Add sticky headers to saved posts

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

app/src/main/java/dev/msfjarvis/lobsters/ui/posts/Header.kt

@@ -0,0 +1,40 @@
+package dev.msfjarvis.lobsters.ui.posts
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.wrapContentHeight
+import androidx.compose.material.MaterialTheme
+import androidx.compose.material.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.tooling.preview.Preview
+import androidx.compose.ui.unit.dp
+import dev.msfjarvis.lobsters.ui.theme.LobstersTheme
+import dev.msfjarvis.lobsters.ui.theme.titleColor
+import java.time.Month
+import java.util.Locale
+import java.time.format.TextStyle as JTextStyle
+
+@Composable
+fun MonthHeader(month: Month) {
+  Box(
+    Modifier
+      .fillMaxWidth()
+      .wrapContentHeight()
+  ) {
+    Text(
+      text = month.getDisplayName(JTextStyle.FULL, Locale.getDefault()),
+      style = MaterialTheme.typography.h4.copy(color = titleColor),
+      modifier = Modifier.padding(horizontal = 12.dp),
+    )
+  }
+}
+
+@Preview
+@Composable
+fun MonthHeaderPreview() {
+  LobstersTheme {
+    MonthHeader(month = Month.JULY)
+  }
+}

app/src/main/java/dev/msfjarvis/lobsters/ui/posts/SavedPosts.kt

@@ -1,5 +1,6 @@
 package dev.msfjarvis.lobsters.ui.posts
 
+import androidx.compose.foundation.ExperimentalFoundationApi
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
 import androidx.compose.foundation.lazy.rememberLazyListState
@@ -7,7 +8,9 @@ import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import dev.msfjarvis.lobsters.data.local.LobstersPost
 import dev.msfjarvis.lobsters.ui.urllauncher.LocalUrlLauncher
+import dev.msfjarvis.lobsters.util.asZonedDateTime
 
+@OptIn(ExperimentalFoundationApi::class)
 @Composable
 fun SavedPosts(
   posts: List<LobstersPost>,
@@ -22,8 +25,13 @@ fun SavedPosts(
   } else {
     LazyColumn(
       state = listState,
-      modifier = Modifier.then(modifier)
+      modifier = Modifier.then(modifier),
     ) {
+      val grouped = posts.groupBy { it.created_at.asZonedDateTime().month }
+      grouped.forEach { (month, posts) ->
+        stickyHeader {
+          MonthHeader(month = month)
+        }
         items(posts) { item ->
           LobstersItem(
             post = item,
@@ -35,4 +43,5 @@ fun SavedPosts(
         }
       }
     }
+  }
 }

app/src/main/java/dev/msfjarvis/lobsters/util/DateExtensions.kt

@@ -0,0 +1,16 @@
+package dev.msfjarvis.lobsters.util
+
+import java.text.SimpleDateFormat
+import java.time.ZoneId
+import java.time.ZonedDateTime
+import java.util.Locale
+
+/**
+ * Parses a given [String] into a [ZonedDateTime]. This method only works on dates in the format
+ * returned by the Lobsters API, and is not a general purpose parsing solution.
+ */
+fun String.asZonedDateTime(): ZonedDateTime {
+  val sdf = SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS", Locale.US)
+  val date = checkNotNull(sdf.parse(this))
+  return date.toInstant().atZone(ZoneId.systemDefault())
+}

app/src/test/java/dev/msfjarvis/lobsters/util/DateExtensionsTest.kt

@@ -0,0 +1,13 @@
+package dev.msfjarvis.lobsters.util
+
+import java.time.Month
+import org.junit.Assert.*
+import org.junit.Test
+
+class DateExtensionsTest {
+  @Test
+  fun `parses date correctly`() {
+    val datetime = "2021-02-15T21:16:02.000-06:00".asZonedDateTime()
+    assertEquals(Month.FEBRUARY, datetime.month)
+  }
+}