I have an NSXMLElement that is a copy from somewhere else in my code. After it's been used, it no longer is connected to an NSXMLDocument. It's (what I believe to be) not linked to anything.
If I NSLog the NSXMLElement, I am given the contents. I can see it.
But, if I try to use nodesForXPath, it returns nothing. It's blank. It cannot find anything (unless I just search for *, in which case it returns everything with /t/t/n/t/t).
Now, if I make that NSXMLElement the root of a NEW NSXMLDocument, and then search the new document, I can search the XPath perfectly!
I am trying to understand the logic there. I have been reading the documentation, but I haven't found anything that explains what is happening here (or at least, I have no understood it if I did find it in the documentation).
I would really appreciate someone helping me understand exactly why this is happening, and whether or not I need to use this NSXMLDocument.
Here is my code:
(element is an NSXMLElement coming from an NSArray of stored NSXMLElements. They are copies from a document used in another method)
NSArray* scene = [element nodesForXPath:#"scene[1]" error:nil];
NSLog(#"scene: %#", [[scene objectAtIndex:0] stringValue]);
But if I do this, I get a result:
NSXMLDocument* docElement = [[NSXMLDocument alloc] initWithRootElement:element];
NSArray* scene = [docElement nodesForXPath:#"scene[1]" error:nil];
NSLog(#"scene: %#", [[scene objectAtIndex:0] stringValue]);
It's probably because the xpath context used by libxml2 holds a reference to the document. If there is no document, it probably doesn't know how to operate. The behavior if you search for * may just be a special-case because it knows it has to return everything so it doesn't even try to search.
Related
I would like to understand more about the way XCode/Objective-C handle constant strings. I found a related question, but I would like more information. Consider the following code:
NSString *a = [[NSString alloc] initWithUTF8String:[[_textFieldA stringValue] UTF8String]];
NSString *b = [[NSString alloc] initWithUTF8String:[[_textFieldB stringValue] UTF8String]];
NSString *c = [a copy];
NSString *d = [a mutableCopy];
Note that the textFields are just a way to set the strings at runtime ensuring that the compiler doesn't get too smart on me and build in a single instance.
If my text fields are empty, or contain a single character such as "x" or "$", then a == b == c == the same constant NSString instance. If I instead provide "xy", then a == c != b. d is always unique, as one might expect since it is mutable.
Now normally this wouldn't be an issue, I'm not trying to modify the contents of these strings, however, I am working on a system where I frequently use objc_setAssociatedObject. So here now I might come accross an empty string, and then set associated object data on it, and then have another empty string and collide with the first.
I have, for the moment, solved my issue by creating mutable strings instead.
So my questions:
Is this an Objective-C specification, or an XCode excentricity?
Does anyone know how the instance is determined? Why "x" get's one instance, but not "xy"? I would think some internal dictionary is involved and there's no good reason to stop at 1 character.
Is there a way to turn this off, so all empty strings are unique instances, or other suggestions?
I am using XCode 5.1.1, OSX 10.9.4, SDK 10.9.
Thank you!
Is this an Objective-C specification, or an XCode excentricity?
It is just implementation detail. Not documented any where. These kind of behaviour may changed in future without notice.
Does anyone know how the instance is determined? Why "x" get's one instance, but not "xy"? I would think some internal dictionary is involved and there's no good reason to stop at 1 character.
No until someone able to access source code want to share the details with us.
Is there a way to turn this off, so all empty strings are unique instances, or other suggestions?
No way to turn it off. Don't use objc_setAssociatedObject with NSString
As #Ken Thomases said in comment
In general, it probably doesn't make sense to use objc_setAssociatedObject() with any value class.
Some other examples are NSNumber, NSData and NSValue. They are often cached and reused.
I am creating an NSXMLDocument, from which I get a file defined in a constant USER_PRINTXML_URL, as follows:
NSXMLDocument *ads_user_printxml = [[NSXMLDocument alloc] initWithContentsOfURL: [NSURL URLWithString:USER_PRINTXML_URL] options:(NSXMLNodePreserveWhitespace|NSXMLNodePreserveCDATA) error:&ads_err];
Then I use XPath to get to the desired location in the XML file, as follows:
NSArray* ipp_folder = [[ads_user_printxml nodesForXPath:#".//root/printing/IPP"error:&ads_err] objectAtIndex:0];
Now, if I want to add NSXMLElements after the element in the XML file, how do I do it?
