There seems to be a lot of confusion regarding the preprocessor.
What the compiler does when it sees a #include
that it replaces that line with the contents of the included files, no questions asked.
So if you have a file a.h
with this contents:
typedef int my_number;
and a file b.c
with this content:
#include "a.h"
#include "a.h"
the file b.c
will be translated by the preprocessor before compilation to
typedef int my_number;
typedef int my_number;
which will result in a compiler error, since the type my_number
is defined twice. Even though the definition is the same this is not allowed by the C language.
Since a header often is used in more than one place include guards usually are used in C. This looks like this:
#ifndef _a_h_included_
#define _a_h_included_
typedef int my_number;
#endif
The file b.c
still would have the whole contents of the header in it twice after being preprocessed. But the second instance would be ignored since the macro _a_h_included_
would already have been defined.
This works really well, but has two drawbacks. First of all the include guards have to be written, and the macro name has to be different in every header. And secondly the compiler has still to look for the header file and read it as often as it is included.
Objective-C has the #import
preprocessor instruction (it also can be used for C and C++ code with some compilers and options). This does almost the same as #include
, but it also notes internally which file has already been included. The #import
line is only replaced by the contents of the named file for the first time it is encountered. Every time after that it is just ignored.
Compare method
Either you implement a compare-method for your object:
- (NSComparisonResult)compare:(Person *)otherObject {
return [self.birthDate compare:otherObject.birthDate];
}
NSArray *sortedArray = [drinkDetails sortedArrayUsingSelector:@selector(compare:)];
NSSortDescriptor (better)
or usually even better:
NSSortDescriptor *sortDescriptor;
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"birthDate"
ascending:YES];
NSArray *sortedArray = [drinkDetails sortedArrayUsingDescriptors:@[sortDescriptor]];
You can easily sort by multiple keys by adding more than one to the array. Using custom comparator-methods is possible as well. Have a look at the documentation.
Blocks (shiny!)
There's also the possibility of sorting with a block since Mac OS X 10.6 and iOS 4:
NSArray *sortedArray;
sortedArray = [drinkDetails sortedArrayUsingComparator:^NSComparisonResult(Person *a, Person *b) {
return [a.birthDate compare:b.birthDate];
}];
Performance
The -compare:
and block-based methods will be quite a bit faster, in general, than using NSSortDescriptor
as the latter relies on KVC. The primary advantage of the NSSortDescriptor
method is that it provides a way to define your sort order using data, rather than code, which makes it easy to e.g. set things up so users can sort an NSTableView
by clicking on the header row.
Best Answer
You need to import
ProductName-Swift.h
. Note that it's the product name - the other answers make the mistake of using the class name.This single file is an autogenerated header that defines Objective-C interfaces for all Swift classes in your project that are either annotated with
@objc
or inherit fromNSObject
.Considerations:
If your product name contains spaces, replace them with underscores (e.g.
My Project
becomesMy_Project-Swift.h
)If your target is a framework, you need to import
<ProductName/ProductName-Swift.h>
Make sure your Swift file is member of the target