Data (char*) appears corrupt when logged after pressing button - ios

Question

I'm quite newbie to iphone programming.
The problem is when I push a button, a data will be destroyed.
I can't find where my code is wrong and why. Please help me.
This is overview of my program.
1. load text data from "sample.txt"
2. log the data
3. When I push a button, it logs the data again.
AppDelegate.h
#import <UIKit/UIKit.h>
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#property (strong, nonatomic) UIButton *myButton1;
#property (assign) unsigned char* bytePtr;
#end
AppDelegate.m:
~snip~
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
// Override point for customization after application launch.
self.window.backgroundColor = [UIColor whiteColor];
[self.window makeKeyAndVisible];
self.myButton1 = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[self.myButton1 setFrame:CGRectMake(0, 0, 100 ,100)];
[self.myButton1 addTarget:self action:#selector(button1DidPushed) forControlEvents:UIControlEventTouchUpInside];
[self.myButton1 setTitle:#"push" forState:UIControlStateNormal];
[self.window addSubview:self.myButton1];
[self load];
return YES;
}
- (void) load
{
NSString* path = [[NSBundle mainBundle] pathForResource:#"sample" ofType:#"txt"];
NSData* data = [NSData dataWithContentsOfFile:path];
self.bytePtr = (unsigned char *)[data bytes];
NSLog(#"%s", self.bytePtr);
}
- (void)button1DidPushed
{
UIAlertView *alerView = [[UIAlertView alloc] initWithTitle:#"Enter something..." message:#"" delegate:self cancelButtonTitle:#"Cancel" otherButtonTitles:#"OK", nil];
alerView.alertViewStyle = UIAlertViewStylePlainTextInput;
[alerView show];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
NSString *title = [alertView buttonTitleAtIndex:buttonIndex];
if([title isEqualToString:#"OK"])
{
}
NSLog(#"%s", self.bytePtr);
}
~snip~
sample.txt:
abcdefghijklmnopqrstuvwxyz
output:
abcdefghijklmnopqrstuvwxyz
<----- When I push the button the mark similar to "?" will appear on output window.(I couldn't show it here(It's probabily ascii code 2(STX))) That is why I think the data is destroyed.
Environment:
xcode 6.0.1
Thanks.

Answer 1

The issue is that you are not retaining the NSData object:
- (void) load
{
NSString* path = [[NSBundle mainBundle] pathForResource:#"sample" ofType:#"txt"];
NSData* data = [NSData dataWithContentsOfFile:path]; // here
self.bytePtr = (unsigned char *)[data bytes];
NSLog(#"%s", self.bytePtr);
}
Once that method returns the NSData object will be destroyed and therefore the buffer pointed to by self.bytePtr is no longer valid.
To solve this issue change self.bytePtr to an NSData object and store that instead.

Answer 2

First of all, in order to receive your alertView:clickedButtonAtIndex: method call, you need to set your UIAlertView delegate to self.
[alertView setDelegate:self] // make sure you set UIAlertViewDelegate protocol on the method owner
Otherwise you should use an NSString object initialized using initWithData:encoding:.