I have tried to do the following:
NSXMLElement *s= [ipp_folder objectAtIndex:0];
But this is generating a run time error. I also tried using an NSXMLNode instead of putting the data in an NSArray but again to no avail. I believe the solution is quite simple, but I can't for the life of me find an answer in the docs.
Thanks for any help.
-nodesForXPath:error: returns an NSArray*, but your second code snippet has already applied -objectAtIndex: to that. So, ipp_folder doesn't hold an array, it holds a pointer to an NSXMLNode. Fix its declared type.
Then, you can identify its parent using the -parent method and its index within the parent's children using the -index method. Then, assuming the parent is an NSXMLElement, you can do:
[[ipp_folder parent] insertChild:someNewNode atIndex:[ipp_folder index] + 1];
Probably some easy answer out there, but I wasn't able to find it. I am using RegexKitLite to manipulate some NSStrings in my iPhone app. I want to get the set of links from an entire HTML source from a webpage. I want it to return the array of links, not the rest.
My code:
NSString *regex = #"[0-9]{1,}.htm";
for (NSString *match in [sourcePage componentsSeperatedByRegex: regex]{
NSLog (#"%#", match); // Just to debug, will use data later.
}
The problem here is that the enire page is returned in small pieces, but exactly without the strings I want.
Any help is appreciated!
Use -componentsMatchedByRegex: instead.
I have a Mac application that keeps it's own log file. It appends info to the file using NSString's writeToFile method. One of the things that it logs are URL's of web services that it is interacting with. To encode the URL, I'm doing this:
searchString = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL, (CFStringRef)searchString, NULL, (CFStringRef)#"!*'();:#&=+$,/?%#[]", kCFStringEncodingUTF8 );
The app then appends searchString to the rest of the URL and writes it to the log file. Now the problem is that after adding that URL encoding line, nothing seems to be getting written to the file. The program functions as expected otherwise however. Removing the line of code above results in all of the correct information being logged to the file (removing that line is not an option because searchString must be URL encoded).
Oh and I am using NSUTF8StringEncoding when writing the NSString to the file.
Thanks for any help.
EDIT: I know there's also a similar function to CFURLCreateStringByAddingPercentEscapes in NSString, but I've read that it doesn't always work. Can anyone shed some light on this if my original question cannot be answered? Thanks! (EDIT: same problem occurs when using stringByAddingPercentEscapesUsingEncoding:)
EDIT 2: Here's the code that I'm using to append messages to the log file.
+(void)logText:(NSString *)theString{
NSString *docsDirectory = [NSSearchPathForDirectoriesInDomains(NSApplicationSupportDirectory,NSUserDomainMask,YES) objectAtIndex:0];
NSString *path = [docsDirectory stringByAppendingPathComponent:#"Folder/File.log"];
NSString *fileContents = [[[NSString alloc] initWithContentsOfFile:path] autorelease];
if([fileContents lengthOfBytesUsingEncoding:NSUTF8StringEncoding] >= 204800){
fileContents = #"";
}
NSString *timeStamp = [[NSDate date] description];
timeStamp = [timeStamp stringByAppendingString:#": "];
timeStamp = [timeStamp stringByAppendingString:theString];
fileContents = [fileContents stringByAppendingString:timeStamp];
fileContents = [fileContents stringByAppendingString:#"\n"];
[fileContents writeToFile:path atomically:YES encoding:NSUTF8StringEncoding error:NULL];
}
Because after almost a whole day no one else has offered any answers, I'm going to post a wild guess here: you're not accidentally using the string you want to output (with percent characters in it) as a format string are you?
That is, making the mistake of doing:
NSLog(#"In format strings you can use %# as a placeholder for an object, and %i for a plain C integer.")
Instead of:
NSLog(#"%#", #"In format strings you can use %# as a placeholder for an object, and %i for a plain C integer.");
But I'm going to be surprised if this turns out to be the cause of your problem, as it usually causes random-looking output, rather than absolutely no output. And in some cases, Xcode also gives compiler warnings about it (when I tried NSLog(myString), I got "warning: format not a string literal and no format arguments").
