Question

我尝试修改简单 CS193P iTunes U示例/家庭作业，只在UIScrollView 中显示 UIImage。我是Obj-C区块的新手。

一旦我尝试将下载Image的代码行包装在* dispatch_async * Block中，我就会打破缩放并获得

Uncaught exception: CALayer position contains NaN: [nan nan]

（显示图像和“滚动”工作）

- (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; [self.activityIndicatorView startAnimating]; [self resetImage]; } - (void)resetImage { if (self.detailItem && self.scrollView) { //reset self.scrollView.contentSize = CGSizeZero; self.imageView.image = nil; NSURL *imageURL = [FlickrFetcher urlForPhoto:self.detailItem format:FlickrPhotoFormatLarge]; __block NSData *imageData; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ imageData = [[NSData alloc] initWithContentsOfURL:imageURL]; dispatch_async(dispatch_get_main_queue(), ^{ //because of UI Stuff UIImage *image = [[UIImage alloc] initWithData:imageData]; if (image) { [self.activityIndicatorView stopAnimating]; [self.activityIndicatorView removeFromSuperview]; self.scrollView.zoomScale = 1.0; self.scrollView.contentSize = image.size; self.imageView.image = image; CGRect frame = { CGPointZero , image.size }; self.imageView.frame = frame; [self addToRecentPhotosList:self.detailItem]; NSLog(@"\n self.imageView.frame:%@ \n self.imageView.bounds: %@ \n self.scrollView.contentSize: %@ \n self.scrollView.frame: %@ \n self.scrollView.bounds: %@ ", NSStringFromCGRect(self.imageView.frame), NSStringFromCGRect(self.imageView.bounds), NSStringFromCGSize(self.scrollView.contentSize), NSStringFromCGRect(self.scrollView.frame), NSStringFromCGRect(self.scrollView.bounds)); } }); }); } } - (void)viewDidLoad { [super viewDidLoad]; [self.scrollView addSubview:self.imageView]; [self.scrollView addSubview:self.activityIndicatorView]; self.scrollView.minimumZoomScale = 0.2; self.scrollView.maximumZoomScale = 5.0; self.scrollView.delegate = self; } - (void)viewDidLayoutSubviews { [super viewDidLayoutSubviews]; if (self.imageView.image) [self.scrollView zoomToRect:self.imageView.bounds animated:YES]; else self.activityIndicatorView.center = self.view.center; } - (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView { return self.imageView; }

记录的帧/边界：

without dispatch_async blocks: self.imageView.frame: {{0, 0}, {1024, 768}} self.imageView.bounds: {{0, 0}, {1024, 768}} with dispatch_async blocks: self.imageView.frame: {{-7.62939e-06, -1.52588e-05}, {1024, 768}} self.imageView.bounds: {{0, 0}, {204.8, 153.6}} self.scrollView.contentSize: {1024, 768} self.scrollView.frame: {{0, 0}, {320, 367}} self.scrollView.bounds: {{0, 0}, {320, 367}}

