Future<List<ui.Image>> _extractFramesAt( VideoSource source, { required List<Duration> timestamps, ui.Size? preferredSize, }) async { return using<Future<List<ui.Image>>>((alloc) async { // Prepare the asset image generator final avAsset = videoSourceToAVAsset(source); final generator = AVAssetImageGenerator.assetImageGeneratorWithAsset_(avAsset); generator.appliesPreferredTrackTransform = true; // Omitted... // This closure is called every time an image is generated final closure = ObjCBlock_ffiVoid_CMTime_CGImage_CMTime_AVAssetImageGeneratorResult_NSError.listener( (requestedTime, cgImagePointer, actualTime, result, error) { if (error != null) { imageStream.addError(error); return; } if (cgImagePointer == nullptr) { imageStream.addError(Exception('No image generated')); return; } // Paint the image onto a CGBitmap final width = darwinLib.CGImageGetWidth(cgImagePointer); final height = darwinLib.CGImageGetHeight(cgImagePointer); final colorSpace = darwinLib.CGColorSpaceCreateDeviceRGB(); if (width == 0 || height == 0) { // <- This gets called often with both width and height equal to 0. imageStream.addError(Exception('Invalid image size: $width x $height')); return; } // Omitted... }, ); // Run the generator. The closure will be called for each timestamp generator.generateCGImagesAsynchronouslyForTimes_completionHandler_( cmTimeArray, closure, ); // Omitted... }); }