ios - App Crashes after executing background fetch completionHandler

Question

I have updated our app for iOS 7 and added the background fetch functionality. However, i'm noticing some crashes because of this feature. After background fetch call is completed, the completion handler block is executed with either new/no data. This results in a call to 'layoutSubviews' of my custom navigation NavigationBar and sometimes app is crashing with the following stack trace. I could not reproduce this issue myself but constantly receiving these crash logs from our users. Any help would be appreciated.

Thread 0 Crashed:
0   libsystem_platform.dylib            0x39cba486 _platform_memmove$VARIANT$Swift + 102
1   Foundation                          0x2fce8f47 -[NSData(NSData) getBytes:range:] + 419
2   CoreFoundation                      0x2f30637d CFDataGetBytes + 205
3   ImageIO                             0x300e7357 CGImageReadGetBytesAtOffset + 283
4   ImageIO                             0x300e7231 CGImageReadSessionGetBytes + 29
5   ImageIO                             0x3022baeb read_fn + 23
6   ImageIO                             0x300e7189 png_read_sig + 45
7   ImageIO                             0x300e6e0d _cg_png_read_info + 33
8   ImageIO                             0x300ef62f copyImageBlockSetPNG + 1123
9   ImageIO                             0x300eec4b ImageProviderCopyImageBlockSetCallback + 539
10  CoreGraphics                        0x2f43577d CGImageProviderCopyImageBlockSetWithOptions + 137
11  CoreGraphics                        0x2f43a145 subImageProviderCopyImageBlockSet + 305
12  CoreGraphics                        0x2f43577d CGImageProviderCopyImageBlockSetWithOptions + 137
13  CoreGraphics                        0x2f435433 img_blocks_create + 383
14  CoreGraphics                        0x2f4286fd img_data_lock + 1053
15  CoreGraphics                        0x2f427ae1 CGSImageDataLock + 89
16  libRIP.A.dylib                      0x2f774da7 ripc_AcquireImage + 99
17  libRIP.A.dylib                      0x2f774131 ripc_DrawImage + 601
18  CoreGraphics                        0x2f4279f3 CGContextDelegateDrawImage + 51
19  CoreGraphics                        0x2f427879 CGContextDrawImage + 285
20  WebCore                             0x373120db WebCore::GraphicsContext::drawNativeImage(CGImage*, WebCore::FloatSize const&, WebCore::ColorSpace, WebCore::FloatRect const&, WebCore::FloatRect const&, float, WebCore::CompositeOperator, WebCore::BlendMode, WebCore::ImageOrientation) + 1127
21  WebCore                             0x37310e79 WebCore::BitmapImage::draw(WebCore::GraphicsContext*, WebCore::FloatRect const&, WebCore::FloatRect const&, WebCore::ColorSpace, WebCore::CompositeOperator, WebCore::BlendMode, WebCore::RespectImageOrientationEnum) + 349
22  WebCore                             0x3735d897 WebCore::BitmapImage::draw(WebCore::GraphicsContext*, WebCore::FloatRect const&, WebCore::FloatRect const&, WebCore::ColorSpace, WebCore::CompositeOperator, WebCore::BlendMode) + 31
23  WebCore                             0x3735892f WebCore::Image::drawTiled(WebCore::GraphicsContext*, WebCore::FloatRect const&, WebCore::FloatPoint const&, WebCore::FloatSize const&, WebCore::ColorSpace, WebCore::CompositeOperator, WebCore::BlendMode) + 499
24  WebCore                             0x37358735 WebCore::GraphicsContext::drawTiledImage(WebCore::Image*, WebCore::ColorSpace, WebCore::IntRect const&, WebCore::IntPoint const&, WebCore::IntSize const&, WebCore::CompositeOperator, bool, WebCore::BlendMode) + 193
25  WebCore                             0x3728bc11 WebCore::RenderBoxModelObject::paintFillLayerExtended(WebCore::PaintInfo const&, WebCore::Color const&, WebCore::FillLayer const*, WebCore::LayoutRect const&, WebCore::BackgroundBleedAvoidance, WebCore::InlineFlowBox*, WebCore::LayoutSize const&, WebCore::CompositeOperator, WebCore::RenderObject*) + 3321
26  WebCore                             0x3728addd WebCore::RenderBox::paintFillLayers(WebCore::PaintInfo const&, WebCore::Color const&, WebCore::FillLayer const*, WebCore::LayoutRect const&, WebCore::BackgroundBleedAvoidance, WebCore::CompositeOperator, WebCore::RenderObject*) + 449
