조건부 탐색 레시피

이 레시피에서는 조건이 충족되는 경우 (이 경우 사용자가 로그인한 경우)에만 특정 대상에 액세스할 수 있는 조건부 탐색을 구현하는 방법을 보여줍니다.

작동 방식

이 예시에는 사용자가 로그인해야 하는 Profile 대상이 있습니다. 사용자가 로그인하지 않고 Profile로 이동하려고 하면 Login 화면으로 리디렉션됩니다. 로그인에 성공하면 Profile 화면으로 자동 이동합니다.

AppBackStack

이 레시피의 핵심은 조건부 탐색을 위한 로직을 캡슐화하는 맞춤 AppBackStack 클래스입니다.

  • RequiresLogin 인터페이스: 마커 인터페이스 RequiresLogin은 사용자가 로그인해야 하는 대상을 식별하는 데 사용됩니다. Profile 대상은 이 인터페이스를 구현합니다.

  • 로그인으로 리디렉션: RequiresLogin을 구현하는 대상으로 add 함수가 호출되고 사용자가 로그인하지 않은 경우 AppBackStack은 의도한 대상을 저장하고 대신 Login 경로를 백 스택에 추가합니다.

  • 로그인 처리: login 함수가 호출되면 사용자의 상태가 로그인됨으로 설정됩니다. 사용자가 액세스하려고 했던 저장된 대상이 있으면 백 스택에 해당 대상을 추가하고 Login 화면을 삭제합니다.

  • 로그아웃 처리: logout 함수가 호출되면 사용자의 상태가 로그아웃으로 설정되고 사용자가 로그인해야 하는 대상이 백 스택에서 삭제됩니다.

이 접근 방식은 맞춤 백 스택 구현에서 로직을 중앙 집중화하여 조건부 탐색을 깔끔하게 처리할 수 있습니다.

/*
 * Copyright 2025 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.nav3recipes.conditional

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.setValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.saveable.rememberSerializable
import androidx.lifecycle.compose.dropUnlessResumed
import androidx.navigation3.runtime.NavBackStack
import androidx.navigation3.runtime.NavKey
import androidx.navigation3.runtime.entryProvider
import androidx.navigation3.runtime.serialization.NavBackStackSerializer
import androidx.navigation3.runtime.serialization.NavKeySerializer
import androidx.navigation3.ui.NavDisplay
import com.example.nav3recipes.content.ContentBlue
import com.example.nav3recipes.content.ContentGreen
import com.example.nav3recipes.content.ContentYellow
import kotlinx.serialization.Serializable


/**
 * Class for representing navigation keys in the app.
 *
 * Note: We use a sealed class because KotlinX Serialization handles
 * polymorphic serialization of sealed classes automatically.
 *
 * @param requiresLogin - true if the navigation key requires that the user is logged in
 * to navigate to it
 */
@Serializable
sealed class ConditionalNavKey(val requiresLogin: Boolean = false) : NavKey

/**
 * Key representing home screen
 */
@Serializable
private data object Home : ConditionalNavKey()

/**
 * Key representing profile screen that is only accessible once the user has logged in
 */
@Serializable
private data object Profile : ConditionalNavKey(requiresLogin = true)

/**
 * Key representing login screen
 *
 * @param redirectToKey - navigation key to redirect to after successful login
 */
@Serializable
private data class Login(
    val redirectToKey: ConditionalNavKey? = null
) : ConditionalNavKey()

class ConditionalActivity : ComponentActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {

            val backStack = rememberNavBackStack<ConditionalNavKey>(Home)
            var isLoggedIn by rememberSaveable {
                mutableStateOf(false)
            }
            val navigator = remember {
                Navigator(
                    backStack = backStack,
                    onNavigateToRestrictedKey = { redirectToKey -> Login(redirectToKey) },
                    isLoggedIn = { isLoggedIn }
                )
            }

            NavDisplay(
                backStack = backStack,
                onBack = { navigator.goBack() },
                entryProvider = entryProvider {
                    entry<Home> {
                        ContentGreen("Welcome to Nav3. Logged in? ${isLoggedIn}") {
                            Column {
                                Button(onClick = dropUnlessResumed { navigator.navigate(Profile) }) {
                                    Text("Profile")
                                }
                                Button(onClick = dropUnlessResumed { navigator.navigate(Login()) }) {
                                    Text("Login")
                                }
                            }
                        }
                    }
                    entry<Profile> {
                        ContentBlue("Profile screen (only accessible once logged in)") {
                            Button(onClick = dropUnlessResumed {
                                isLoggedIn = false
                                navigator.navigate(Home)
                            }) {
                                Text("Logout")
                            }
                        }
                    }
                    entry<Login> { key ->
                        ContentYellow("Login screen. Logged in? $isLoggedIn") {
                            Button(onClick = dropUnlessResumed {
                                isLoggedIn = true
                                key.redirectToKey?.let { targetKey ->
                                    backStack.remove(key)
                                    navigator.navigate(targetKey)
                                }
                            }) {
                                Text("Login")
                            }
                        }
                    }
                }
            )
        }
    }
}


// An overload of `rememberNavBackStack` that returns a subtype of `NavKey`.
// See https://issuetracker.google.com/issues/463382671 for a discussion of this function
@Composable
fun <T : NavKey> rememberNavBackStack(vararg elements: T): NavBackStack<T> {
    return rememberSerializable(
        serializer = NavBackStackSerializer(elementSerializer = NavKeySerializer())
    ) {
        NavBackStack(*elements)
    }
}
ConditionalActivity.kt
 
/*
 * Copyright 2025 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.example.nav3recipes.conditional

import androidx.navigation3.runtime.NavBackStack

/**
 * Provides navigation events with built-in support for conditional access. If the user attempts to
 * navigate to a [ConditionalNavKey] that requires login ([ConditionalNavKey.requiresLogin] is true)
 * but is not currently logged in, the Navigator will redirect the user to a login key.
 *
 * @property backStack The back stack that is modified by this class
 * @property onNavigateToRestrictedKey A lambda that is called when the user attempts to navigate
 * to a key that requires login. This should return the key that represents the login screen. The
 * user's target key is supplied as a parameter so that after successful login the user can be
 * redirected to their target destination.
 * @property isLoggedIn A lambda that returns whether the user is logged in.
 */
class Navigator(
    private val backStack: NavBackStack<ConditionalNavKey>,
    private val onNavigateToRestrictedKey: (targetKey: ConditionalNavKey?) -> ConditionalNavKey,
    private val isLoggedIn: () -> Boolean,
) {
    fun navigate(key: ConditionalNavKey) {
        if (key.requiresLogin && !isLoggedIn()) {
            val loginKey = onNavigateToRestrictedKey(key)
            backStack.add(loginKey)
        } else {
            backStack.add(key)
        }
    }

    fun goBack() = backStack.removeLastOrNull()
}
Navigator.kt