Cocoa – What’s the optimum way of storing an NSDate in NSUserDefaults

cocoadatetimensdatenstimeintervalnsuserdefaults

There's two ways of storing an NSDate in NSUserDefaults that I've come across.

Option 1 – setObject:forKey:

// Set
NSDate *myDate = [NSDate date];
[[NSUserDefaults standardUserDefaults] setObject:myDate forKey:@"myDateKey"];

// Get
NSDate *myDate = (NSDate *)[[NSUserDefaults standardUserDefaults] objectForKey:@"myDateKey"];

Option 2 – timeIntervalSince1970

// Set
NSDate *myDate = [NSDate date];
NSTimeInterval myDateTimeInterval = [myDate timeIntervalSince1970];
[[NSUserDefaults standardUserDefaults] setFloat:myDateTimeInterval forKey:@"myDateKey"];

// Get
NSTimeInterval myDateTimeInterval = [[NSUserDefaults standardUserDefaults] floatForKey:@"myDateKey"];
NSDate *myDate = [NSDate dateWithTimeIntervalSince1970:myDateTimeInterval];

Pros and Cons

Option 1

This seems to be compact and logical. However, I've got worries about this going wrong because of Date Formatter bugs.

Option 2

This seems to be clumsy. I'm also unsure of the accuracy of it – in one test I did, when I retrieved the date back it was out by 48 seconds, despite the Apple Docs saying that NSTimeInterval has "subsecond accuracy".

Requirements

Whatever method I choose, it must be:

Precise to within a second.
Readable and reliable.

My Question

Is the inaccuracy with Option 2 because I'm doing something wrong?

Which out of these two options would you use?

Is there another option that I'm not aware of?

Thanks!

Best Answer

You are needlessly complicating things. Why are you converting the date to a time interval (then the time interval to a different primitive)? Just [sharedDefaults setObject:theDate forKey:@"theDateKey"] and be done with it. NSDate is one of the "main types" supported by the PLIST format (dates, numbers, strings, data, dictionaries, and arrays), so you can just store it directly.

See the documentation for proof.

Just store and retrieve the date directly and watch it Do The Right Thing (including time zones, precision, etc). There is no formatter involved, as others have said.

Related Solutions

Objective-c – What’s the Best Way to Shuffle an NSMutableArray

I solved this by adding a category to NSMutableArray.

Edit: Removed unnecessary method thanks to answer by Ladd.

Edit: Changed (arc4random() % nElements) to arc4random_uniform(nElements) thanks to answer by Gregory Goltsov and comments by miho and blahdiblah

Edit: Loop improvement, thanks to comment by Ron

Edit: Added check that array is not empty, thanks to comment by Mahesh Agrawal

//  NSMutableArray_Shuffling.h

#if TARGET_OS_IPHONE
#import <UIKit/UIKit.h>
#else
#include <Cocoa/Cocoa.h>
#endif

// This category enhances NSMutableArray by providing
// methods to randomly shuffle the elements.
@interface NSMutableArray (Shuffling)
- (void)shuffle;
@end


//  NSMutableArray_Shuffling.m

#import "NSMutableArray_Shuffling.h"

@implementation NSMutableArray (Shuffling)

- (void)shuffle
{
    NSUInteger count = [self count];
    if (count <= 1) return;
    for (NSUInteger i = 0; i < count - 1; ++i) {
        NSInteger remainingCount = count - i;
        NSInteger exchangeIndex = i + arc4random_uniform((u_int32_t )remainingCount);
        [self exchangeObjectAtIndex:i withObjectAtIndex:exchangeIndex];
    }
}

@end

Cocoa: What’s the difference between the frame and the bounds

The bounds of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to its own coordinate system (0,0).

The frame of an UIView is the rectangle, expressed as a location (x,y) and size (width,height) relative to the superview it is contained within.

So, imagine a view that has a size of 100x100 (width x height) positioned at 25,25 (x,y) of its superview. The following code prints out this view's bounds and frame:

// This method is in the view controller of the superview
- (void)viewDidLoad {
    [super viewDidLoad];

    NSLog(@"bounds.origin.x: %f", label.bounds.origin.x);
    NSLog(@"bounds.origin.y: %f", label.bounds.origin.y);
    NSLog(@"bounds.size.width: %f", label.bounds.size.width);
    NSLog(@"bounds.size.height: %f", label.bounds.size.height);

    NSLog(@"frame.origin.x: %f", label.frame.origin.x);
    NSLog(@"frame.origin.y: %f", label.frame.origin.y);
    NSLog(@"frame.size.width: %f", label.frame.size.width);
    NSLog(@"frame.size.height: %f", label.frame.size.height);
}

And the output of this code is:

bounds.origin.x: 0
bounds.origin.y: 0
bounds.size.width: 100
bounds.size.height: 100

frame.origin.x: 25
frame.origin.y: 25
frame.size.width: 100
frame.size.height: 100

So, we can see that in both cases, the width and the height of the view is the same regardless of whether we are looking at the bounds or frame. What is different is the x,y positioning of the view. In the case of the bounds, the x and y coordinates are at 0,0 as these coordinates are relative to the view itself. However, the frame x and y coordinates are relative to the position of the view within the parent view (which earlier we said was at 25,25).

There is also a great presentation that covers UIViews. See slides 1-20 which not only explain the difference between frames and bounds but also show visual examples.