The following document is a record of useful pointers and tips as I learn Objective-C. The style is concise and straight to the point, with many basic aspects missing. This is meant for programmers who know another language, so to act as a tip sheet for future reference. There is an emphasis on syntax, differences and best practices.
Beware! If you are a seasoned programmer in Objective-C, quickly `cmd + w` your way out of here.
I'm very sure you will find lots of mistakes and inaccuracies, things which aren't strictly true,
or are just plain wrong. Save yourselves!
Objective-C is a super-set of the C language. Any C code is valid Objective-C code. Objective-C provides Object-Oriented capabilities to C. We will be discussing syntax in the rest of this document, rather than the core OOP principles.
Some examples will make use of Apple's Cocoa framework, mostly the foundations library.
Use the #import directive, rather than #include directive to include Objective-C libraries.
Instead of the usual stdlib, etc, C libraries, use the Foundations library which is part of the Apple Cocoa framework.
#import <Foundations/Foundations.h>
The Foundations library provides the NSLog() method, which performs the same role as printf() in C. The syntax is identical.
String literals in Objective-C are prefixed by the @ symbol. So coupled with the NSLog() method, we get
NSLog(@"This line will be printed to console.");
An additional loop structure is provided by Objective-C. This is the for-in loop. This is referred to as the fast-enumeration syntax which is more efficient than the standard for/while loops when used with Objective-C Collections.
NSArray *models = @[@"Ford", @"Honda", @"Nissan", @"Porsche"];
for (id model in models) {
NSLog(@"%@", model);
}
All Objective-C objects are referenced as pointers. The syntax therefore works naturally with pointers.
The macro nil is provided in Objective-C to represent null objects. It is identical to the C NULL macro. Though it is good practice to use nil for Objective-C objects, and NULL for C pointers.
To call a method in Objective-C, we pass messages. Use the schema
[ClassInstance Message]
Objective-C separates interface from implementation. Most classes are structured so that there is an implementation file (.m), and an interface file (.h).
The basic syntax for declaring a class is
// Filename: Class.h
@interface ClassName : InheritedClass {
// Attributes
}
// Method definitions
- (returnType) methodName: (paramType) paramName;
- (returnType) first: (firstparamType) firstParamName
andSecond: (secondParamType) secondParamName;
@end
The - means this method is an instance method. To declare a class method use a +. Most Objective-C names are given as plain english.
The basic syntax for an implementation file is
// Filename: Class.m
#import "Class.h"
@implementation ClassName
- (returnType) methodName: (paramType) paramName {
}
@end