Loading text file by AJAX call in UIWebView using custom NSURLProtocol fails

Question

I want to display a website on an iOS app using a UiWebView. Some components of the site (namely the webservice results loaded using AJAX calls) should be replaced by local data.

Consider the following example:

text.txt:

foo

page1.html:

<html><head>
    <title>test</title>
    <script type="text/javascript" src="jquery.js"></script>
</head>
<body>
<div id="target"></div>
<script type="text/javascript">
    function init(){
        $.get("text.txt",function(text){    
            $("#target").text(text);
        });
    }
    $(init);
</script>
</body></html>

View Controller:

@interface ViewController : UIViewController <UIWebViewDelegate>
    @property (nonatomic,assign) IBOutlet UIWebView *webview;
@end


@implementation ViewController
    @synthesize webview;
    //some stuff here
    - (void)viewDidLoad
    {
        [super viewDidLoad];
        [NSURLProtocol registerClass:[MyProtocol class]];
        NSString *url = @"http://remote-url/page1.html";
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url]];
        [request setCachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData];
        [webview loadRequest:request];
    }
@end

MyProtocol:

@interface MyProtocol : NSURLProtocol

@end

@implementation MyProtocol

+ (BOOL) canInitWithRequest:(NSURLRequest *)req{
    NSLog(@"%@",[[req URL] lastPathComponent]);
    return [[[req URL] lastPathComponent] caseInsensitiveCompare:@"text.txt"] == NSOrderedSame;
}

+ (NSURLRequest*) canonicalRequestForRequest:(NSURLRequest *)req{
    return req;
}

- (void) startLoading{

    NSLog(@"Request for: %@",self.request.URL);
    NSString *response_ns = @"bar";
    NSData *data = [response_ns dataUsingEncoding:NSASCIIStringEncoding];
    NSURLResponse *response = [[NSURLResponse alloc] initWithURL:[self.request URL] MIMEType:@"text/plain" expectedContentLength:[data length] textEncodingName:nil];

    [[self client] URLProtocol: self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageNotAllowed];
    [[self client] URLProtocol:self didLoadData:data];
    [[self client] URLProtocolDidFinishLoading:self];
    [response release];
}

- (void) stopLoading{
    NSLog(@"stopLoading");
}

@end

If I don't register my custom URLProtocol the page is displayed properly. If I do startLoading() is called, the content is loaded and stopLoading() is triggered afterwards. But on the UIWebView nothing happends at all. I tried to do some error-handling, but neither is a JS AJAX error thrown nor is didFailLoadWithError of the UIWebViewDelegate called.

I tried another scenario and created a HTML page that just loads an image:

<img src="image.png" />

and modified my URLProtocol to just handle the loading of the image - this works properly. Maybe this has anything to do with AJAX calls?

Do you have any idea what the problem might be?

Thanks in advance!

score 9 · Accepted Answer · edited May 23 '17 at 10:32

I had the same problem and finally solved it after days of hair pulling :

Your problem comes from the way you create the response, you have to create a status 200 response and force the WebView to allow cross-domain request if necessary :

NSDictionary *headers = @{@"Access-Control-Allow-Origin" : @"*", @"Access-Control-Allow-Headers" : @"Content-Type"};
NSHTTPURLResponse *response = [[NSHTTPURLResponse alloc] initWithURL:request.URL statusCode:200 HTTPVersion:@"1.1" headerFields:headers];

You can see my full working implementation in my answer here :

How to mock AJAX call with NSURLProtocol?

Hope this helps, Vincent

Loading text file by AJAX call in UIWebView using custom NSURLProtocol fails

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