So don't shoot me down if this answer doesn't help. It would be easier to answer your question if you could show us more of your logging code. As for the one line you provided, I can't detect anything wrong with it.
Edit: Oops, I kind of missed that you mentioned you're using writeToFile:atomically:encoding:error: to write the string to the file, so it's even more unlikely you're accidentally treating it as a format string somewhere. But I'm going to leave this answer up for now. Again, you should really show us more of your code though ...
Edit: Regarding your question on a method in NSString that has similar percent encoding functionality, that would be stringByAddingPercentEscapesUsingEncoding:. I'm not sure what kind of problems you're thinking of when you say you've heard it doesn't always work. But one thing is that CFURLCreateStringByAddingPercentEscapes allows you to specify extra characters that don't normally have to be escaped but which you still want to be escaped, while the method of NSString doesn't allow you to specify this.
I'm new with the whole QTKit and I was looking for some feedback on the following code that I am attempting to use to display the camera's image and record movies.
- (void)initializeMovie {
NSLog(#"Hi!");
QTCaptureSession* mainSession = [[QTCaptureSession alloc] init];
QTCaptureDevice* deviceVideo = [QTCaptureDevice defaultInputDeviceWithMediaType:#"QTMediaTypeVideo"];
QTCaptureDevice* deviceAudio = [QTCaptureDevice defaultInputDeviceWithMediaType:#"QTMediaTypeSound"];
NSError* error;
[deviceVideo open:&error];
[deviceAudio open:&error];
QTCaptureDeviceInput* video = [QTCaptureDeviceInput deviceInputWithDevice:deviceVideo];
QTCaptureDeviceInput* audio = [QTCaptureDeviceInput deviceInputWithDevice:deviceAudio];
[mainSession addInput:video error:&error];
[mainSession addInput:audio error:&error];
QTCaptureMovieFileOutput* output = [[QTCaptureMovieFileOutput alloc] init];
[output recordToOutputFileURL:[NSURL URLWithString:#"Users/chasemeadors/Desktop/capture1.mov"]];
[mainSession addOutput:output error:&error];
[movieView setCaptureSession:mainSession];
[mainSession startRunning];
}
Also, I'm not sure about the whole error parameter that the methods keep calling for, I saw the "&error" symbol in an example but I don't know what it means.
I'm also getting an error "cannot initialize a device that is not open" when I explicitly open the devices.
If anyone could help me sort this out, it'd be a great help, thanks.
QTCaptureDevice* deviceVideo = [QTCaptureDevice defaultInputDeviceWithMediaType:#"QTMediaTypeVideo"];
QTCaptureDevice* deviceAudio = [QTCaptureDevice defaultInputDeviceWithMediaType:#"QTMediaTypeSound"];
Pass the actual constants here, not string literals containing their names. There's no guarantee that QTMediaTypeVideo is defined to #"QTMediaTypeVideo"; it could be #"Ollie ollie oxen free", and even if it is what you expect now, it could change at any time.
[output recordToOutputFileURL:[NSURL URLWithString:#"Users/chasemeadors/Desktop/capture1.mov"]];
Don't assume that the current working directory is /. Always use absolute paths. (I know this is test code; in real code, of course, you would have run an NSSavePanel and gotten the path from there.)
Also, I'm not sure about the whole error parameter that the methods keep calling for, I saw the "&error" symbol in an example but I don't know what it means.
The & means you're taking the address of a variable, which in this case is error. You're passing this address (a.k.a. pointer) to the error: argument of one of QTKit's methods. The method will, if it encounters an error, create an NSError object and store it at that address—i.e., in your variable. This is called “return-by-reference” (the “reference” being the pointer you provided).
I'm also getting an error "cannot initialize a device that is not open" when I explicitly open the devices.
Which method returns the error? Are you talking about an NSError, or just a Console message? If the latter, check your NSError variable and see what the problem method left behind.
This, incidentally, is why you should bail out if any of the QTKit methods returns an error: one of the subsequent messages may clobber it with a new error if you don't.
Also, you may want to look at the MyRecorder sample code. It's a fully functional video recorder based on the QTKit Capture API. The code is reasonably simple and should be easy to understand.