Detecting JavaScript errors in a UIWebView [duplicate]

Question

I need to have my iPhone Objective-C code catch Javascript errors in a UIWebView. That includes uncaught exceptions, syntax errors when loading files, undefined variable references, etc.

This is for a development environment, so it doesn't need to be SDK-kosher. In fact, it only really needs to work on the simulator.

I've already found used some of the hidden WebKit tricks to e.g. expose Obj-C objects to JS and to intercept alert popups, but this one is still eluding me.

[NOTE: after posting this I did find one way using a debugging delegate. Is there a way with lower overhead, using the error console / web inspector?]

Answer 1

I have now found one way using the script debugger hooks in WebView (note, NOT UIWebView). I first had to subclass UIWebView and add a method like this:

- (void)webView:(id)webView windowScriptObjectAvailable:(id)newWindowScriptObject {
    // save these goodies
    windowScriptObject = newWindowScriptObject;
    privateWebView = webView;

    if (scriptDebuggingEnabled) {
        [webView setScriptDebugDelegate:[[YourScriptDebugDelegate alloc] init]];
    }
}

Next you should create a YourScriptDebugDelegate class that contains methods like these:

// in YourScriptDebugDelegate

- (void)webView:(WebView *)webView       didParseSource:(NSString *)source
 baseLineNumber:(unsigned)lineNumber
        fromURL:(NSURL *)url
       sourceId:(int)sid
    forWebFrame:(WebFrame *)webFrame
{
    NSLog(@"NSDD: called didParseSource: sid=%d, url=%@", sid, url);
}

// some source failed to parse
- (void)webView:(WebView *)webView  failedToParseSource:(NSString *)source
 baseLineNumber:(unsigned)lineNumber
        fromURL:(NSURL *)url
      withError:(NSError *)error
    forWebFrame:(WebFrame *)webFrame
{
    NSLog(@"NSDD: called failedToParseSource: url=%@ line=%d error=%@\nsource=%@", url, lineNumber, error, source);
}

- (void)webView:(WebView *)webView   exceptionWasRaised:(WebScriptCallFrame *)frame
       sourceId:(int)sid
           line:(int)lineno
    forWebFrame:(WebFrame *)webFrame
{
    NSLog(@"NSDD: exception: sid=%d line=%d function=%@, caller=%@, exception=%@", 
          sid, lineno, [frame functionName], [frame caller], [frame exception]);
}

There is probably a large runtime impact for this, as the debug delegate can also supply methods to be called for entering and exiting a stack frame, and for executing each line of code.

See http://www.koders.com/noncode/fid7DE7ECEB052C3531743728D41A233A951C79E0AE.aspx for the Objective-C++ definition of WebScriptDebugDelegate.

Those other methods:

// just entered a stack frame (i.e. called a function, or started global scope)
- (void)webView:(WebView *)webView    didEnterCallFrame:(WebScriptCallFrame *)frame
      sourceId:(int)sid
          line:(int)lineno
   forWebFrame:(WebFrame *)webFrame;

// about to execute some code
- (void)webView:(WebView *)webView willExecuteStatement:(WebScriptCallFrame *)frame
      sourceId:(int)sid
          line:(int)lineno
   forWebFrame:(WebFrame *)webFrame;

// about to leave a stack frame (i.e. return from a function)
- (void)webView:(WebView *)webView   willLeaveCallFrame:(WebScriptCallFrame *)frame
      sourceId:(int)sid
          line:(int)lineno
   forWebFrame:(WebFrame *)webFrame;

Note that this is all hidden away in a private framework, so don't try to put this in code you submit to the App Store, and be prepared for some hackery to get it to work.