Missing NSDate in scheduledTimerWithTimeInterval method?

A simple test --

- (void)viewDidLoad
{
    [super viewDidLoad];

    date2 = [NSDate dateWithTimeIntervalSinceNow:3];
    myTimer = [NSTimer scheduledTimerWithTimeInterval:1
                                                   target:self
                                         selector:@selector(timerFires)
                                                 userInfo:nil
           开发者_StackOverflow                                       repeats:YES];

}

- (void)timerFires{
    date1 = [NSDate date];
    NSTimeInterval timeBetween = [date1 timeIntervalSinceDate:date2];
    NSLog(@"follow: %f", timeBetween);  
}

It cause an "EXC_BAD_ACCESS" error in "timerFires"

Thread 1: Program received signal: "EXC_BAD_ACCESS".

I have declared 'date1' 'date2' and 'myTimer' in .h file.

It seems -(void)timerFires cannot got the value of 'date2'. Could you help me fix it please!

i'm guessing that your date variables are not staying around because you've never explicitly retained them. [NSDate dateWithTimeIntervalSinceNow:3] returns an autoreleased object. are you declaring the date instance variables as properties that are retained and synthesizing their getters and setters? for example, in your interface file:

@interface MyClass {
   NSDate *date1;
   NSDate *date2;
}

@property (nonatomic, retain) NSDate *date1;
@property (nonatomic, retain) NSDate *date2;

then in your implementation file:

@implementation
@synthesize date1, date2

then you should access them using self so they are properly retained:

self.date2 = [NSDate dateWithTimeIntervalSinceNow:3];

On doing date2 = [NSDate dateWithTimeIntervalSinceNow:3];, you are assigning an autoreleased object to date2. This is getting released before the timer fires. You will have to retain the object if you want to use it elsewhere. So do this –

date2 = [[NSDate dateWithTimeIntervalSinceNow:3] retain];

Remember, since you are retaining this you will need to release it afterwards.