The language Objective-C notes

Time: 31 March 2014
Categories:
Other 9
Tags:
objective-C 1

###calling method

[object method];
[object methodWithInput:input];
output = [object method];

define var

id myObj = [NSString string];
// pointer type var
NSString* myStr = [NSString string]; 

###Nested Msg

[NSString Func1: [Func2]]

multiple method

- (BOOL) writeFile:(NSString*)path atomically:(BOOL)useXFile;
BOOL ret = [myData writeFile:@"path" atomically:@"file"];

accessors (getter/setter)

[Phone setCaption:@"pNmae"];
output = [Phone caption]; // leave out get prefix

// we can use dot syntax since oc 2.0 (only for setter and getter)
Phone.caption = @"pName";
output = Phone.caption

create obj

// automatic style, created an AutoReleased object
NSString* myStr = [NSString string];

// munual style , should release it later
NSString* myStr = [[NSString alloc] init];

// use another init which takes input
NSNumber* myNum = [[NSNumber alloc] initWithFloat:1.0];

memory management

//str1 should be released automatically
NSString* str1 = [NSString string];

// must release str2 when done
NSString* str2 = [[NSString alloc] init];
[str2 release];

(1) setter

In most cases, the setter for an instance variable should just autorelease the old object, and retain the new one. You then just make sure to release it in dealloc as well:

- (void) setTotalAmount: (NSNumber*)input
{
    [totalAmount autorelease];
    totalAmount = [input retain];
}

- (void) dealloc
{
    [totalAmount release];
    [super dealloc];
}

(2) alloc

the only real work is managing local references inside a function. And there’s only one rule: if you create an object with alloc or copy, send it a release or autorelease message at the end of the function. If you create an object any other way, do nothing.

NSNumber* value1 = [[NSNumber alloc] initWithFloat:8.75];
NSNumber* value2 = [NSNumber numberWithFloat:14.78];

// only release value1, not value2
[value1 release];

Design Class

(1) Class Interface

phone.h code :

#import <Cocoa/Cocoa.h>
@interface Phone : NSObjet {
    NSString* str1
    NSString* str2
}
// getter
// single dash means that it's a instance method , plus means class method
- (NSString*) str1
- (NSString*) str2

// settter
- (void) setStr1: (NSString*)input;
- (void) setStr2: (NSString*)input;

@end

(2) Class Implement

phone.m code:

#import "Phone.h"
@implementation Phone 

// getter
- (NSString*) str1 {
    return str1 ;
}
- (NSString*) str2 {
    return str2 ;
}

//setter
- (void) setStr1: (NSString*)input {
    [str1 autorelease] ;
    str1 = [input retain];
}
- (void) setStr2: (NSString*)input {
    [str2 autorelease] ;
    str2 = [input retain];
    // in garbage collection environment we can set value directly
    // str2 = input ;
}

@end

init

- (id) init {
    if (self = [super init]){
        [self setStr1:@"string 1"];
        [self setStr2:@"string 2"];
    }
    return self;
}

dealloc

- (void) dealloc {
    [str1 release]  ;
    [str2 release]  ;
    [super dealloc]  ;
}

log

NSLog ( @"The current date and time is: %@", [NSDate date] );

properties

Properties are a feature in Objective-C that allow us to automatically generate accessors.

old code code:

#import <Cocoa/Cocoa.h>

@interface Photo : NSObject {
    NSString* caption;
    NSString* photographer;
}
- (NSString*) caption;
- (NSString*) photographer;

- (void) setCaption: (NSString*)input;
- (void) setPhotographer: (NSString*)input;
@end

new code code:

#import <Cocoa/Cocoa.h>

@interface Photo : NSObject {
    NSString* caption;
    NSString* photographer;
}
// the setter shoudld retain the input value
@property (retain) NSString* caption;
@property (retain) NSString* photographer;

@end

and then, the implementation code:

#import "Photo.h"
        
@implementation Photo

// pay attention AutoGenerate setter and getter
@synthesize caption;
@synthesize photographer;

- (void) dealloc
{
    [caption release];
    [photographer release];
    [super dealloc];
}

@end

Nil

You can call methods on nil without crashing or throwing an exception. you usually don’t need to check for nil before calling a method on an object.If you call a method on nil that returns an object, you will get nil as a return value.

Use Nil to improve our dealloc method slightly:

- (void) dealloc
{
    // setter just retain nil (which does nothing) and release old value 
    self.caption = nil;
    self.photographer = nil;
    [super dealloc];
}