.net barcode sdk open source Figure 5-14 The debugger notifies you when a watchpoint s value changes in Objective-C

Maker QR Code ISO/IEC18004 in Objective-C Figure 5-14 The debugger notifies you when a watchpoint s value changes

Figure 5-14 The debugger notifies you when a watchpoint s value changes
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1 Create a new Utility Application named Debug 2 Open FlipsideViewxib in Interface Builder and remove the File s Owner s outlet connected
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to the view (Figure 5-15)
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3 Select Build and Run from the Build menu to run the application Click the Info button,
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and the application crashes Although the console displays the exact error message, suspend disbelief and pretend like it didn t
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4 Quit the iPhone Simulator and return to Xcode 5 Open MainViewControllerm and add a breakpoint at the second line in showInfo
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(Figure 5-16)
Figure 5-15 Remove the view from the view controller
(continued)
iPhone SDK Programming: A Beginner s Guide
Figure 5-16 Debugger window stopped at breakpoint in showInfo
6 Select Build and Debug from the Run menu to run the application in the debugger If the
debugger window is not visible, from the Run menu select Debugger
NOTE
Build and Go builds an application and then either runs or debugs the application, depending upon your previous choice 7 Try stepping over the next few lines and the application crashes You now know exactly
which line in your code causes the application to crash Something about presenting the FlipsideViewController caused the crash
Forgetting to set a File s Owner view outlet is a common mistake
5:
Deploying to an iPhone, Debugging, and Testing
Figure 5-17 Adding objc_exception_throw as a breakpoint
8 Stop the iPhone Simulator and return to Xcode Remove the breakpoint 9 From the Run menu, select Show | Breakpoints 10 On the Debug-Breakpoints window, double-click Double-Click For Symbol and add a
new breakpoint at the objc_exception_throw method by entering this method to the list (Figure 5-17)
11 Click Build And Go on the Debug-Breakpoints window to launch the application in the
debugger
12 Click the Info button, and the application halts at the newly set breakpoint Open the
debugger window, if it is not already open (Figure 5-18)
13 Notice the upper-left window This contains the stack listing Follow the stack down several
items, and you see the last thing to occur prior to an NSException is loading the view from the nib Follow the stack to row 6, and you see the last line of code in the view controller that was executed in the lower pane (Figure 5-19) So you know trying to load the view from the nib caused the crash
iPhone SDK Programming: A Beginner s Guide
Figure 5-18 Debugger window paused at objc_exception_throw breakpoint
NSZombieEnabled
When an object is deallocated, if there are any objects with a reference to the deallocated object, they are no longer referencing a valid object Any messages sent to the deallocated object result in errors Often, the error is rather cryptic For instance, the following code fragment is obviously an error
FooBar * myFooBar = [[FooBar alloc] init]; NSMutableArray * myArray = [[NSMutableArray alloc] initWithObjects: myFooBar,nil]; [myFooBar dealloc]; [[myArray objectAtIndex:0] sayHello];
FooBar is allocated, initialized, and added to myArray There are two references to myFooBar, so its retainCount is two However, deallocating myFooBar makes both references invalid, pointing to deallocated memory space The sayHello message is sent to the first object in myArray the problem is that the object no longer exists Although in this simple example
5:
Deploying to an iPhone, Debugging, and Testing
Figure 5-19 Following the stack trace takes you to an error s source
it is easy enough to surmise the cause of the error message, in a real application, finding this type of error s source is often difficult
objc[1289]: FREED(id): message sayHello sent to freed object=0x521a90 Program received signal: "EXC_BAD_INSTRUCTION"
Zombies help avoid this nebulous error, helping you track down an error s source You enable zombies by setting the NSZombieEnabled environment variable in Xcode Then, when debugging the application, rather than releasing an object, the debugger creates a zombie object The zombie knows its original identity before joining the undead The result, though, is you usually receive a more descriptive error message
2009-02-28 12:28:38749 Zombie[1316:20b] *** -[FooBar sayHello]: message sent to deallocated instance 0x52c6a0
iPhone SDK Programming: A Beginner s Guide
Again, in this simple example, the difference is trivial; in a real-world project, the difference is not trivial The following task illustrates using NSZombieEnabled
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Enabling Zombies
1 Create a new View-based Application named Zombie 2 Create a new Objective-C class called FooBar 3 Create one method called helloThere (Listing 5-2) Don t forget to put the method s
signature in FooBar s interface (Listing 5-3) Listing 5-2 FooBarm
#import "FooBarh" @implementation FooBar - (void) sayHello { NSLog(@"Hello there"); } - (void) dealloc { [super dealloc]; } @end
Listing 5-3
FooBarh
@interface FooBar : NSObject { - (void) sayHello; @end 4 Modify applicationDidFinishLaunching in ZombieAppDelegate (Listing 5-4) Don t forget
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