http://github.com/gabriel/yajl-objc
I've already tried SBJSON, and while it works, I'm looking into alternative options to improve on parsing speed and memory consumption. Usage of this library doesn't seem to be as straightforward as SBJSON though, and I'm not sure how to begin using yajl.
Something like this:
NSArray *parsed = [data yajl_JSON];
Results in the following error:
-[NSConcreteMutableData yajl_JSON]: unrecognized selector sent to instance 0x5372180
Attempting to parse an NSString object results in the same problem. I can see the interface, but there doesn't seem to be an implementation... Am I not linking into the static lib properly?
Google turns up very little on usage examples. The doc on for the project itself only says the following about generating objects from json data/strings.
#import "NSObject+YAJL.h"
NSData *JSONData = [NSData dataWithContentsOfFile:#"example.json"];
NSArray *arrayFromData = [JSONData yajl_JSON];
NSString *JSONString = #"[\"Test\"]";
NSArray *arrayFromString = [JSONString yajl_JSON];
Which looks pretty much the same as what I've tried. What am I missing here?
You must have missed that part:
Under 'Other Linker Flags' in the Test target, add -ObjC and -all_load (So NSObject+YAJL category is loaded).
I had no problems with using YAJL, until I ran the app in Release mode, then I got the same error as you - turns out, I've only added these flags to Debug mode, not to all of them.
In general, I can recommend YAJL, it's definitely faster than all other alternatives (see benchmark results on my blog).
Related
I have a very bad code written in my program, was just playing around as I am learning Objective C and iOS platform. What I did is,
I have created NSMutableArray like this,
placeInfo = [NSMutableArray array];
and than later in my code I am doing something like this, basically I am manipulating Google places api response(JSON).
NSDictionary *results = [responseString JSONValue];
placeInfo = [results objectForKey:#"result"];
self.phoneNumber = (NSString *)[placeInfo objectForKey:#"formatted_phone_number"]; // In this line compiler warns me that NSMutableArray might not response to this.
I checked documentation but I didn't find objectForKey in NSMutableArray.
So what could be the reason? Why my code isn't crashing? Why it is returning phone number by "formatted_phone_number" key?
EDIT
After first answer I have edited my code and added type casting like this, but it still works.
NSDictionary *results = [responseString JSONValue];
placeInfo = (NSMutableArray *)[results objectForKey:#"result"];
self.phoneNumber = (NSString *)[placeInfo objectForKey:#"formatted_phone_number"];
I’ve never used the Google Places API, but I’d guess [results objectForKey:#"result"] actually returns another dictionary, so the objectForKey: works.
Because objective-c just uses pointers to refer to objects, it’s never actually being converted to an NSMutableArray. Also, objective-c doesn't know at compile time if a method will exist, due to its dynamic nature (you can actually add methods and even whole classes at runtime).
Depending on the compiler settings, it may just show a warning that objectForKey: might not be found at runtime, and let it continue compiling anyway. It ends up working just fine if you actually passed it an NSDictionary.
Even when you put the (NSMutableArray *) cast in front of it, it won’t change anything. That simply casts the pointer type, and doesn’t actually change the object in any way.
It's doing this because [results objectForKey:#"result"] is returning you something that is not an NSMutableArray. That something that's being returned is likely another NSDictionary which, of course, does respond to objectForKey: To find out what you've got, set a breakpoint after result = [placeInfo objectForKey:#"result"] and inspect result. The debugger will tell you what kind of object you're dealing with. I'll bet you anything you like that it's an NSDictionary.
Objective C allows you to send any message (called a selector) to any object at any time; the runtime does not care whether a particular object implements a given selector. If the target object does not respond to a given selector it will ignore it. It will not crash. In this respect it's utterly unlike most other OOP languages, including C++ and Java, which will fail to compile if you try to call a method that a particular class doesn't implement. You can find out if an object responds to a given selector (which is analagous to using introspection to see if a given class implements a certain method) by saying [result respondsToSelector:#selector(objectForKey:)]. This difference between methods and messages is critically important to understanding Objective C. I'd recommend reading The Objective C Programming Language before doing anything else.
