Understanding XCode debug area error messages - objective-c

Question

For example, I get this error in debug area:
2014-05-01 05:01:28.052 MyApp[93187:303] *** setObjectForKey: object cannot be nil (key: -1)
2014-05-01 05:01:28.057 MyApp[93187:303] (
0 CoreFoundation 0x00007fff8cb3f25c __exceptionPreprocess + 172
1 libobjc.A.dylib 0x00007fff89202e75 objc_exception_throw + 43
2 CoreFoundation 0x00007fff8ca3a46e -[__NSDictionaryM setObject:forKey:] + 1102
3 MyApp 0x0000000100009725 +[SystemTools getData:] + 5509
4 MyApp 0x0000000100001c4b -[AppDelegate drawCanvas:] + 1467
5 MyApp 0x0000000100001675 -[AppDelegate repeatingTimer:] + 85
6 Foundation 0x00007fff8ec480f4 __NSFireTimer + 96
7 CoreFoundation 0x00007fff8caa6564 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
8 CoreFoundation 0x00007fff8caa609f __CFRunLoopDoTimer + 1151
9 CoreFoundation 0x00007fff8cb175aa __CFRunLoopDoTimers + 298
10 CoreFoundation 0x00007fff8ca618e5 __CFRunLoopRun + 1525
11 CoreFoundation 0x00007fff8ca610b5 CFRunLoopRunSpecific + 309
12 HIToolbox 0x00007fff875dca0d RunCurrentEventLoopInMode + 226
13 HIToolbox 0x00007fff875dc7b7 ReceiveNextEventCommon + 479
14 HIToolbox 0x00007fff875dc5bc _BlockUntilNextEventMatchingListInModeWithFilter + 65
15 AppKit 0x00007fff907853de _DPSNextEvent + 1434
16 AppKit 0x00007fff90784a2b -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 122
17 AppKit 0x00007fff90778b2c -[NSApplication run] + 553
18 AppKit 0x00007fff90763913 NSApplicationMain + 940
19 MyApp 0x0000000100016aa2 main + 34
20 libdyld.dylib 0x00007fff9440b5fd start + 1
21 ??? 0x0000000000000003 0x0 + 3
)
Unfortunately, I have saveral "setObject:forKey:" calls in "SystemTools getData:" function.
How can I determine to which call this error is related? What that numbers (after plus) means?

Answer 1

Add an Execption breakpoint in your break points. This will halt at the line of code which is causing the app to crash. That way you dont have to debug with NSLogs.
The default behavior of Xcode is to stop where the exception is
caught, not where the exception is raised!
To explain those lines in the crash logs:
From Ray Wenderlich's blog:
2 XYZLib 0x34648e88 0x83000 + 8740
It is basically four columns:
The frame number – in this case, 2.
The name of the binary – in this case, XYZLib.
The address of the function that was called – in this case, 0x34648e88.
The fourth column is divided into two sub-columns, a base address and an offset. Here it is 0×83000 + 8740, where the first number points to the file, and the second points to the line of code in that file.

Answer 2

Use breakpoints in all of your setObject:forKey calls. Use NSLog for print some useful information in log. You are a developer so you need to know debugging also.

Answer 3

The first line of the output is useful here:
setObjectForKey: object cannot be nil (key: -1)
The NSDictionary doesn't like nil values, so look for a place in your getData method that could be assigning a nil value to a key of "-1".
Good luck!