If user navigates away from a page before OCR completes, then cancel …

…the OCR task. Tested on 13 page Japanese document.

Classes/Session/OCRVision/OCRTracker.m

@@ -71,6 +71,9 @@ @interface OCRDatum : NSObject
 
 @property(weak) NSImage *image;
 
+/// non-nil while this is actively OCRing.
+@property OCRVision *activeOCR API_AVAILABLE(macos(10.15));
+
 @property(weak) CALayer *selectionLayer;
 
 /// <VNRecognizedTextObservation *> - 10.15 and newer
@@ -317,16 +320,18 @@ - (CGRect)boundBoxOfPiece:(VNRecognizedTextObservation *)piece range:(NSRange)ch
 - (void)ocrDidFinish:(id<OCRVisionResults>)results image:(NSImage *)image index:(NSInteger)index
 {
 	NSArray *textPieces = @[];
+	NSError *error = results.ocrError;
 	if (@available(macOS 10.15, *)) {
+		self.datums[index].activeOCR = nil;
 		textPieces = results.textObservations;
 	}
 	// Since we are changing state that affects the U.I., we do it on the main thread in the future,
 	// but `complete` isn't guaranteed to exist then, so we assign to locals so it will be captured
 	// by the block.
 	dispatch_async(dispatch_get_main_queue(), ^{
 		OCRDatum *datum = self.datums[index];
-		datum.image = image;
-		datum.textPieces = textPieces;
+		datum.image = (error == nil) ? image : nil;
+		datum.textPieces = (error == nil) ? textPieces : @[];
 		[datum.selectionPieces removeAllObjects];
 		[self.view setNeedsDisplay:YES];
 		self.needsInvalidateCursorRects = YES;
@@ -338,14 +343,18 @@ - (void)ocrImage:(NSImage *)image index:(NSInteger)index
 	if (@available(macOS 10.15, *)) {
 		OCRDatum *datum = self.datums[index];
 		datum.image = nil;
+		[datum.activeOCR cancel];
+		datum.activeOCR = nil;
 		datum.textPieces = @[];
 		[datum.selectionPieces removeAllObjects];
 		if (image)
 		{
 			__block OCRVision *ocrVision = [[OCRVision alloc] init];
+			datum.activeOCR = ocrVision;
+			__weak typeof(self) weakSelf = self;
 			dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
 				[ocrVision ocrImage:image completion:^(id<OCRVisionResults> _Nonnull complete) {
-					[self ocrDidFinish:complete image:image index:index];
+					[weakSelf ocrDidFinish:complete image:image index:index];
 					ocrVision = nil;
 				}];
 			});

Classes/Session/OCRVision/OCRVision.h

@@ -58,6 +58,9 @@ API_AVAILABLE(macos(10.15))
 /// @param completion - a block passed an object that corresponds to the OCRVision protocol.
 - (void)ocrCGImage:(CGImageRef)cgImage completion:(void (^)(id<OCRVisionResults> _Nonnull ocrResults))completion;
 
+/// if an OCR task is currently running, attempt to cancel it. Always safe to call.
+- (void)cancel;
+
 @end
 
 NS_ASSUME_NONNULL_END

Classes/Session/OCRVision/OCRVision.m

@@ -16,6 +16,12 @@
 
 /// Rather than allocate a new object to pass the results, just make the OCRVision object do double duty.
 @interface OCRVision()<OCRVisionResults>
+
+/// non nil while processing the request
+///
+/// Assumes caller will create a new OCRVision for each image to analyze.
+@property(nullable) VNRecognizeTextRequest *activeTextRequest;
+
 @property(readwrite) NSArray<VNRecognizedTextObservation *> *textObservations;
 @property(readwrite, nullable, setter=setOCRError:) NSError *ocrError;
 @end
@@ -110,6 +116,10 @@ - (void)handleTextRequest:(nullable VNRequest *)request
 	else if ([request isKindOfClass:[VNRecognizeTextRequest class]])
 	{
 		VNRecognizeTextRequest *textRequests = (VNRecognizeTextRequest *)request;
+		if (textRequests == self.activeTextRequest)
+		{
+			self.activeTextRequest = nil;
+		}
 		NSMutableArray<VNRecognizedTextObservation *> *pieces = [NSMutableArray array];
 		NSArray *results = textRequests.results;
 		for (id rawResult in results)
@@ -152,9 +162,10 @@ - (void)ocrCGImage:(CGImageRef)cgImage completion:(void (^)(id<OCRVisionResults>
 		}
 		NSError *error = nil;
     VNImageRequestHandler *handler = [[VNImageRequestHandler alloc] initWithCGImage:cgImage options:@{}];
+		self.activeTextRequest = textRequest;
 		if (![handler performRequests:@[textRequest] error:&error])
 		{
-			[self callCompletion:completion observations:@[] error:error];
+			[weakSelf callCompletion:completion observations:@[] error:error];
 		}
   } else {
 		NSString *desc = @"Could not create text request";
@@ -184,4 +195,9 @@ - (void)ocrImage:(NSImage *)image completion:(void (^)(id<OCRVisionResults> _Non
 	}
 }
 
+- (void)cancel
+{
+	[self.activeTextRequest cancel];
+}
+
 @end