completion(.success(data))
    }
    task.resume()
}

// การใช้กับ [weak self]
fetchData { [weak self] result in
    guard let self else { return }
    switch result {
    case .success(let data):
        self.updateUI(data)
    case .failure(let error):
        self.showError(error)
    }
}

interface Callback<T> {
    fun onSuccess(data: T)
    fun onError(error: Throwable)
}

class Repository {
    fun loadData(callback: Callback<List<User>>) {
        thread {
            try {
                val result = api.fetchUsers()
                runOnUiThread { callback.onSuccess(result) }
            } catch (e: Exception) {
                runOnUiThread { callback.onError(e) }
            }
        }
    }
}

// การใช้กับ lambda
repository.loadData(object : Callback<List<User>> {
    override fun onSuccess(data: List<User>) { showUsers(data) }
    override fun onError(error: Throwable) { showError(error.message) }
})

typealias NetworkResult = (Result<[String: Any], Error>) -> Void

func performRequest(
    url: URL,
    then handler: @escaping NetworkResult
) {
    let task = URLSession.shared.dataTask(with: url) { data, _, error in
        handler(Result {
            guard let json = try JSONSerialization.jsonObject(with: data)
            else { throw NetworkError.invalidData }
            return json as! [String: Any]
        })
    }
    task.resume()
}

performRequest(url: url) { result in
    switch result {
    case .success(let json): process(json)
    case .failure(let error): log(error.localizedDescription)
    }
}

fun <T, R> processWithCallback(
    input: T,
    transform: (T) -> R,
    onResult: (R) -> Unit
) {
    thread {
        val result = transform(input)
        runOnUiThread { onResult(result) }
    }
}

// ตัวอย่างกับ lambda
processWithCallback(
    input = "Hello",
    transform = { it.length },
    onResult = { length ->
        textView.text = "Length: $length"
    }
)

class DataController {
    var onDataUpdate: ((String) -> Void)?

    func setupCallback() {
        // Retain cycle!
        onDataUpdate = { text in
            self.process(text)
        }

        // แก้ไขด้วย [weak self]
        onDataUpdate = { [weak self] text in
            guard let self else { return }
            self.process(text)
        }
    }

    func process(_ input: String) { }
}

class SafeCallbackManager {
    private val listeners = mutableListOf<WeakReference<(String) -> Unit>>()

    fun addListener(callback: (String) -> Unit) {
        listeners.add(WeakReference(callback))
    }

    fun notifyAll(data: String) {
        val iterator = listeners.iterator()
        while (iterator.hasNext()) {
            val ref = iterator.next().get()
            if (ref != null) ref(data)
            else iterator.remove()
        }
    }
}

// การใช้ใน Fragment
manager.addListener { result ->
    // WeakReference ไม่เก็บ Fragment
    updateUI(result)
}

// callback ที่ซ้อนกัน — Callback Hell
loginUser(credentials) { user in
    fetchProfile(user.id) { profile in
        downloadAvatar(profile.avatarUrl) { image in
            cacheImage(image) { success in
                updateUI(user, profile, image)
            }
        }
    }
}

// async/await — วิธีแก้ไข
func loadUserExperience() async throws {
    let user = try await loginUser(credentials)
    let profile = try await fetchProfile(user.id)
    let image = try await downloadAvatar(profile.avatarUrl)
    try await cacheImage(image)
    updateUI(user, profile, image)
}

// callback ตามลำดับ — Callback Hell
api.login(credentials) { user ->
    api.fetchProfile(user.id) { profile ->
        api.download(profile.avatarUrl) { bytes ->
            file.save(bytes) { result ->
                textView.text = result.toString()
            }
        }
    }
}

// Coroutines — วิธีแก้ไข
suspend fun loadUserData() {
    val user = withContext(Dispatchers.IO) { api.login(credentials) }
    val profile = withContext(Dispatchers.IO) { api.fetchProfile(user.id) }
    val bytes = withContext(Dispatchers.IO) { api.download(profile.avatarUrl) }
    withContext(Dispatchers.IO) { file.save(bytes) }
    textView.text = "Done"
}