NSScanner Loop Question

问题

I have an NSScanner object that scans through HTML documents for paragraph tags. It seems like the scanner stops at the first result it finds, but I need all the results in an array.

How can my code be improved to go through an entire document?

- (NSArray *)getParagraphs:(NSString *) html 
{
    NSScanner *theScanner;
    NSString *text = nil;

    theScanner = [NSScanner scannerWithString: html];

    NSMutableArray*paragraphs = [[NSMutableArray alloc] init];

    // find start of tag
    [theScanner scanUpToString: @"<p>" intoString: NULL];
    if ([theScanner isAtEnd] == NO) {
        NSInteger newLoc = [theScanner scanLocation] + 10;
        [theScanner setScanLocation: newLoc];

        // find end of tag
        [theScanner scanUpToString: @"</p>" intoString: &text];

        [paragraphs addObject:text];
    }

    return text;
}

回答1:

Disclaimer: To parse HTML, it's better to use a HTML parser like libxml's HTML 4 parser, especially to deal with arbitrary possibly malformed HTML. Anyway, since the question asks how to improve existing code using NSParser, I provide the following example. This will work in most cases but there are some corner cases where it won't. For seriuos HTML parsing, use a HTML parser.

Iterate until the scanner has exhausted all characters:

NSScanner* scanner = [NSScanner scannerWithString:html];
NSMutableArray *paragraphs = [[NSMutableArray alloc] init];
[scanner scanUpToString:@"<p" intoString:nil];
while (![scanner isAtEnd]) {
    [scanner scanUpToString:@">" intoString:nil];
    [scanner scanString:@">" intoString:nil];
    NSString * text = nil;
    [scanner scanUpToString:@"</p>" intoString:&text];
    if (text) { // if html contains empty paragraphs <p></p>, text could be nil
        [paragraphs addObject:text];
    }
    [scanner scanUpToString:@"<p" intoString:nil];
}
...
[paragraphs release];

回答2:

Do not use a scanner to parse HTML (and don't use regular expressions, either.... oh, the pain)*. The whole point of HTML is that it is a structured document that is designed to be traversed as a tree of nodes or object. Pretty much the entire DOM [Document Object Model] based industry is built around this.

Just use an XML parser as [well structured HTML is really just XML anyway]. NSXMLDocument (or -- if you need event driven -- NSXMLParser) will work grand.

Or, if you have to deal with malformed HTML (i.e. arbitrary server sewage), use a proper HTML parser.

This question/answer describes exactly that, with a solid example.

*Not to mention that parsing HTML is a "solved problem" in the industry. There is no need to roll a new one.

来源：https://stackoverflow.com/questions/6323677/nsscanner-loop-question

标签

iphone

objective-c

nsscanner