[Bug]: share_plus decodes full-resolution images on the main thread when preparing iOS share previews #4001

Plugin: share_plus 13.3.1
Current stable releases: Flutter 3.47.7 and iOS 27.0.1. These are current-release references; the original report’s exact OS and Flutter versions haven’t been confirmed.

Sharing a high-resolution photo can stall the app while the share sheet prepares its preview. We encountered this with a 48 MP image; the hang trace pointed to synchronous image drawing in the share metadata callback.

The placeholder, thumbnail, and link-metadata callbacks load the original image using UIImage imageWithContentsOfFile:. The metadata callback then draws it into a 120 × 120 context. This limits the resulting preview size, but still requires decoding and drawing the full-resolution source on the calling thread.

PR #3320, which addressed #3322, added that resize. The same implementation remains in 13.3.1 and current main.

Proposed solution

One possible approach is to generate small previews with ImageIO downsampling on a background queue. This could avoid decoding and resizing the full-resolution image inside UIKit’s preview callbacks.