I was wondering if there is an effective way to debug problems in xcode while coding in Objective-C. I create webpages constantly and code in jquery and javascript where you can set various alert boxes in different places in your code to determine if your script is properly executing sections. Can you do something like that in xcode to make sure that your script is executing methods properly and creating variables properly?
Thanks
Use the debugger - that's what it is there for! Set breakpoints by clicking in the grey are next to the line of code you want to break on. When this line of code is going to be excuted, the debugger will kick in and highlight the current place in execution. You can hover the cursor over variables in the IDE to examine their values, view the current call-stack (to see here this code has been called from) and get a list of local variables to help track program state. You can modify variable properties here too which often makes debugging simpler.
Execute code line by line by 'Stepping Over' (cmd+shift+o), which executes the current line, 'Stepping Into' (cmd_shift+i) which steps into the current line of code (if it is a function), or 'Stepping Out' to return back up the call stack.
If you want to stick to 'old-school' printf style debugging, go with NSLoging output to console.
NSLog(#"this text appears");
prints the following to the console:
this text appears
To print some basic variable values:
CGFloat pi = 3.14;
NSString *aFloatString = [NSString stringWithFormat:#"%.2f", pi];
NSLog(#"pi is equal to: %#", aFloatString);
Prints:
pi is equa to: 3.14
Standard c formatters can be used in NSLog i.e %d for int, %.2f for a float to 2 decimal places etc. Use %# for NSString*s.
Remember that NSLog will remain in production code unless you #IFDEF it out of release builds (or something similar), so if you don't want a performance hit, or embarrassing console logs to accompany the app you will want to remove them.
I've been known to litter functions that dump the following to console - and it isn't good:
OUTPUT:
Number of vertices is: 1200
<Requested reduction>
Can I kick it?
....
....
YES. I. CAN!
Number of vertices is: 800
Could have done with removing things like that :|
yes, the debugger can do all the things you want to do (just set some breakpoints - right click where you want them - then build&debug)
You can try writing to the console.
NSLog(#"Some status message here");
NSLog(#"The value of myObject is:%#", myObject);
To view the output of your application, while running with Xcode, click Run->Console and you will see all of the output from your application.
Related
i try badly to extend (im a beginner with Objective-C) a pdf-viewer with PDF-included Outlines. The viewer is based on Apples PDFKit. (https://developer.apple.com/documentation/pdfkit/pdfoutline)
Thats what i have done so far:
PDFPage *page = [_pdfDocument pageAtIndex:_pdfDocument.pageCount-1];
PDFOutline *pdfOutline = [_pdfDocument outlineRoot];
NSLog(#"LOG of pdfOutline");
NSLog(#"%#", pdfOutline);
NSLog(#"%i", pdfOutline.numberOfChildren);
Thats gives me the following Output:
[3685:9776989] LOG of pdfOutline
[3685:9776989] <PDFOutline: 0x60c000203370>
[3685:9776989] 4
So far so good, but I need somehow the labels and the page numbers in an jsonObject (its necessary cause of using it later in a react-native callback). Im even not sure what the output of "pdfOutline" is.
I really have no idea how to start. The goal is clear, generate an json-object from the outlines.
That's just given you a pointer to the object. You need to use the pdfOutline.label method to get the text of the outline's label.
Outlines don't contain a page number, but a Destination object, which you can read using the .destination method; or an Action object. A Destination is a page number, page co-ordinates, and optional zoom level. An Action may be "Go to page", or a URL, or other.
Don't forget that page numbers in PDFKit start at 0, not 1. !!
I'm trying to build something that will only fire a command once per keyboard input (as opposed to every frame like QC does natively). In order to do so, I'm trying to listen in on the keyboard inputs (via Freeboard) and compare the current input versus a previous version.
What seems to be happening is the previous version is getting wiped every time the patch executes, so my conditional to compare strings is failing every time. Here's some code to make it a bit clearer:
- (BOOL)execute:(id <QCPlugInContext>)context atTime:(NSTimeInterval)time withArguments:(NSDictionary *)arguments
{
self.outputPrevious=previousCharacter;
if ([self.inputCharacter caseInsensitiveCompare:previousCharacter]){
self.outputText=#"SAME";
}
else {
self.outputText=#"CHANGE";
}
previousCharacter = [NSString stringWithString:self.inputCharacter];
[previousCharacter retain];
return YES;
}
where self.outputText is the text that tells me the result of the if, self.outputPrevious is telling me what the previous character input was, and self.inputCharacter is the current keyboard input.
previousCharacter is defined in the header and instantiated in -init, so it shouldn't be being reset every time.
I've tried pretty much everything with this, so if you have any ideas or insights, that would be awesome. Thanks!
Figured it out eventually. Full solution can be found here
When I use NSLog, I get output similar to the following:
2012-01-24 17:05:32:860 App[21856:71939] {logging goes here}
I recognize that '2012-01-24 17:05:32:860' is the date, 'App' is the app name, but I have no clue what '[21856:71939]' means. Can someone fill me in on what that is and where it's generated at?
All I'm trying to do is get logging that lines up nicely so it's easy to read, but the '[21856:71939]' varies in digits enough to mess up any alignment attempts. If I knew how the numbers in '[21856:71939]' were generated, I could add spaces as needed to make it line up correctly, but that's my only idea at this point.
Any help would be much appreciated :)
21856 is the process id. 71939 is the thread id.