27  WebCore                             0x3728ab41 WebCore::RenderBox::paintBackground(WebCore::PaintInfo const&, WebCore::LayoutRect const&, WebCore::BackgroundBleedAvoidance) + 317
28  WebCore                             0x37289ee5 WebCore::RenderBox::paintBoxDecorations(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 329
29  WebCore                             0x3728806f WebCore::RenderBlock::paintObject(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 55
30  WebCore                             0x37289c3f WebCore::RenderBlock::paint(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 187
31  WebCore                             0x37288cc9 WebCore::RenderBlock::paintFloats(WebCore::PaintInfo&, WebCore::LayoutPoint const&, bool) + 305
32  WebCore                             0x3728813d WebCore::RenderBlock::paintObject(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 261
33  WebCore                             0x37289c3f WebCore::RenderBlock::paint(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 187
34  WebCore                             0x37288cfb WebCore::RenderBlock::paintFloats(WebCore::PaintInfo&, WebCore::LayoutPoint const&, bool) + 355
35  WebCore                             0x3728813d WebCore::RenderBlock::paintObject(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 261
36  WebCore                             0x37289c3f WebCore::RenderBlock::paint(WebCore::PaintInfo&, WebCore::LayoutPoint const&) + 187
37  WebCore                             0x37288861 WebCore::RenderLayer::paintForegroundForFragmentsWithPhase(WebCore::PaintPhase, WTF::Vector<WebCore::LayerFragment, 1ul, WTF::CrashOnOverflow> const&, WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int, WebCore::RenderObject*) + 269
38  WebCore                             0x372886e7 WebCore::RenderLayer::paintForegroundForFragments(WTF::Vector<WebCore::LayerFragment, 1ul, WTF::CrashOnOverflow> const&, WebCore::GraphicsContext*, WebCore::GraphicsContext*, WebCore::LayoutRect const&, bool, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int, WebCore::RenderObject*, bool, bool) + 235
39  WebCore                             0x37286435 WebCore::RenderLayer::paintLayerContents(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 1585
40  WebCore                             0x37285ca9 WebCore::RenderLayer::paintLayer(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 621
41  WebCore                             0x372885d9 WebCore::RenderLayer::paintList(WTF::Vector<WebCore::RenderLayer*, 0ul, WTF::CrashOnOverflow>*, WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 101
42  WebCore                             0x3728647f WebCore::RenderLayer::paintLayerContents(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 1659
43  WebCore                             0x37285ca9 WebCore::RenderLayer::paintLayer(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 621
44  WebCore                             0x372885d9 WebCore::RenderLayer::paintList(WTF::Vector<WebCore::RenderLayer*, 0ul, WTF::CrashOnOverflow>*, WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 101
45  WebCore                             0x3728647f WebCore::RenderLayer::paintLayerContents(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 1659
46  WebCore                             0x37285ca9 WebCore::RenderLayer::paintLayer(WebCore::GraphicsContext*, WebCore::RenderLayer::LayerPaintingInfo const&, unsigned int) + 621
47  WebCore                             0x372859b3 WebCore::RenderLayer::paint(WebCore::GraphicsContext*, WebCore::LayoutRect const&, unsigned int, WebCore::RenderObject*, WebCore::RenderRegion*, unsigned int) + 103
48  WebCore                             0x3728541b WebCore::FrameView::paintContents(WebCore::GraphicsContext*, WebCore::IntRect const&) + 499
49  WebKit                              0x37c4578d -[WebFrame(WebInternal) _drawRect:contentsOnly:] + 357
50  WebKit                              0x37c455b9 -[WebHTMLView drawSingleRect:] + 137
51  WebKit                              0x37c454db -[WebHTMLView drawRect:] + 43
52  WebCore                             0x37284d93 -[WAKView _drawRect:context:lockFocus:] + 275
53  WebCore                             0x37284ec7 -[WAKView _drawRect:context:lockFocus:] + 583
