Use IDBCursor methods in the beginning of reading a cursor (#73)

core/src/jsMain/kotlin/CursorStart.kt

@@ -0,0 +1,26 @@
+package com.juul.indexeddb
+
+import com.juul.indexeddb.external.IDBCursor
+
+public sealed class CursorStart {
+
+    internal abstract fun apply(cursor: IDBCursor)
+
+    public data class Advance(val count: Int) : CursorStart() {
+        override fun apply(cursor: IDBCursor) {
+            cursor.advance(count)
+        }
+    }
+
+    public data class Continue(val key: Key) : CursorStart() {
+        override fun apply(cursor: IDBCursor) {
+            cursor.`continue`(key.toJs())
+        }
+    }
+
+    public data class ContinuePrimaryKey(val key: Key, val primaryKey: Key) : CursorStart() {
+        override fun apply(cursor: IDBCursor) {
+            cursor.continuePrimaryKey(key.toJs(), primaryKey.toJs())
+        }
+    }
+}

core/src/jsMain/kotlin/Transaction.kt

@@ -49,19 +49,23 @@ public open class Transaction internal constructor(
     public suspend fun Queryable.openCursor(
         query: Key? = null,
         direction: Cursor.Direction = Cursor.Direction.Next,
+        cursorStart: CursorStart? = null,
     ): Flow<CursorWithValue> = openCursorImpl(
         query,
         direction,
+        cursorStart,
         open = this::requestOpenCursor,
         wrap = ::CursorWithValue,
     )
 
     public suspend fun Queryable.openKeyCursor(
         query: Key? = null,
         direction: Cursor.Direction = Cursor.Direction.Next,
+        cursorStart: CursorStart? = null,
     ): Flow<Cursor> = openCursorImpl(
         query,
         direction,
+        cursorStart,
         open = this::requestOpenKeyCursor,
         wrap = ::Cursor,
     )
@@ -70,14 +74,19 @@ public open class Transaction internal constructor(
     private suspend fun <T : Cursor, U : IDBCursor> openCursorImpl(
         query: Key?,
         direction: Cursor.Direction,
+        cursorStart: CursorStart?,
         open: (Key?, Cursor.Direction) -> Request<U?>,
         wrap: (U) -> T,
     ): Flow<T> = callbackFlow {
+        var cursorStartAction = cursorStart
         val request = open(query, direction).request
         val onSuccess: (Event) -> Unit = { event ->
             @Suppress("UNCHECKED_CAST")
             val cursor = (event.target as IDBRequest<U?>).result
-            if (cursor != null) {
+            if (cursorStartAction != null && cursor != null) {
+                cursorStartAction?.apply(cursor)
+                cursorStartAction = null
+            } else if (cursor != null) {
                 val result = trySend(wrap(cursor))
                 when {
                     result.isSuccess -> cursor.`continue`()

core/src/jsTest/kotlin/Samples.kt

@@ -2,6 +2,7 @@ package com.juul.indexeddb
 
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.toList
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
@@ -68,5 +69,25 @@ class Samples {
             objectStore("customers").index("age").count(upperBound(32))
         }
         assertEquals(2, countBelowThirtyTwo)
+
+        val skipTwoYoungest = database.transaction("customers") {
+            objectStore("customers")
+                .index("age")
+                .openCursor(cursorStart = CursorStart.Advance(2))
+                .map { it.value as Customer }
+                .map { it.name }
+                .toList()
+        }
+        assertEquals(listOf("Alice", "Bill"), skipTwoYoungest)
+
+        val skipUntil33 = database.transaction("customers") {
+            objectStore("customers")
+                .index("age")
+                .openCursor(cursorStart = CursorStart.Continue(Key(33)))
+                .map { it.value as Customer }
+                .map { it.name }
+                .toList()
+        }
+        assertEquals(listOf("Alice", "Bill"), skipUntil33)
     }
 }

external/src/jsMain/kotlin/IDBCursor.kt

@@ -6,6 +6,12 @@ public external interface IDBCursor {
     public val primaryKey: dynamic
 
     public fun `continue`()
+
+    public fun advance(count: Int)
+
+    public fun `continue`(key: dynamic)
+
+    public fun continuePrimaryKey(key: dynamic, primaryKey: dynamic)
 }
 
 /** https://developer.mozilla.org/en-US/docs/Web/API/IDBCursorWithValue */