Merge pull request #65 from PureSwift/feature/asyncimage-cache

Author: colemancda

Demo/swiftui/src/commonMain/kotlin/com/pureswift/swiftui/ImageCache.kt

@@ -0,0 +1,28 @@
+package com.pureswift.swiftui
+
+import androidx.compose.ui.graphics.ImageBitmap
+import java.util.Collections
+
+// A small process-wide cache of decoded images, keyed by URL. Without it, a
+// re-keyed AsyncImage (URL change, a row scrolled off then back, a re-navigated
+// screen) drops its bitmap and re-fetches over the network — the spinner flashes
+// every time. A hit returns the decoded bitmap synchronously, so a URL seen
+// before renders with no network and no spinner.
+internal object ImageCache {
+
+    private const val MAX_ENTRIES = 64
+
+    // access-ordered so the least-recently-used entry is evicted first
+    private val store = Collections.synchronizedMap(
+        object : LinkedHashMap<String, ImageBitmap>(16, 0.75f, true) {
+            override fun removeEldestEntry(eldest: Map.Entry<String, ImageBitmap>): Boolean =
+                size > MAX_ENTRIES
+        }
+    )
+
+    fun get(url: String): ImageBitmap? = store[url]
+
+    fun put(url: String, bitmap: ImageBitmap) {
+        store[url] = bitmap
+    }
+}

Demo/swiftui/src/commonMain/kotlin/com/pureswift/swiftui/Render.kt

@@ -1314,11 +1314,19 @@ private fun RenderAsyncImage(node: ViewNode) {
         Text("[image]", modifier = node.composeModifiers())
         return
     }
-    var bitmap by remember(url) { mutableStateOf<ImageBitmap?>(null) }
+    // Seed from the cache synchronously: a URL already fetched renders on the
+    // first frame with no spinner. Only a miss touches the network.
+    var bitmap by remember(url) { mutableStateOf(ImageCache.get(url)) }
     var failed by remember(url) { mutableStateOf(false) }
     LaunchedEffect(url) {
+        if (bitmap != null) return@LaunchedEffect
         val loaded = loadRemoteImage(url)
-        if (loaded != null) bitmap = loaded else failed = true
+        if (loaded != null) {
+            ImageCache.put(url, loaded)
+            bitmap = loaded
+        } else {
+            failed = true
+        }
     }
     val image = bitmap
     when {