Also, Objective C's type system is less stringently enforced than those other languages. It's quite legal (although a very bad idea) to do what you have done here, which is to declare a pointer of type NSMutableArray and then assign it to (I'm guessing) an NSDictionary.
The line below has an error, can you see it with a naked eye?
NSString *title = [sender titleForState:UIControlStateNormal];
NSString *newText = [[NSString alloc] initWithFormat:"%# button pressed.", title];
When the Iphone simulator executes this line it crashes without any other information. I must be missing something about debugging. The console showed me nothing.
I was able to figure out the error, my question is not about that. My question is why I get a hard crash with no help from XCode. Without any clue it took me precious 5 minutes before I could realize my typo.
Any suggestions? Or when programming for IPhone I just need to be very careful with typos?
EDIT: I guess some people did not see it immediately like me. The error is the lack of '#' for the string constant. So now my question is why XCode/Simulator did not show me any kind of error message, just crashed without any clues. Am I missing something about debugging?
Objective-C does not strongly verify that the arguments you pass to messages are of the right type during compilation nor at runtime. It should gives you a warning though. Here you pass a c string instead of a NSString. Because NSString are objects (thus referenced using pointer), your method uses it as a pointer while you feed it with a simple string. You then probably try to access unaccessible memory blocks...
I think you miss a # before the "%# button pressed".
The correct one should be:
NSString *newText = [[NSString alloc] initWithFormat:#"%# button pressed.", title];
All the constant NSString should be #"SOMETHING HERE";
Check you compilation warnings. That's all you need. On the case you are showing, you will get a proper warning that will alert you that bad things might happen at that line.
I get the following Error when compiling your code:
error: cannot convert 'const char*' to 'NSString*' in argument passing
Not sure what you need to do to get it to show you that, I'm working in Obj-C++.
Try adding "-Wall" to your "OtherWarningFlags" under your target's build settings.
A web service provides data in the form of a plist. After the download, I have all of the data in an NSData object, which I want to convert to an NSDictionary. Right now the only way I know of to do that without parsing it by hand (yuck) is this:
static NSString *fileName = #"tempFile";
[data writeToFile: fileName atomically: NO];
NSDictionary *dict = [NSDictionary dictionaryWithContentsOfFile: fileName];
I'm surprised NSDictionary doesn't have a dictionaryWithData: method. I wouldn't even mind converting the NSData to an NSString first, if dictionaryWithString: existed. Clearly the SDK is doing something similar under the hood, but it isn't exposed to developers.
Can anyone suggest a better way to do this? One of the files I need to convert can potentially be pretty big, and doing the write-to-file+read-from-file operations could be a bit slow.
Use one of the following:
+[NSPropertyListSerialization propertyListFromData:mutabilityOption:format:errorDescription:]
+[NSPropertyListSerialization propertyListWithData:options:format:error:]
The former is available in iOS 2.0+ but has been deprecated in 4.0.
My code crashes at this function (at the stringByAppendingFormat: with error objc_msgSend() selector name: stringByAppendingFormat).
This is that line:
// imagesPath = ...iPhone Simulator/4.0/Applications/NUMBERS/Documents/images
UIImage *image = [[UIImage alloc] initWithContentsOfFile:[imagesPath stringByAppendingFormat:#"/%d.png", [[self.postsArrayID objectAtIndex:row] intValue]]];
Could it have something to do with the retaining of objects?
Thanks :)
> rootPath =
> [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
> NSUserDomainMask, YES)
> objectAtIndex:0]; imagesPath =
> [rootPath
> stringByAppendingPathComponent:#"/images/"];
Hah! Setting a property and setting
value using self.imagesPath =... fixed
it. Obj-c is so hard to understand
sometimes...
