Add PageCurl override with page keys

demo/src/main/kotlin/eu/wewox/pagecurl/screens/PagingPageCurlScreen.kt

@@ -3,7 +3,9 @@
 package eu.wewox.pagecurl.screens
 
 import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material.CircularProgressIndicator
 import androidx.compose.material.MaterialTheme
@@ -12,7 +14,7 @@ import androidx.compose.runtime.Composable
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
-import androidx.paging.LoadState
+import androidx.paging.LoadState.Loading
 import androidx.paging.Pager
 import androidx.paging.PagingConfig
 import androidx.paging.PagingData
@@ -21,6 +23,7 @@ import androidx.paging.PagingState
 import androidx.paging.compose.collectAsLazyPagingItems
 import eu.wewox.pagecurl.ExperimentalPageCurlApi
 import eu.wewox.pagecurl.page.PageCurl
+import eu.wewox.pagecurl.page.rememberPageCurlState
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 
@@ -30,67 +33,86 @@ import kotlinx.coroutines.flow.Flow
  */
 @Composable
 fun PagingPageCurlScreen() {
-    val items = rememberPager().collectAsLazyPagingItems()
+    val startPage = 4 // in range 0..9
+    val startPageOffset = 5 // in range 0..9
 
-    val extraItem = items.loadState.append is LoadState.Loading || items.loadState.refresh is LoadState.Loading
+    val items = rememberPager(startPage).collectAsLazyPagingItems()
-    val count = items.itemCount + (if (extraItem) 1 else 0)
+
+    val loading = with(items.loadState) { refresh is Loading || append is Loading || prepend is Loading }
 
     Box(Modifier.fillMaxSize()) {
-        PageCurl(count = count) { index ->
+        if (items.loadState.refresh is Loading) {
             Box(
                 contentAlignment = Alignment.Center,
                 modifier = Modifier
                     .fillMaxSize()
                     .background(MaterialTheme.colors.background)
             ) {
-                if (index < items.itemCount) {
-                    Text(items[index]?.content.orEmpty())
-                } else {
                 CircularProgressIndicator()
             }
+        } else {
+            PageCurl(
+                count = items.itemCount,
+                key = { index -> items.peek(index).hashCode() },
+                state = rememberPageCurlState(initialCurrent = startPageOffset),
+            ) { index ->
+                Column(
+                    verticalArrangement = Arrangement.Center,
+                    horizontalAlignment = Alignment.CenterHorizontally,
+                    modifier = Modifier
+                        .fillMaxSize()
+                        .background(MaterialTheme.colors.background)
+                ) {
+                    Text(items[index]?.content.orEmpty())
+                    Text(if (loading) "Loading..." else "")
+                }
             }
         }
     }
 }
 
 @Composable
-private fun rememberPager(): Flow<PagingData<Item>> =
+private fun rememberPager(startPage: Int): Flow<PagingData<Item>> =
     remember {
         Pager(
             config = PagingConfig(
                 pageSize = 10,
                 prefetchDistance = 3,
             ),
-            pagingSourceFactory = { ItemPagingSource(BackendService()) }
+            pagingSourceFactory = { ItemPagingSource(startPage, BackendService()) }
         ).flow
     }
 
 private class Item(val content: String)
 
-private class Response(val items: List<Item>, val nextPageNumber: Int)
+private class Response(val items: List<Item>, val prePageNumber: Int, val nextPageNumber: Int)
 
 private class BackendService {
 
     suspend fun searchItems(page: Int): Response {
-        delay(5_000L)
+        delay(1_000L)
         return Response(
             items = List(10) { Item("Content #${page * 10 + it}") },
-            nextPageNumber = page + 1
+            prePageNumber = page - 1,
+            nextPageNumber = page + 1,
         )
     }
 }
 
-private class ItemPagingSource(private val backend: BackendService) : PagingSource<Int, Item>() {
+private class ItemPagingSource(
+    private val startPage: Int,
+    private val backend: BackendService
+) : PagingSource<Int, Item>() {
 
     @Suppress("TooGenericExceptionCaught") // It is only for demo purpose
     override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Item> =
         try {
-            val nextPageNumber = params.key ?: 0
+            val nextPageNumber = params.key ?: startPage
             val response = backend.searchItems(nextPageNumber)
             LoadResult.Page(
                 data = response.items,
-                prevKey = null, // Only paging forward.
+                prevKey = response.prePageNumber.takeIf { it >= 0 },
-                nextKey = response.nextPageNumber
+                nextKey = response.nextPageNumber.takeIf { it < 10 }
             )
         } catch (e: Exception) {
             LoadResult.Error(e)

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/PageCurl.kt

@@ -4,8 +4,11 @@ import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.BoxWithConstraints
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.rememberUpdatedState
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import eu.wewox.pagecurl.ExperimentalPageCurlApi
 
@@ -85,6 +88,44 @@ public fun PageCurl(
     }
 }
 
+/**
+ * Shows the pages which may be turned by drag or tap gestures.
+ *
+ * @param count The count of pages.
+ * @param key The lambda to provide stable key for each item. Useful when adding and removing items before current page.
+ * @param state The state of the PageCurl. Use this to programmatically change the current page or observe changes.
+ * @param modifier The modifier for this composable.
+ * @param content The content lambda to provide the page composable. Receives the page number.
+ */
+@ExperimentalPageCurlApi
+@Composable
+public fun PageCurl(
+    count: Int,
+    key: (Int) -> Any,
+    modifier: Modifier = Modifier,
+    state: PageCurlState = rememberPageCurlState(),
+    content: @Composable (Int) -> Unit
+) {
+    var lastKey by remember(state.current) { mutableStateOf(if (count > 0) key(state.current) else null) }
+
+    remember(count) {
+        val newKey = if (count > 0) key(state.current) else null
+        if (newKey != lastKey) {
+            val index = List(count, key).indexOf(lastKey).coerceIn(0, count - 1)
+            lastKey = newKey
+            state.current = index
+        }
+        count
+    }
+
+    PageCurl(
+        count = count,
+        state = state,
+        content = content,
+        modifier = modifier,
+    )
+}
+
 /**
  * Shows the pages which may be turned by drag or tap gestures.
  *

pagecurl/src/main/kotlin/eu/wewox/pagecurl/page/PageCurlState.kt

@@ -125,7 +125,7 @@ public class PageCurlState(
     internal fun setup(count: Int, constraints: Constraints) {
         max = count
         if (current >= count) {
-            current = count - 1
+            current = (count - 1).coerceAtLeast(0)
         }
 
         if (internalState?.constraints == constraints) {