Objective-C 對比學習

   在我們學習一門新的語言時，總要把它和我們熟悉的語言對比著來學習，就象學習英語時，都要記單詞的漢語意思，來協助我們對單詞的理解和記憶。

下面通過與C#的對比來學習Objective-C, 首先對比一下語言的定義:
  Objective-C，通常寫作ObjC和較少用的Objective C或Obj-C，是在C的基礎上，加入物件導向特性擴充而成的程式設計語言。
目前，Objective-C主要應用於Mac OS X和iOS這兩個NeXTSTEP的派生系統，而在NeXTSTEP和OpenStep中它更是基礎語言。Objective-C可以在任何gcc支援的平台上進行編譯，因為gcc本地支援Objective-C。
  C#是微軟推出的一種基於.NET架構的、物件導向的進階程式設計語言。C#由C語言和C++派生而來，繼承了其強大的效能，同時又以.NET 架構類庫作為基礎，擁有類似Visual Basic的快速開發能力。

Objective-C是在C的基礎上加入了物件導向的特性，而C#是由C和C++派生而來，可見C#顯得更“進階”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#是全新設計的，沒有什麼superset。
2. 單繼承。
3. 感覺就象C#的interface.
4. 應該是相對C語言來說的，不同於link的方式，ObjC使用了基於訊息傳遞(message-passing)的方式進行方法調用。
5. C#中的用object定義和傳遞變數應該算上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;

對於C#開發人員，這樣的代碼看起來很詭異，別緊張，把它想象成方法調用就對了。

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;

也許是為了ObjC看上去不那麼“詭異”，終於看上去“正常”了一些吧~！

id anObject;Persion  *aPersion = (Persion *)anObject;

這是所謂的“Loosely typed”吧，id應該是一個指象對象的指標吧。

nil

if(persion == nil) return;if(!persion) return;persion = nil;[button setTarget: nil];persion = nil;[persion castBallot];

nil就是C#裡的null，不同的是OjbC中nil上調用方法不出錯，對照mesage-passing方式，就好理解了，不就是沒人接收訊息嘛，那就什麼也不做好了。

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沒有bool類型，通常使用YES和NO，也可以使用TRUE和FALSE，或者乾脆使用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:);

感覺很象C#中的委託(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, 上面的代碼在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#中好象沒有直接比較對象屬性值的功能，得自已override Equals方法。

description:

- (NSString*)description;[NSString stringWithFormat: @"The answer is: %@", myObject];NSLog([anObject description]);

這不就是object的ToString()方法嗎？！

NSObject

對等於C#中的Object類。

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;

基本上沒什麼好說的，這就是System.Stirng.

Common NSMutableString methods:

+ (id)string;- (void)appendString:(NSString*)string;- (void)appendFormat:(NSString*)format, ...;

樣本：

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嗎？！

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#好象沒Set這樣的東東，怎麼沒什麼印象呢？！不過看樣子搞一個也不是很麻煩。

Enumeration

for (Persion *persion in array){     NSLog([persion description]);}

C#的foreach就對了。

Common NSNumber methods:

+ (NSNumber*)numberWithInt:(int)value;+ (NSNumber*)numberWithDouble:(double)value;- (int)intValue;- (double)doubleValue;

NSData/NSMutableData
NSDate/NSMutableDate

byte[] 和 DateTime就對了。

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

還是感覺一個類一個檔案簡潔。

SuperClass methods

- (void)doSomething{  // Call superclass implementation first  [super doSometing];  // Then do our custom behavior  int foo = bar;  // ...}

ObjC super 等於 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個對象嗎？！還搞成這樣？？！！！

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

建構函式和解構函式。

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，優勢盡顯，這也表現出來的“進階”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沒什麼說的。

Method type identifier
類方法：+
執行個體方法: -

ObjC 的+ 等於 C#中的static, 不過ObjC中沒有public和 private。

參考學習資料：

http://v.163.com/special/opencourse/iphonekaifa.html

http://rotator.youkulabs.com/?f5527199o1p0