Vincent Gable’s Blog

June 15, 2009

Ignoring Just One Deprecated Warning

Filed under: Bug Bite,C++,iPhone,Objective-C,Programming,Tips | , , , , ,
― Vincent Gable on June 15, 2009

Switching projects over to iPhone OS 3.0 means discovering that functions I’m using are deprecated. Occasionally there isn’t a totally straightforward replacement, and the best thing to do is to file a bug/TODO/note for myself, and ignore the warning until a later version, when major refactoring will be possible. But bitter experience has taught me to have Xcode treat warnings as errors1, so it’s necessary to trick the compiler into ignoring the warning for things to build.

-Wno-deprecated

The -Wno-deprecated flag tells GCC to suppress warnings about deprecated code. But adding it to an Xcode project means you won’t get useful warnings about other depreciated code.

You could file a bug telling yourself to turn that warning back on after the deprecated functionality has been updated. That should work just fine. But it feels like bad project hygiene to me.

Casting and Protocols

Type casting is a dangerous old-C technique that’s earned its infamy. But it’s undeniably fitting to use a deprecated language feature to get deprecated code to build. The basic idea is to declare a protocol that includes the method you want to suppress warnings for,

@protocol DeprecatedHack
- (void) myDeprecatedMethod;
@end

then just cast your objects so the compiler thinks they implement the protocol,

[foo myDeprecatedMethod]; //warnings
[(id<DeprecatedHack>>)foo myDeprecatedMethod]; //no warnings

Although having to declare a protocol is somewhat heavyweight, it leaves a nice artifact in the code reminding you to replace deprecated functionality.

Protocols Not Required

Sometimes just casting to id is enough. This happens if another object has a non-deprecated method with the same name.

1For experimental or prototyping projects I let warnings slide. But in the main project I always treat warnings as errors. Ignoring them in production code has never worked — warnings fester and grow on each other.

Because Objective-C is so dynamic, there are many errors that the compiler can warn you about, but can’t be totally sure are errors. For example, methods can be added to a class at runtime, so if you call -someMethodThatDoesNotExistAnywhere, the compiler will warn you that something is up, but won’t stop the build, because the necessary code could magically appear at runtime. Of course, 99% of the time, it’s me accidentally using count when I meant length, etc. What I’m really trying to say here is that treating warnings as errors is an even better idea in Objective-C.

June 11, 2009

Early Adopters Wanted!

Filed under: Announcement,iPhone | , , ,
― Vincent Gable on June 11, 2009

I am wrapping up work on Prometheus, an iPhone app that edits the Simple English Wikipedia.

Unfortunately, I am having trouble finding people to take the pre-release version for a spin, and tell me what they think. I want to be sure I’ve fixed any glaring issues before I push my work out to the App Store.

If you are interested in helping, please visit the Prometheus webpage.

Thank you!

May 1, 2009

NSXMLParser and HTML/XHTML

Filed under: Bug Bite,iPhone,Objective-C,Programming | , , , ,
― Vincent Gable on May 1, 2009

NSXMLParser converts HTML/XML-entities in the string it gives the delegate callback -(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string. So if an XML file contains the string, "&lt; or &gt;", the converted string "< or >" would be reported to the delegate, not the string that you would see if you opened the file with TextEdit.

This is correct behavior for XML files, but it can cause problems if you are trying to use an NSXMLParser to monkey with XHTML/HTML.

I was using an NSXMLParser to modify an XHTML webpage from Simple Wikipedia, and it was turning: “#include &lt;stdio&gt;” into “#include <stdio>“, which then displayed as “#include “, because WebKit thought <stdio> was a tag.

Solution: Better Tools

For scraping/reading a webpage, XPath is the best choice. It is faster and less memory intensive then NSXMLParser, and very concise. My experience with it has been positive.

For modifying a webpage, JavaScript might be a better fit then Objective-C. You can use
- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script to execute JavaScript inside a UIWebView in any Cocoa program. Neat stuff!

My Unsatisfying Solution

Do not use this, see why below:

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string;
{
	string = [string stringByReplacingOccurrencesOfString:@"<" withString:@"&lt;"];
	string = [string stringByReplacingOccurrencesOfString:@">" withString:@"&gt;"];

	/* ... rest of the method */
}

Frankly that code scares me. I worry I’m not escaping something I should be. Experience has taught me I don’t have the experience of the teams who wrote HTML libraries, so it’s dangerous to try and recreate their work.

(UPDATED 2009-05-26: And indeed, I screwed up. I was replacing & with &amp;, and that was causing trouble. While my “fix” of not converting & seems to work on one website, it will not in general.)

I would like to experiment with using JavaScript instead of an NSXMLParser, but at the moment I have a working (and surprisingly compact) NSXMLParser implementation, and much less familiarity with JavaScript then Objective-C. And compiled Obj-C code should be more performant then JavaScript. So I’m sticking with what I have, at least until I’ve gotten Prometheus 1.0 out the door.

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