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 0x00         


        

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;