Override UIWebView user agent - but not for Facebook SDK - ios

Question

I'm setting a custom UserAgent for the webviews in my app as per the instructions in this other question. Specifically, I'm setting
NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:#"MyApp/MyLongVersionInfoString", #"UserAgent", nil];
[[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
at app launch. (Just setting the appropriate headers for the NSMutableURLRequest to be used by the UIWebView - UserAgent, User-Agent, User_Agent - does not work.)
This causes my embedded webviews to use the correct user agent. However, it also breaks the embedded webviews used by the Facebook SDK for dialogs - after I post to my wall, for instance, the contents of the FB dialog webview are replaced with text similar to window.location.href="fbconnect:\/\/success?post_id=100002469633196_43677789308119... and the webview does not close as it normally would (the user has to manually close it). This only occurs when I have my custom user agent set.
I thought I could circumvent the problem by unsetting the user agent before Facebook calls and resetting it afterwards, but it seems I can't unset the default defaults; I tried calling [[NSUserDefaults standardUserDefaults] removeObjectForKey:#"UserAgent"] and [[NSUserDefaults standardUserDefaults] removePersistentDomainForName:NSRegistrationDomain] before each Facebook call and setting them again in the call's result handler, but I still see the same misbehavior.
I tried switching my initial settings to [[NSUserDefaults standardUserDefaults] setObject:newUA forKey:#"UserAgent"];, but then my webviews don't pick up the user agent.
Surely, surely, someone has used the Facebook SDK before in an app with non-Facebook embedded webviews. What am I missing? I've gone a number of rounds on this, each of which seems to almost-but-not-quite fix everything.

Answer 1

I ended up having to do this by implementing an NSURLProtocol subclass. While my implementation was messy, I'm including a scrubbed version below because I was surprised not to be able to find an example already on StackOverflow.
#import <Foundation/Foundation.h>
#interface MyProtocol : NSURLProtocol {
NSURLConnection *connection;
}
#property (nonatomic, retain) NSURLConnection *connection;
#end
#implementation MyProtocol
#synthesize connection;
#pragma mark -
#pragma mark custom methods
+ (NSString *)myUA {
return [[NSUserDefaults standardUserDefaults] stringForKey:#"MyCustomUserAgent"];
}
+ (BOOL)canInitWithRequest:(NSURLRequest *)request
{
NSString *scheme = [request.URL.scheme stringByAppendingString:#"://"];
NSString *baseRequestString = [[[request.URL absoluteString] stringByReplacingOccurrencesOfString:request.URL.path withString:#""]
stringByReplacingOccurrencesOfString:scheme withString:#""];
if (request.URL.query != nil) {
NSString *query = [#"?" stringByAppendingString:request.URL.query];
baseRequestString = [baseRequestString stringByReplacingOccurrencesOfString:query withString:#""];
}
BOOL shouldIntercept = [baseRequestString isEqualToString:myWebHost];
BOOL alreadyIntercepted = [[NSURLProtocol propertyForKey:#"UserAgent" inRequest:request] isEqualToString:[self myUA]];
return shouldIntercept && !alreadyIntercepted;
}
+ (NSURLRequest *) canonicalRequestForRequest:(NSURLRequest *)request
{
return request;
}
- (id)initWithRequest:(NSURLRequest *)request cachedResponse:(NSCachedURLResponse *)cachedResponse client:(id<NSURLProtocolClient>)client
{
NSMutableURLRequest *mutableRequest = [NSMutableURLRequest requestWithURL:request.URL
cachePolicy:request.cachePolicy
timeoutInterval:request.timeoutInterval];
[mutableRequest setAllHTTPHeaderFields:[request allHTTPHeaderFields]];
[mutableRequest setHTTPMethod:[request HTTPMethod]];
if ([request HTTPBody] != nil)
[mutableRequest setHTTPBody:[request HTTPBody]];
if ([request HTTPBodyStream] != nil)
[mutableRequest setHTTPBodyStream:[request HTTPBodyStream]];
[NSURLProtocol setProperty:[[self class] myUA] forKey:#"UserAgent" inRequest:mutableRequest];
[mutableRequest setValue:[[self class] myUA] forHTTPHeaderField:#"User-Agent"];
[mutableRequest setValue:[[self class] myUA] forHTTPHeaderField:#"User_Agent"];
self = [super initWithRequest:mutableRequest cachedResponse:cachedResponse client:client];
return self;
}
- (void) dealloc
{
[connection release];
}
#pragma mark -
#pragma mark boilerplate NSURLProtocol subclass requirements
- (void) startLoading
{
self.connection = [[NSURLConnection connectionWithRequest:[self request] delegate:self] retain];
}
- (void)stopLoading
{
[self.connection cancel];
}
- (void)connection:(NSURLConnection*)conn didReceiveResponse:(NSURLResponse*)response
{
[[self client] URLProtocol:self didReceiveResponse:response cacheStoragePolicy:[[self request] cachePolicy]];
}
- (void)connection:(NSURLConnection*)connection didReceiveData:(NSData*)data
{
[[self client] URLProtocol:self didLoadData:data];
}
- (void)connectionDidFinishLoading:(NSURLConnection*)conn
{
[[self client] URLProtocolDidFinishLoading:self];
}
- (NSURLRequest*)connection:(NSURLConnection*)connection willSendRequest:(NSURLRequest*)theRequest redirectResponse:(NSURLResponse*)redirectResponse
{
return theRequest;
}
- (void)connection:(NSURLConnection*)conn didFailWithError:(NSError*)error
{
[[self client] URLProtocol:self didFailWithError:error];
}
#end
Note that the protocol must be registered with [NSURLProtocol registerClass:[MyProtocol class]]; before you make any web requests to which you want it to apply - for instance, in applicationDidFinishLaunchingWithOptions:.