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POThread is executing lines, but unaffective
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copied!<p>Okay this is strange. I have a multi threaded app, thread calls back some method in the main view. I step through with the debugger, the lines are run, but nothing happens. Here is the flow:</p> <p>I have AViewController, it invokes Wrapper class W with callbackTarget (AViewController self) and UpdateScreen Selector parameters. Class W opens the UIImagePickerController, grabs the image, and pass into an Image Processing (IP) class, passing along callbackTarget and Selector. IP class then spawns a thread to process the image. When this is done, the thread calls callbackTarget.Selector, which is to update the view with the results in AViewController.</p> <p>I have a break point in UpdateScreen. All the lines are executed, but nothing happens on the screen. I am suspecting some variables are not visible across threads, but I don't know how to make it work. Help please?</p> <p><strong>EDIT adding sample code. It's messy that's why I didn't include it to begin with</strong></p> <p><strong>CODE SNIPPET</strong></p> <pre><code> // AViewController, this is entry point -(IBAction) callCardScanner_tapped{ [testResultLabel setText:@"ocr started"]; // this is shown CardScaner* scanner = [[CardScaner alloc] init] ; [scanner scanWithCameraSendResultTo:self selector:@selector(updateScreenWithResultFromCardScanner:)]; [scanner release]; } //this method is used to pass into the thread so it can call back -(void) updateScreenWithResultFromCardScanner:(OCRResult *)result{ // ... update labels with result // ... [resultLabel setText: result.resultString]; // these lines seem to be executed by main thread according to debugger, but the screen remain blank } </code></pre> <p><strong>CardScanner wrapper class</strong></p> <pre><code>-(void) scanWithCameraSendResultTo:(NSObject*) target selector :(SEL) selector{ // ... UIViewController *rootViewController = [[[UIApplication sharedApplication] keyWindow] rootViewController]; [rootViewController presentModalViewController:imagePicker animated:true]; } - (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info{ // ... ImageProcessor* recognizer = [[ImageProcessoralloc] init]; [recognizer ocrImageInThread:img callbackTarget:callbackTarget selector:self.callbackMethod]; [imagePicker release]; } </code></pre> <p><strong>Image Processor class</strong></p> <pre><code> - (void) ocrImageInThread:(UIImage*) photo callbackTarget:(NSObject*) target selector:(SEL) selector{ NSArray* args = [NSArray arrayWithObjects:photo, target, NSStringFromSelector(selector), nil]; NSThread* ocrThread = [[[NSThread alloc] initWithTarget:self selector: @selector(ocrImageThread_start:) object:args] autorelease]; [ocrThread start]; } -(void) ocrImageThread_start:(NSArray*) args{ //.. do image processing to acquire result object, here in the end invoke the callback method [callbackTarget performSelectorOnMainThread:NSSelectorFromString(selector) withObject:resultObject waitUntilDone:NO]; [resultObject release]; [pool release]; } </code></pre>

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