54  WebCore                             0x37284ec7 -[WAKView _drawRect:context:lockFocus:] + 583
55  WebCore                             0x37284ec7 -[WAKView _drawRect:context:lockFocus:] + 583
56  WebCore                             0x37284ec7 -[WAKView _drawRect:context:lockFocus:] + 583
57  WebCore                             0x37284c47 -[WAKView displayRect:] + 59
58  WebCore                             0x37284c03 -[WAKWindow displayRect:] + 55
59  WebCore                             0x37284b49 WebCore::TileCache::drawWindowContent(TileLayer*, CGContext*, CGRect) + 673
60  WebCore                             0x37284525 WebCore::TileCache::drawLayer(TileLayer*, CGContext*) + 505
61  QuartzCore                          0x317d3813 CABackingStoreUpdate_ + 1859
62  QuartzCore                          0x318ad735 ___ZN2CA5Layer8display_Ev_block_invoke + 53
63  QuartzCore                          0x317d30c3 x_blame_allocations + 83
64  QuartzCore                          0x317d2d77 CA::Layer::display_() + 1119
65  QuartzCore                          0x317b6969 CA::Layer::display_if_needed(CA::Transaction*) + 209
66  QuartzCore                          0x317b6601 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 25
67  QuartzCore                          0x317b600d CA::Context::commit_transaction(CA::Transaction*) + 229
68  QuartzCore                          0x317b5e1f CA::Transaction::commit() + 315
69  UIKit                               0x31d973c9 _UIWindowUpdateVisibleContextOrder + 209
70  UIKit                               0x31d9729d +[UIWindow _prepareWindowsPassingTestForAppResume:] + 13
71  UIKit                               0x31db0ac5 -[UIApplication _updateSnapshotAndStateRestorationArchiveForBackgroundEvent:saveState:exitIfCouldNotRestoreState:] + 201
72  UIKit                               0x31db0d81 -[UIApplication _replyToBackgroundFetchRequestWithResult:remoteNotificationToken:sequenceNumber:updateApplicationSnapshot:] + 105
73  UIKit                               0x31db138d __61-[UIApplication _handleOpportunisticFetchWithSequenceNumber:]_block_invoke + 37
74  MyApp                               0x001456b7 -[List fetched:error:withBackgroundFetchCompletionHandler:] (List.m:421)
75  MyApp                               0x00144f87 __71-[List fetchFromServerWithBackgroundFetchCompletionHandler:]_block_invoke (List.m:292)
76  MyApp                               0x0021ed07 APICompletionBlock_block_invoke_6 (APIRequest.m:305)
77  libdispatch.dylib                   0x39b8c103 _dispatch_call_block_and_release + 11
78  libdispatch.dylib                   0x39b8c0ef _dispatch_client_callout + 23
79  libdispatch.dylib                   0x39b8e9a9 _dispatch_main_queue_callback_4CF + 269
80  CoreFoundation                      0x2f3785b9 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
81  CoreFoundation                      0x2f376e85 __CFRunLoopRun + 1309
82  CoreFoundation            

Answer 1

When you leave your app to the background, it saves the snapshot of the current view to the memory so it could be shown when you double-click the home button. With the iOS7 background refresh feature ((void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler), you must execute the completionHandler function after you finish executing your fetch request.

The execution of that function also triggers updates to the UI - [UIApplication _updateSnapshotAndStateRestorationArchiveForBackgroundEvent:saveState:exitIfCouldNotRestoreState:], and in your case it triggers calculating the frame for a UIWebView from a background thread. UIWebView can be updated only from the UIThread, so to overcome this crash I would suggest to wrap the completionHandler with execution on the main thread:

- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler
{
    void (^safeHandler)(UIBackgroundFetchResult) = ^(UIBackgroundFetchResult result){
        dispatch_async(dispatch_get_main_queue(), ^{
            completionHandler(result);
        });
    };
    [someInstance doSomethingInBackground:userInfo withHandler:safeHandler];
}

And you will no longer have to worry where you invoke the completionHandler (now safeHandler) from.