从UITableView委托方法访问实例变量

问题:

我正在尝试从UITableView访问变量 tableView:didSelectRowAtIndexPath:

applicationDidFinishLaunching

奇怪的是,如果我这样声明变量,问题就不会出现:

helloWorld = @"Hello World!";

helloWorld = [NSString stringWithFormat: @"Hello World!"];

对这里可能发生的事情有什么想法吗?我错过了什么?

UntitledAppDelegate.h:

#import <UIKit/UIKit.h>

@interface UntitledAppDelegate : NSObject <UIApplicationDelegate, UITableViewDelegate, UITableViewDataSource>  {
    UIWindow *window;
    NSString *helloWorld;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

#import "UntitledAppDelegate.h"

@implementation UntitledAppDelegate

@synthesize window;


- (void)applicationDidFinishLaunching:(UIApplication *)application {

    helloWorld = [NSString stringWithFormat: @"Hello World!"];

    NSLog(@"helloWorld: %@", helloWorld); // As expected, logs "Hello World!" to console.

    [window makeKeyAndVisible];
}

- (void)dealloc {
    [window release];
    [super dealloc];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
     return 1;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *MyIdentifier = @"MyIdentifier";    
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier]; 
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:MyIdentifier] autorelease];
    }   
    cell.textLabel.text = @"Row";
    return cell;
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
    NSLog(@"helloWorld: %@", helloWorld); // App crashes
}

@end