Error in dateFormat returns nil

问题

As an exercise to practice Objective-C, I am converting Swift code to Objective-C. After 1.5k lines converted, I find myself blocked for the past few days on one last issue.

In a few cells of my TableViewController, the description labels don't appear at all. I checked my code, and it seems the strings that are passed to the cells are actually empty or nil.

I seem to have made a syntax error in converting the Swift code that returns the date to my cells. What error have I made that makes the date nil?

THE CODE:

ViewController.m

...

NSDate const *now = [[NSDate alloc] init];
NSDate *date = [[NSDate alloc] init];

self.sections =  [[NSMutableArray<NSString *> alloc]init];
self.items = [[NSMutableArray<NSMutableArray<TableItem *> *> alloc]init];
self.sectionItems = [[NSMutableArray<TableItem *> alloc]init];

...

This is the cell data that doesn't load correctly (empty strings are passed to theDescription):

anItem = [[TableItem alloc ]initWithTitle: @"Custom: dd MMM yyyy HH:mm:ss" theDescription: [NSString stringWithFormat:@"%@", [now toString: [DateFormat CustomDateFormat:@"dd MMM yyyy HH:mm:ss"]]]];
        [_sectionItems addObject: anItem];

//theDescription should be: @"25 April 2016 15:04:57" after this ^, but is actually nil or @""

anItem = [[TableItem alloc ]initWithTitle: @"ISO8601(Year)" theDescription: [NSString stringWithFormat:@"%@", [now toString: [DateFormat ISODateFormat: ISOFormatYear]]]];
        [_sectionItems addObject: anItem];

//theDescription should be: @"2016" after this ^, but is actually nil or @""

Here's where I think I've made a mistake:

Original Swift syntax:

Extension.swift

toString() function

 case .ISO8601(let isoFormat):
        dateFormat = (isoFormat != nil) ? isoFormat!.rawValue : ISO8601Format.DateTimeMilliSec.rawValue
        zone = NSTimeZone.localTimeZone()

What I tried:

Extension.m

else if([format.dateFormatType compare: ISO8601DateFormatType] == NSOrderedSame) {
    NSString *isoFormat = ISO8601DateFormatType;
    dateFormat =  (isoFormat != nil) ? isoFormat : ISOFormatDateTimeMilliSec;
    zone = [NSTimeZone localTimeZone];
}

回答1:

The problem is in this line:

NSDateFormatter *formatter = [NSDate formatter : dateFormat : [NSTimeZone localTimeZone] : [NSLocale currentLocale]];

dateFormat is @"Custom", as it was set here:

else if([format.dateFormatType compare: CustomDateFormatType] == NSOrderedSame) {
    NSString *string = CustomDateFormatType;
    dateFormat = string;
}

This is passed into your NSDateFormatter constructor here:

formatter.dateFormat = format;

@"Custom" is not a valid date format string. Instead, pass in format.formatDetails:

NSDateFormatter *formatter = [NSDate formatter : format.formatDetails : [NSTimeZone localTimeZone] : [NSLocale currentLocale]];

I should note that not using named arguments in Objective-C makes your code much harder to read.

来源：https://stackoverflow.com/questions/36753954/error-in-dateformat-returns-nil