app: setup pagination and a viewmodel for posts

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/java/dev/msfjarvis/lobsters/MainActivity.kt

@@ -1,47 +1,42 @@
 package dev.msfjarvis.lobsters
 
 import android.os.Bundle
+import androidx.activity.viewModels
 import androidx.appcompat.app.AppCompatActivity
 import androidx.compose.foundation.Text
-import androidx.compose.foundation.lazy.LazyColumnFor
+import androidx.compose.foundation.lazy.LazyColumnForIndexed
 import androidx.compose.material.Scaffold
 import androidx.compose.material.TopAppBar
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.Providers
 import androidx.compose.runtime.ambientOf
-import androidx.compose.runtime.mutableStateListOf
+import androidx.compose.runtime.collectAsState
 import androidx.compose.ui.platform.setContent
 import androidx.compose.ui.res.stringResource
-import androidx.lifecycle.lifecycleScope
 import dagger.hilt.android.AndroidEntryPoint
 import dev.msfjarvis.lobsters.api.LobstersApi
-import dev.msfjarvis.lobsters.model.LobstersPost
+import dev.msfjarvis.lobsters.data.LobstersViewModel
 import dev.msfjarvis.lobsters.ui.LobstersItem
 import dev.msfjarvis.lobsters.ui.LobstersTheme
 import dev.msfjarvis.lobsters.urllauncher.UrlLauncher
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.withContext
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import javax.inject.Inject
 
 val UrlLauncherAmbient = ambientOf<UrlLauncher> { error("Needs to be provided") }
 
 @AndroidEntryPoint
+@ExperimentalCoroutinesApi
 class MainActivity : AppCompatActivity() {
   @Inject lateinit var urlLauncher: UrlLauncher
   @Inject lateinit var apiClient: LobstersApi
+  private val viewModel: LobstersViewModel by viewModels()
 
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
     setContent {
       Providers(UrlLauncherAmbient provides urlLauncher) {
         LobstersTheme {
-          val posts = mutableStateListOf<LobstersPost>()
-          lifecycleScope.launchWhenCreated {
-            withContext(Dispatchers.IO) {
-              posts.addAll(apiClient.getHottestPosts(1))
-            }
-          }
-          LobstersApp(posts)
+          LobstersApp(viewModel)
         }
       }
     }
@@ -49,15 +44,21 @@ class MainActivity : AppCompatActivity() {
 }
 
 @Composable
+@ExperimentalCoroutinesApi
 fun LobstersApp(
-  items: List<LobstersPost>,
+  viewModel: LobstersViewModel
 ) {
   val urlLauncher = UrlLauncherAmbient.current
+  val state = viewModel.posts.collectAsState()
+  val lastIndex = state.value.lastIndex
 
   Scaffold(
     topBar = { TopAppBar({ Text(text = stringResource(R.string.app_name)) }) },
     bodyContent = {
-      LazyColumnFor(items) { item ->
+      LazyColumnForIndexed(state.value) { index, item ->
+        if (lastIndex == index) {
+          viewModel.getMorePosts()
+        }
         LobstersItem(item) { post ->
           urlLauncher.launch(post.url)
         }

app/src/main/java/dev/msfjarvis/lobsters/data/LobstersViewModel.kt

@@ -0,0 +1,32 @@
+package dev.msfjarvis.lobsters.data
+
+import androidx.hilt.lifecycle.ViewModelInject
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import dev.msfjarvis.lobsters.api.LobstersApi
+import dev.msfjarvis.lobsters.model.LobstersPost
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.launch
+
+
+@ExperimentalCoroutinesApi
+class LobstersViewModel @ViewModelInject constructor(
+  private val lobstersApi: LobstersApi,
+) : ViewModel() {
+  private var apiPage = 1
+  private val _posts = MutableStateFlow<List<LobstersPost>>(emptyList())
+  val posts: StateFlow<List<LobstersPost>> get() = _posts
+
+  init {
+    getMorePosts()
+  }
+
+  fun getMorePosts() {
+    viewModelScope.launch {
+      _posts.value += lobstersApi.getHottestPosts(apiPage)
+      apiPage += 1
+    }
+  }
+}