You can generate this portion of the log on your own using:
[NSString stringWithFormat:#"[%ld,%lx]",
(long) getpid(),
(long) pthread_mach_thread_np(pthread_self())];
Edit 2014-09-23:
At least on the simulator in iOS 8, the second number is now the pthread_threadid_np of the thread.
__uint64_t threadId;
if (pthread_threadid_np(0, &threadId)) {
threadId = pthread_mach_thread_np(pthread_self());
}
[NSString stringWithFormat:#"[%ld,%llu]", (long) getpid(), threadId]
IIRC, the 21856 is the process PID, and the 71939 is some sort of thread identifier.
It's a thread ID, but I don't actually know how they are generated.
you can grab it yourself using this
pthread_mach_thread_np(pthread_self())
but that doesn't answer how you would set it up with correct lining. On the other note, when you're debugging, go to the last tab (Show the log Navigator), double click on Debug App, it'll show up nicely
I know how to do this in gdb. I'd attach, and follow with:
break myfunction
commands
return
cont
end
cont
I'm wondering if there's a way of doing this in c? I already have my code working for reading memory addresses and writing to memory addresses. And it automatically finds the pid and does related stuff. I'm stuck with implementing that use of breakpoints.
If you are talking about some sort of hand-written debugger, you can use IP value to set a breakpoint; Literally, when IP hits some certain value, you stop the program being debugged and perform some routine (for example, heading away to debugger process). To use function names, you should use symbol tables like it is done in GDB.
It's not quite clear what you are trying to achieve.
The GDB sequence you've show will simply make myfunction immediately return.
Assuming you want your mini-debugger to have the same effect, simply write the opcode for ret (0xC3 on x86) to the address of myfunction; no need to do the breakpoint at all.
I'm writing a simple QR code generator (just for fun and to learn some Obj-C), and I'm working on tracing the outline of connected "modules" (i.e. the black squares that make up a QR code). This is in order to have nicer vector output than simply making a bunch of rects for each module.
Long story short, my outline-tracing code works - BUT ONLY if I make sure to call NSLog in a specific place! If I remove the NSLog-call, the code loops! I'm literally doing nothing but logging. And it doesn't matter what I log; I just have to call NSLog or things break.
The tracing algorithm is simple enough: Go clockwise around the shape of connected modules. When you hit a corner, turn right until you're back to following the outline of the shape. Stop when you reach the starting point again. The shape can have two modules that share a corner-point. The tracing-loop will thus hit that point twice. This is expected, and the code handles it correctly - if I call NSLog.
Otherwise, the code will say that a certain point is a corner the first time it sees it, and not a corner the second time, which causes the tracing to loop around. Detecting if something's a corner-point is not dependent on anything except the x and the y coordinates of the point and an array of module objects - but neither the modules nor the array changes while the tracing is going on, so given the same x,y you should always get the same result. And it does – if I call NSLog.
Without NSLog, the coordinates – e.g. (10,9) – is corner on moment, and a moment later (10,9) is suddenly not a identified as a corner. But with an NSLog-call, (10,9) is correctly seen as a corner-point every time.
Again: I change absolutely nothing; I just log something - anything! And suddenly it works. It's like it's saying that 2 == 2 is true or false, unless I tell it to log 2 and 2, in which case 2 == 2 is always true, as it should be.
Here's the flaky code. It's hard to understand out of context, but there's a lot of context, so I hope this is enough. Everything is integers (no fuzzy floating point values).
do { // start going around the shape
// If this isn't here or simply commented out, the code loops.
NSLog(#"foobar"); // doesn't matter what I log - I just need to log something
// Branch: Is current x,y a corner-point? This should
// always return the same result given the same X and Y
// values, but it only does if NSLog is there!
if( [self cornerAtX:x Y:y] ) {
// add the point to the path
[path addPoint:NSMakePoint(x, y)];
// rotate the direction clockwise, until
// the direction is following the edge of the
// the shape again.
do {
dt = dx;
dx = -dy;
dy = dt;
} while( ![self boundaryFromX:x Y:y inDirectionX:dx Y:dy] );
}
// continue along direction
x += dx;
y += dy;
} while( !(sx == x && sy == y) ); // we're back to the start of the shape, so stop
If anyone can tell me why NSLog can make code work (or rather: Why not using NSLog makes working code break), I'd be happy to hear it! I hope someone can make sense of it.
Make sure cornerAtX:Y: always returns something—i.e., that there's no code path that fails to return a value.
Otherwise, it may very well “return” whatever the last function you called returns, in which case calling NSLog (which doesn't return a value, but may ultimately call a function that does) causes it to “return” something different, which may always be something that's considered true.
The compiler should warn you if you fail to return a value from a function or method that you declared as doing so. You should listen to it. Turn on all the warnings you can get away with and fix all of them.
You should also turn on the static analyzer (also included in that post), as it, too, may tell you about this bug, and if it does, it will tell you step-by-step how it's happening.
There's not much to go on here, but I'd guess that it's either an uninitialized variable or some sort of memory stomping. NSLog probably uses both stack and heap memory, so it could affect those.
Have you tried replacing NSLog with some other meaningless operation? If that will also work then I suppose problem is linked to [self cornerAtX: x Y: y].
Another possibility is that the problem is time-related. NSLog takes time to execute, so if QR code is loaded in another thread you can see this kind of behavior.