UIWebDocumentView _updateSubviewCaches crash in iOS10

Question

I am getting the following crash in HockeyApp more seriously in iOS10. Please find the crash log as given below.

Thread 4 Crashed:

0 libobjc.A.dylib 0x0000000187242f30 objc_msgSend + 16
1 UIKit 0x000000018e86e914 -[UIWebDocumentView _updateSubviewCaches] + 36
2 UIKit 0x000000018e69093c -[UIWebDocumentView subviews] + 88
3 UIKit 0x000000018e941bd4 -[UIView(CALayerDelegate) _wantsReapplicationOfAutoLayoutWithLayoutDirtyOnEntry:] + 68
4 UIKit 0x000000018e63d770 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1248
5 QuartzCore 0x000000018bb0640c -[CALayer layoutSublayers] + 144
6 QuartzCore 0x000000018bafb0e8 CA::Layer::layout_if_needed(CA::Transaction*) + 288
7 QuartzCore 0x000000018bafafa8 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 28
8 QuartzCore 0x000000018ba77c64 CA::Context::commit_transaction(CA::Transaction*) + 248
9 QuartzCore 0x000000018ba9f0d0 CA::Transaction::commit() + 508
10 QuartzCore 0x000000018ba9faf0 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) + 116
11 CoreFoundation 0x00000001887a57dc __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 28
12 CoreFoundation 0x00000001887a340c __CFRunLoopDoObservers + 368
13 CoreFoundation 0x00000001886d2068 CFRunLoopRunSpecific + 472
14 WebCore 0x000000018d273a2c RunWebThread(void*) + 452
15 libsystem_pthread.dylib 0x000000018788b860 _pthread_body + 236
16 libsystem_pthread.dylib 0x000000018788b770 _pthread_start + 280
17 libsystem_pthread.dylib 0x0000000187888dbc thread_start + 0

Any idea what is going on here ? Seems like the following crash groups are also related.

Crash Group 1

[UIWebBrowserView _collectAdditionalSubviews]
objc_msgSend() selector name: autorelease

Crash Group 2

[UIWebDocumentView subviews]
objc_msgSend() selector name: arrayByAddingObjectsFromArray:

Answer 1

I am answering to my own question, since I am able to replicate this issue and found the root cause.

Please note the following.

1. I am using a UIWebView. First of all, this is not recommended.

2. The following way of implementation causes this issue. I am writing pseudo-code here to demonstrate the problem.

@implementation MyViewController

    - (void)loadView {

        //creating UIWebView Here
        self.webView = [[UIWebView alloc] initWithFrame:_some_frame];

        //setting the web view properties here.

        //Adding to the view
        [self.view addSubView: self.webView];

        [self populateWebView];
    }

    - (void) populateWebView {
        [self.webView loadHTMLString:_some_html 
                             baseURL:[NSURL fileURLWithPath:_some_path]];
    }


    - (void)viewDidLayoutSubviews {
        [super viewDidLayoutSubviews];

        if (self.webView) {
           //The following two lines causing this issue.
           self.webView.scrollView.contentInset = _some_insets;
           self.webView.scrollView.scrollIndicatorInsets = _some_insets;
        }
    }

@end

3. The solution is given below.

@implementation MyViewController

    - (void)loadView {

        //creating UIWebView Here
        self.webView = [[UIWebView alloc] initWithFrame:_some_frame];

        //setting the web view properties here.

        //Adding to the view
        [self.view addSubView: self.webView];

        [self populateWebView];
    }

    - (void) populateWebView {
        [self.webView loadHTMLString:_some_html 
                             baseURL:[NSURL fileURLWithPath:_some_path]];
    }


    - (void)viewDidLayoutSubviews {
        [super viewDidLayoutSubviews];
    }

    - (void)webViewDidFinishLoad:(UIWebView *)webView {
        webView.scrollView.contentInset = _some_insets;
        webView.scrollView.scrollIndicatorInsets = _some_insets;
    }

@end

In my actual code, I was using a custom WebView class here by subclassing the UIWebView. Not think that will create some issue. The root cause is setting the "contentInset" and "scrollIndicatorInsets" in viewDidLayoutSubviews which is not a good idea. When I put the break points, "viewDidLayoutSubviews" called several times and eventually App crashes.

As a solution I moved the following part of the code into "webViewDidFinishLoad" which will call only when the WebView is ready after finished loading. So it makes sense to add this code under this delegate method.

webView.scrollView.contentInset = _some_insets;
webView.scrollView.scrollIndicatorInsets = _some_insets;