feat(share_plus): implement previewThumbnail preview on Android and Windows

packages/share_plus/share_plus/README.md

@@ -222,6 +222,40 @@ ShareParams(
 )
 ```
 
+#### Preview Thumbnail
+
+Sets a preview thumbnail shown in the share UI when sharing `text` or `uri`.
+
+- On **Android**, rendered by the system Sharesheet (API 29+). It is ignored
+  for file shares, where the system builds its own preview from the files.
+- On **Windows**, set as the `DataPackage` thumbnail.
+- Ignored on other platforms.
+
+> **Important:** the `XFile` **must carry a correct image MIME type**, or the
+> platform treats it as a generic binary file and shows **no preview** (the
+> share itself still succeeds). The plugin does **not** infer the type from the
+> file contents — it is the caller's responsibility to set it. This is the most
+> common reason a thumbnail does not appear.
+
+Provide the MIME type via `XFile.mimeType`, or via a file name/path that ends in
+a matching image extension. An `XFile.fromData(bytes)` created **without** a
+`mimeType` falls back to `application/octet-stream` and will not render a preview.
+
+```dart
+// From in-memory bytes — you MUST pass mimeType (set it to the actual image
+// type, e.g. image/jpeg or image/webp):
+ShareParams(
+  text: 'Check this out',
+  previewThumbnail: XFile.fromData(bytes, mimeType: 'image/png'),
+)
+
+// From a file path — make sure the path/name has an image extension:
+ShareParams(
+  text: 'Check this out',
+  previewThumbnail: XFile('/path/to/thumbnail.png'),
+)
+```
+
 ## Known Issues
 
 ### Sharing data created with XFile.fromData

packages/share_plus/share_plus/android/src/main/kotlin/dev/fluttercommunity/plus/share/MethodCallHandler.kt

@@ -4,50 +4,45 @@ import android.os.Build
 import io.flutter.plugin.common.MethodCall
 import io.flutter.plugin.common.MethodChannel
 
-/** Handles the method calls for the plugin.  */
+/** Handles the method calls for the plugin. */
 internal class MethodCallHandler(
     private val share: Share,
     private val manager: ShareSuccessManager,
 ) : MethodChannel.MethodCallHandler {
 
-    override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
-        expectMapArguments(call)
-
-        // We don't attempt to return a result if the current API version doesn't support it
-        val isWithResult =
-            Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP_MR1
-
-        if (isWithResult)
-            manager.setCallback(result)
-
-        try {
-            when (call.method) {
-                "share" -> {
-                    share.share(
-                        arguments = call.arguments<Map<String, Any>>()!!,
-                        withResult = isWithResult,
-                    )
-                    success(isWithResult, result)
-                }
-                else -> result.notImplemented()
-            }
-        } catch (e: Throwable) {
-            manager.clear()
-            result.error("Share failed", e.message, e)
-        }
-    }
+  override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
+    expectMapArguments(call)
+
+    // We don't attempt to return a result if the current API version doesn't support it
+    val isWithResult = Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP_MR1
 
-    private fun success(
-        isWithResult: Boolean,
-        result: MethodChannel.Result
-    ) {
-        if (!isWithResult) {
-            result.success("dev.fluttercommunity.plus/share/unavailable")
+    if (isWithResult) manager.setCallback(result)
+
+    try {
+      when (call.method) {
+        "share" -> {
+          share.share(
+              arguments = call.arguments<Map<String, Any>>()!!,
+              withResult = isWithResult,
+          )
+          success(isWithResult, result)
         }
+        else -> result.notImplemented()
+      }
+    } catch (e: Throwable) {
+      manager.clear()
+      result.error("Share failed", e.message, e)
     }
+  }
 
-    @Throws(IllegalArgumentException::class)
-    private fun expectMapArguments(call: MethodCall) {
-        require(call.arguments is Map<*, *>) { "Map arguments expected" }
+  private fun success(isWithResult: Boolean, result: MethodChannel.Result) {
+    if (!isWithResult) {
+      result.success("dev.fluttercommunity.plus/share/unavailable")
     }
+  }
+
+  @Throws(IllegalArgumentException::class)
+  private fun expectMapArguments(call: MethodCall) {
+    require(call.arguments is Map<*, *>) { "Map arguments expected" }
+  }
 }