检查某些东西是否为零的更好方法？

Question

I'm trying to parse a XML file using TBXML . However, this parser has no error-checking built in so if an element doesn't exist it crashes. Here's how I'm parsing one of my XML files:

TBXML *XML = [[TBXML tbxmlWithXMLData:myxmlfile] retain];
if (XML.rootXMLElement) {
    TBXMLElement *XMLRoot = XML.rootXMLElement;
    if ([TBXML childElementNamed:@"blah" parentElement:XMLRoot]) {
        TBXMLElement *Blah = [TBXML childElementNamed:@"blah" parentElement:XMLRoot];   
        if ([TBXML childElementNamed:@"stuff" parentElement:Blah]) {
            TBXMLElement *Item = [TBXML childElementNamed:@"item" parentElement:Blah;
            if ([TBXML childElementNamed:@"stuff:blah" parentElement:Item]) {
                TBXMLElement *something = [TBXML childElementNamed:@"stuff:blah" parentElement:Item];
                NSString *Something = [TBXML textForElement:something];
                //do something here...
            }
            else {
               [self showFetchError];
                [XML release];
                return;}
        } else {
            [self showFetchError];
            [XML release];
            return;}
    } else {
        [self showFetchError];
        [XML release];
        return;}
} else {
    [self showFetchError];
    [XML release];
    return;
}

As you can see, it's making a call twice for each item. That seems like a huge waste of overhead to me. Any way I can do the same verification of each item without doing what I'm doing right now?

Answer 1

Here's a shorter version:

BOOL success = NO;
TBXML *XML = [[TBXML tbxmlWithXMLData:myxmlfile] retain];
if (XML.rootXMLElement) {
    TBXMLElement *XMLRoot = XML.rootXMLElement;
    if ([TBXML childElementNamed:@"blah" parentElement:XMLRoot]) {
        TBXMLElement *Blah = [TBXML childElementNamed:@"blah" parentElement:XMLRoot];   
        if ([TBXML childElementNamed:@"stuff" parentElement:Blah]) {
            TBXMLElement *Item = [TBXML childElementNamed:@"item" parentElement:Blah;
            if ([TBXML childElementNamed:@"stuff:blah" parentElement:Item]) {
                success = YES;
                TBXMLElement *something = [TBXML childElementNamed:@"stuff:blah" parentElement:Item];
                NSString *Something = [TBXML textForElement:something];
                //do something here...
            }
        }
    }
}

if (!success) {
    [self showFetchError];
    [XML release];
    return;
}

Updated version using enumerator:

TBXML *XML = [[TBXML tbxmlWithXMLData:myxmlfile] retain];
NSArray *path = [NSArray arrayWithObjects:@"blah", @"item", @"stuff:blah", nil];
NSEnumerator *e = [path objectEnumerator];
TBXMLElement *currentNode = XML.rootXMLElement;
BOOL success = NO;
while ((NSString *node = [e nextObject]) && currentNode) {
    if ([node isEqualToString:[path lastObject]]) {
        success = YES;
        NSString *Something = [TBXML textForElement:currentNode];
        // do the last element thing with string
    } else {
        currentNode = [TBXML childElementNamed:node parentElement:currentNode];
    }
}

if (!success) {
    [self showFetchError];
    [XML release];
    return;
}

I'm not sure if enumerator version works, but off top of my head it should solve your problem.

Answer 2

You can do things like:

TBXMLElement *Blah = [TBXML childElementNamed:@"blah" parentElement:XMLRoot];
if (Blah) {
    ...
}