堆栈追踪：

0 CoreFoundation 0x01ca002e __exceptionPreprocess + 206 1 libobjc.A.dylib 0x010dde7e objc_exception_throw + 44 2 CoreFoundation 0x01c9fdeb +[NSException raise:format:] + 139 3 QuartzCore 0x02293e0b _ZN2CA5Layer12set_positionERKNS_4Vec2IdEEb + 151 4 QuartzCore 0x0229eba1 -[CALayer(CALayerPrivate) setDoublePosition:] + 81 5 UIKit 0x000874a2 -[UIScrollView setZoomScale:withAnchorPoint:validatingScrollOffset:allowRubberbanding:animated:duration:notifyDelegate:force:] + 1476 6 UIKit 0x00083004 -[UIScrollView _updatePinchGestureForState:] + 3499 7 UIKit 0x00083960 -[UIScrollView handlePinch:] + 59 8 UIKit 0x002ec85a _UIGestureRecognizerSendActions + 139 9 UIKit 0x002eb99b -[UIGestureRecognizer _updateGestureWithEvent:] + 333 10 UIKit 0x002ed0df -[UIGestureRecognizer _delayedUpdateGesture] + 46 11 UIKit 0x002efd2d ___UIGestureRecognizerUpdate_block_invoke_0543 + 57 12 UIKit 0x002efcac _UIGestureRecognizerRemoveObjectsFromArrayAndApplyBlocks + 331 13 UIKit 0x002e7a28 _UIGestureRecognizerUpdate + 1348 14 UIKit 0x00054972 -[UIWindow _sendGesturesForEvent:] + 1283 15 UIKit 0x00054e53 -[UIWindow sendEvent:] + 98 16 UIKit 0x00032d4a -[UIApplication sendEvent:] + 436 17 UIKit 0x00024698 _UIApplicationHandleEvent + 9874 18 GraphicsServices 0x01bfbdf9 _PurpleEventCallback + 339 19 GraphicsServices 0x01bfbad0 PurpleEventCallback + 46 20 CoreFoundation 0x01c15bf5 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 53 21 CoreFoundation 0x01c15962 __CFRunLoopDoSource1 + 146 22 CoreFoundation 0x01c46bb6 __CFRunLoopRun + 2118 23 CoreFoundation 0x01c45f44 CFRunLoopRunSpecific + 276 24 CoreFoundation 0x01c45e1b CFRunLoopRunInMode + 123 25 GraphicsServices 0x01bfa7e3 GSEventRunModal + 88 26 GraphicsServices 0x01bfa668 GSEventRun + 104 27 UIKit 0x00021ffc UIApplicationMain + 1211 28 CS193P.4.SPoT 0x00002b84 main + 164 29 CS193P.4.SPoT 0x00002a95 start + 53

Answer 1

或许，如果你想在另一个线程上创建的线程上使用一个对象，那么首先将线程彼此同步是一个好主意。在准备使用之前开始使用imageData并不是一个屡获殊荣的想法。

// ...

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    dispatch_semaphore_t _semaphore = dispatch_semaphore_create(0);
    imageData = [[NSData alloc] initWithContentsOfURL:imageURL];
    dispatch_semaphore_signal(_semaphore);

    dispatch_async(dispatch_get_main_queue(), ^{    //because of UI Stuff

        dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
        dispatch_release(_semaphore);
        UIImage *image = [[UIImage alloc] initWithData:imageData];

        // etc...

    };
};

// ...

<强>更新

尝试此解决方案，它的工作方式与GCD版本应该有效。

<强> SPoTDetailViewController.h

@interface SPoTImageViewController : UIViewController <UISplitViewControllerDelegate, UIScrollViewDelegate> {
    dispatch_semaphore_t _semaphore;
    __block NSData *imageData;
}

// ...

@end

<强> SPoTDetailViewController.m

- (void)resetImage
{
    // Update the user interface for the detail item.
    [self.activityIndicatorView startAnimating];

    if (self.detailItem && self.scrollView) {
        self.scrollView.contentSize = CGSizeZero;
        self.imageView.image = nil;

        [self performSelectorInBackground:@selector(downloadImageDataOnBackgroundThread) withObject:nil];

        dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
        // dispatch_release(_semaphore);
        UIImage *image = [[UIImage alloc] initWithData:imageData];
        if (image) {

            [_activityIndicatorView stopAnimating];

            _scrollView.zoomScale = 1.0;
            _scrollView.contentSize = image.size;
            _imageView.image = image;

            CGRect frame = { CGPointZero , image.size };
            NSLog(@"\n new CGRect frame:%@ \n ", NSStringFromCGRect(frame));

            _imageView.frame = frame;
            NSLog(@"\n self.imageView.frame:%@ \n self.imageView.bounds: %@", NSStringFromCGRect(self.imageView.frame), NSStringFromCGRect(self.imageView.bounds));
        }
    }
}

- (void)downloadImageDataOnBackgroundThread {
    _semaphore = dispatch_semaphore_create(0);
    NSURL *imageURL = [FlickrFetcher urlForPhoto:self.detailItem format:FlickrPhotoFormatLarge];
    imageData = [[NSData alloc] initWithContentsOfURL:imageURL];
    dispatch_semaphore_signal(_semaphore);
}

Blocks / dispatch_async：我的简单代码出了什么问题？

1 个答案: