在我们学习一门新的语言时，总要把它和我们熟悉的语言对比着来学习，就象学习英语时，都要记单词的汉语意思，来帮助我们对单词的理解和记忆。
下面通过与C#的对比来学习Objective-C, 首先对比一下语言的定义:
Objective-C，通常写作ObjC和较少用的Objective C或Obj-C，是在C的基础上，加入面向对象特性扩充而成的编程语言。
目前，Objective-C主要应用于Mac OS X和iOS这两个NeXTSTEP的派生系统，而在NeXTSTEP和OpenStep中它更是基本语言。Objective-C可以在任何gcc支持的平台上进行编译，因为gcc本地支持Objective-C。
C#是微软推出的一种基于.NET框架的、面向对象的高级编程语言。C#由C语言和C++派生而来，继承了其强大的性能，同时又以.NET 框架类库作为基础，拥有类似Visual Basic的快速开发能力。
Objective-C是在C的基础上加入了面向对象的特性，而C#是由C和C++派生而来，可见C#显得更“高级”一些.
1. Objective-C is a strict superset of C
2. Single inheritance
3. Protocols define behavior that cross classes
4. Dynamic runtime
5. Loosely typed
1. C#是全新设计的，没有什么superset。
2. 单继承。
3. 感觉就象C#的interface.
4. 应该是相对C语言来说的，不同于link的方式，ObjC使用了基于消息传递(message-passing)的方式进行方法调用。
5. C#中的用object定义和传递变量应该算上Loosely。
Message Expression
[reciever message] [reciever message:argument] [reciever message:argument andArg:arg2] Metho definition - (void)castBallot; - (int)arg; - (void)setArg:(int)age; - (void)registerForState:(NSString*)state party:(NSString*)party;
对于C#开发人员，这样的代码看起来很诡异，别紧张，把它想象成方法调用就对了。
Objective-C 2.0 introduced dot syntax
float height = [persion height]; float height = persion.height; [persion setHeight:newHeight]; persion.height = newHeight; [[persion child] setHeight:newHeight]; persion.child.height = newHeight;
也许是为了ObjC看上去不那么“诡异”，终于看上去“正常”了一些吧~！
id anObject; Persion *aPersion = (Persion *)anObject;
这是所谓的“Loosely typed”吧，id应该是一个指象对象的指针吧。
nil
if(persion == nil) return; if(!persion) return; persion = nil; [button setTarget: nil]; persion = nil; [persion castBallot];
nil就是C#里的null，不同的是OjbC中nil上调用方法不出错，对照mesage-passing方式，就好理解了，不就是没人接收消息嘛，那就什么也不做好了。
When ObjC was developed, C has no boolean type (C99 introduced one).
ObjC uses a typedef to define BOOL as a type
BOOL flag = NO;
OjbC没有bool类型，通常使用YES和NO，也可以使用TRUE和FALSE，或者干脆使用0和1表示。
SEL
SEL action = [button action]; [button setAction:@selector(start:)]; Conceptually similar to function pointer - (void)setName:(NSString*)name age:(int)age; SEL sel = @selector(setName:age:);
感觉很象C#中的委托(delegate).
Class:
Class myClass = [myObject class];
NSLog(@"My class is %@", [myObject className]);
if ([myObject isKindOfClass:[UIControl class]])
{
// something
}
if ([myObject isMemberOfClass:[NSString class]])
{
// someting string specific
}
C#中有Type, 上面的代码在C#中如下：
Type myClass = myObject.GetType();
Console.WriteLine(string.Format("My class is {0}", myClass.TypeName));
if(myObject.GetType() == typeof(UIControl))
{
// something
}
if (myObject is typeof(String))
{
// someting string specific
}
Identity versus Equality
Identity - testing equality of the pointer values
if (object1 == ojbect2)
{
NSLog(@"Same exact object instance");
}
Equality - testing object attributes
if ([object1 isEqual: object2])
{
NSLog(@"Logically equivalent, but may be different object instances");
}
C#中好象没有直接比较对象属性值的功能，得自已override Equals方法。
description:
- (NSString*)description; [NSString stringWithFormat: @"The answer is: %@", myObject]; NSLog([anObject description]);
NSObject
对等于C#中的Object类。
NSString
In C constant strings are
"simple"
In ObjC constant strings are
@"just as simple"
NSString *aString = @"Hello World!"; NSLog(@"I am a %@, I have %d items", [array className], [array count]); NSString *myString = @"Hello"; NSString *fullString; fullString = [myString stringByAppendString:@" world!"];
fullString would be set to "Hello World!".
- (BOOL) isEqualToString:(NSStirng*)string; - (BOOL) hasPrefix:(NSString*)string; - (int)intValue; - (double)doubleValue;
基本上没什么好说的，这就是System.Stirng.
Common NSMutableString methods:
+ (id)string; - (void)appendString:(NSString*)string; - (void)appendFormat:(NSString*)format, ...;
NSMutableString *newString = [NSMutableString string]; [newString appendString:@"Hi"]; [newString appendFormat:@" , my favorite number is: %d", [self favoriteNumber]];
是不是很象StringBuilder?
Collections
Array, Dictionary, Set
Common NSArray methods:
+ arrayWithObjects:(id)firstObj, ...; //nil terminated!!! - (unsigned)count; - (id)objectAtIndex:(unsigned)index; - (unsigned)indexOfObject:(id)object;
NSNotFound returned for index if not found
if ([array indexOfObject:@"Purple"] == NSNotFound)
{
NSLog(@"No color purple");
}
NSNotFound不就是-1吗？！
NSMutableArray
NSMutalbeArray subclasses NSArray.
Common NSDictionary methods:
+ dictionaryWithObjectsAndKeys:(id)firstObject, ...; - (unsigned)count; - (id)objectForKey:(id)key;
nil returned if no object found for gived key.
NSMutalbeDictionary
NSMutalbeDictionary subclasses NSDictionary.
Common NSSet methods:
+ setWithOjbects:(id)firstObj, ...; //nil terminated - (unsigned)count; - (BOOL)containsObject:(id)object;
No object is ever in there more than once.
NSMutableSet
NSMutableSet subclasses NSSet
C#好象没Set这样的东东，怎么没什么印象呢？！不过看样子搞一个也不是很麻烦。
Enumeration
for (Persion *persion in array)
{
NSLog([persion description]);
}
C#的foreach就对了。
Common NSNumber methods:
+ (NSNumber*)numberWithInt:(int)value; + (NSNumber*)numberWithDouble:(double)value; - (int)intValue; - (double)doubleValue;
NSData/NSMutableData
NSDate/NSMutableDate
byte[] 和 DateTime就对了。
Class interface declared in header file
#import <Foundation/Foundation.h>
@interface Persion : NSObject
{
// instance variables
NSString *name;
int arg;
}
// method declarations
- (NSString *)name;
- (void)setName:(NSString *)value;
- (int)age;
- (void)setAge:(int)age;
- (BOOL)canLegallyVote;
- (void)castBallot;
@end
Class Implementation write in .m file
#import "Persion.h"
@implemetation Person
- (int)age
{
return age;
}
- (void)setAge:(int)value
{
age = value;
}
// ... and other methods
- (BOOL)canLegallyVote
{
return ([self age] >= 18);
}
- (void)castBallot
{
if([self canLegallyVote])
{
// do voting stuff
}
else
{
NSLog(@"I'm not allowed to vote!");
}
}
@end
还是感觉一个类一个文件简洁。
SuperClass methods
- (void)doSomething
{
// Call superclass implementation first
[super doSometing];
// Then do our custom behavior
int foo = bar;
// ...
}
ObjC super 等于 C# base
Object Creation
+ alloc
Class method that knows how much memory is needed.
- init
Instance method to set initial values, perfomr other setup.
Create = Allocate + Initialize
Persion *persion = nil; Persion *persion = [[Persion alloc] init];
不就是new个对象吗？！还搞成这样？？！！！
Implementing your own - init method
#import "Persion.h"
@implementation Person
- (id)init
{
// allow superclass to initialize its state first
if (self == [super init])
{
age = 0;
name = @"Bob";
// do other initialization...
}
return self;
}
@end
Multiple init methods
- (id)init; - (id)initWithName:(NSString *)name; - (id)initWithNameAndAge:(NSString *)name age:(int)age;
Less specific ones typically call more specific with default values
- (id)init
{
return [self initWithName:@"No Name"];
}
- (id)initWithName:(NSString *)name
{
return [self initWithNameAndAge:name age:0];
}
Persion *persion = [[Persion alloc] init]; // ... [persion release]; // Object is deallocated [persion doSomething]; // Crash! persion = nil; [persion doSomething]; // No effect
Implementing a -dealloc method
#import "Persion.h"
@implementation Persion
- (void)dealloc
{
// Do any cleanup that's necessary
// ...
[name release];
// when we're done, call super to clean us up
[super dealloc];
}
@end
构造函数和析构函数。
Object Ownership
#import "Persion.h"
@implementation Persion
- (NSString *)name
{
return name;
}
// retain
- (void)setName:(NSString *)newName
{
if (name != newName)
{
[name release];
name = [newName retain];
// name's retain count has been bumped up by 1
}
}
// copy
- (void)setName:(NSString *)newName
{
if (name != newName)
{
[name release];
name = [newName retain];
// name's retain count has been bumped up by 1
}
}
Returning a newly created object
- (NSString *)fullName
{
NSString *result;
result = [[NSString alloc] initWithFormat:@"%@ %@", firstName, lastName];
[result autorelease];
return result;
}
Autorelease is not garbage collection, Objective-C on iPhone OS does not have garbage collection.
C#的GC，优势尽显，这也表现出来的“高级”的地方吧！
Defining Properites
@property int age; @property (copy) NSString *name; @property (readonly) BOOL canLegallyVote;
Synthesizing Properties
- (int)age
{
return age;
}
- (void)setAge:(int)value
{
age = value;
}
- (NSString *)name
{
return name;
}
- (void)setName:(NSString *)value
{
if(value != name)
{
[name release];
name = [value copy];
}
}
@implementation Person
@synthesize age;
@synthesize name;
- (BOOL)canLegallyVote
{
return (age > 17);
}
Memory namagement policies
@property (assign) NSString *name; // pointer assignment @property (retain) NSString *name; // retain called @property (copy) NSString *name; // copy called
Property Names vs. Instance Variables
@interface Persion : NSObject
{
int numberOfYearsOld;
}
@property int age;
@end
@implemetation Persion @synthesize age = numberOfYearsOld; @end
就是property没什么说的。
Method type identifier
类方法：+
实例方法: -
ObjC 的+ 等于 C#中的static, 不过ObjC中没有public和 private。
参考学习资料：