The methods you used to set the get the paths are autoreleased, so when you tried to access them later they had already died. Using the self.imagesPath property will retain the data (you specified it as (nonatomic, retain) - so it will stay around until you release it (or assign anything else using the property accessor self.imagesPath = ....;
Apple's memory management guide is highly recommended, though it is still easy to fail after reading it a couple of times. :-)
Usually a crash in objc_msgSend() implies the message being passed to the object (in this case, stringByAppendingFormat) is not specified for that object. Quick googling reveals that many top pages for stringByAppendingFormat are quite dated, inferring the API has possibly been deprecated in favor of something else.
As a workaround, it would seem +[NSString stringWithFormat:] would be a viable alternative for your use case.
Why not use stringByAppendingPathComponent:? And surely imagesPath is not ........../NUMBERS/images? Would it not be ................/<random ID>/images?
I've familiarized myself with the NSXMLParser from the iPhone SDK but I find the event-driven nature of it awkward for my purposes. I just want to extract some element values but this concept of having to handle the startElement, foundCharacters, and endElement seems like more work than it really should be. Am I just looking at this the wrong way or is there a simpler tree/DOM-based way of working with XML in the iPhone SDK?
If the advice is to just work with NSXMLParser, are there certain design patterns I can use to keep my code from having 5 levels of nested ifs in the startElement method?
If you're on the iPhone, using tree-based parsing can be a prohibitive memory hog. Trust me, I've been there, and I've tried many different approaches over the last five months of development of my main iPhone application. Tree-based parsing works fine until you download someone's comment stream containing 400 very long comments, clocking in at about 600KB of raw data. Quite aside from the size of the resultant XML tree, the memory allocated internally while creating that tree can be enormous.
I wound up creating a variant of NSXMLParser which pulls data in from a supplied NSInputStream rather than using a single chunk of data, and which passes only 1KB at a time into libxml for handling (NSXMLParser uses libxml too, but passes 100% of the data in one go).
The source code is available on github (look in the StreamingXMLParser folder). You'll also find a delegate superclass in there; for most parsing needs you can subclass AQXMLParserDelegate and implement -start[Element]WithAttributes: (NSDictionary *) attrs and -end[Element] in your subclass. These methods will be called for you as start & end tags are discovered, and inside the end tag you can use self.characters to access the content characters or CDATA of the element.
For more on the relative memory footprints of the different parsers (albeit on the Mac, not the iPhone) see my original blog post here and the followup on NSXMLDocument here.
Consider the following code snippet, that uses libxml2, Matt Gallagher's libxml2 wrappers and Ben Copsey's ASIHTTPRequest to parse an XML document.
The nodes instance of type NSArray* will contain NSDictionary* objects that you can parse recursively to get the data you want.
Or, if you know the scheme of your XML document, you can write an XPath query to get you to a nodeContent or nodeAttribute value directly.
ASIHTTPRequest *request = [ASIHTTPRequest alloc] initWithURL:[NSURL URLWithString:#"http://stackoverflow.com/"];
[request start];
NSError *error = [request error];
if (!error) {
NSData *response = [request responseData];
NSLog(#"Root node: %#", [[self query:#"//" withResponse:response] description]);
}
else
#throw [NSException exceptionWithName:#"kHTTPRequestFailed" reason:#"Request failed!" userInfo:nil];
[request release];
...
- (id) query:(NSString *)xpathQuery withResponse:(NSData *)respData {
NSArray *nodes = PerformXMLXPathQuery(respData, xpathQuery);
if (nodes != nil)
return nodes;
return nil;
}
Repurposing the code from Seismic XML provides a very good API that creates NSObject subclasses from XML.
If the advice is to just work with NSXMLParser, are there certain design patterns I can use to keep my code from having 5 levels of nested ifs in the startElement method?
I depends on what you are trying to do. You could put your element names in a dictionary and take action based on the relevant object in a dictionary - this is effectively what SeismicXML does.