Objc under the_hood_2013

Objective-C - Under the hood
Michael Pan
13年5月21⽇日星期⼆二

Outline
• Pointer & Object
• C Struct for class
• Dynamic feature

Contacts
E-mail : scentsome@gmail.com
Facebook Group : Developer’s Lesson
http://www.facebook.com/groups/developerslesson
Facebook Page : Developer’s Note
http://www.facebook.com/pages/Taipei-Taiwan/Developers-note/226724001803
Blogger : Developer’s Note
http://iosdevelopersnote.blogspot.com/

Book
Objective-C 與 iOS 開發⼊入⾨門
http://www.books.com.tw/exep/prod/
booksﬁle.php?item=0010517912

Pointer & Object

Lets go from C
0x01
0x05
0x09

Lets go from C
int a;0x01
0x05
0x09

Lets go from C
int a;a 0x01
0x05
0x09

Lets go from C
int a;a
a=5;
0x01
0x05
0x09

Lets go from C
int a;a
a=5;
50x01
0x05
0x09

Pointer
int a;
a=5;
a 50x01
0x09
0x05
& => address of

Pointer
int a;
a=5;
int * pa;
a 50x01
0x09
0x05
& => address of

Pointer
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
& => address of

Pointer
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
& => address of

Pointer
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
0x01
& => address of

Pointer - More
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
0x01
printf(“%d”,*pa);

Recap - pointer
Write some thing

Pointer - More
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
0x01

Pointer - More
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
0x01
*pa = 10

Pointer - More
int a;
a=5;
int * pa;
pa
a 50x01
0x09
0x05
pa = &a;
0x01
*pa = 10
printf(“%d”,a);

Pointer - More
int a;
a=5;
int * pa;
pa
a 0x01
0x09
0x05
pa = &a;
0x01
*pa = 10
printf(“%d”,a);
10

Struct
• More complicated type
struct Date {
int day;
int month;
int year;
};
struct Date date = {3,10,1970};
printf("The day is %d, %d/%d", date.year, date.day, date.month);

Struct - Memory
0x10date
struct Date {
int day;
int month;
int year;
};

Struct - Memory
0x10date
struct Date date ;
struct Date {
int day;
int month;
int year;
};

Struct - Memory
0x10date
struct Date date ;
date = {3,10,1970};
struct Date {
int day;
int month;
int year;
};

Struct - Memory
0x10date
struct Date date ;
date = {3,10,1970};
struct Date {
int day;
int month;
int year;
};
3
10
1970

Function
• we know main function
• make another function

Function - Memory
int add(int a, int b){
int result = a+b;
return result;
}
add
result
in Stack

• Text Segment - 唯讀執⾏行區
• Data Segment - 可讀寫區包含 global 變數
• Heap - 可依程式需要產⽣生和消除記憶體(動態配置- malloc)
• Stack - 為了 function 產⽣生，可變動⼤大⼩小，包含區域變數
Objective-C 程式記憶體配置
Text Segment
Data Segment
Heap
Stack
未使⽤用
記憶體位置增加
Stack Pointer

Stack 與 Function
Text Segment
Data Segment
Heap
Stack
Stack Pointer
int main (int argc, const char * argv[]) {
// ... 省略
!
}

Stack 與 Function
Text Segment
Data Segment
Heap
Stack
Stack Pointer
// ... 省略
!
}
int sum = add(5, 6);

Stack 與 Function
Text Segment
Data Segment
Heap
Stack
Stack Pointer
// ... 省略
!
}
int sum = add(5, 6);
// 下⼀一⾏行程式碼

Stack 與 Heap
Text Segment
Data Segment
Heap
Stack local variable
malloc

Pointer - 動態配置記憶體
int * p_a;
p_a = malloc(sizeof(int));
*p_a = 8;
printf("value in pointer a is %dn", *p_a);
#include <stdlib.h>
p_a
Heap
8
Stack
0xA0
0xA0

Static variable
• in Data Segment
Text Segment
Heap
Stack
未使⽤用
Data Segment
int callStaticValue();
callStaticValue();
callStaticValue();
printf("static variable is %d", callStaticValue());
return 0;
}
int callStaticValue(){
! static int sVar=0; // static 變數初始化只會做⼀一次
! return sVar++; // ⼀一直加1
}

Struct + function
• it is what we call Object
struct Date {
int day;
int month;
int year;
void (*formatedDate)( struct Date date);
};
void formatedFunction(struct Date date){
printf("The day is %d, %d/%d",date.year, date.month, date.day );
}
// in main()
struct Date today = {29, 4, 2013};
today.formatedDate = formatedFunction;
today.formatedDate(today);

Objective-C class - C struct
• /usr/include/objc/objc.h
typedef struct objc_class *Class;
typedef struct objc_object {
Class isa;
} *id;
typedef struct objc_selector !*SEL;
typedef id (*IMP)(id, SEL, ...);

SEL
struct objc_selector
{
void *sel_id;
const char *sel_types;
};

Objective-C class - C struct
• /usr/include/objc/runtime.h
struct objc_class {
Class isa;
#if !__OBJC2__
Class super_class OBJC2_UNAVAILABLE;
const char *name OBJC2_UNAVAILABLE;
long version OBJC2_UNAVAILABLE;
long info OBJC2_UNAVAILABLE;
long instance_size OBJC2_UNAVAILABLE;
struct objc_ivar_list *ivars OBJC2_UNAVAILABLE;
struct objc_method_list **methodLists OBJC2_UNAVAILABLE;
/* ignore */
#endif
} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */
/* ignore */
typedef struct {
const char *name;
const char *value;
} objc_property_attribute_t;

Method
struct objc_method {
SEL method_name OBJC2_UNAVAILABLE;
char *method_types OBJC2_UNAVAILABLE;
IMP method_imp OBJC2_UNAVAILABLE;
struct objc_method_list {
struct objc_method_list *obsolete OBJC2_UNAVAILABLE;
int method_count OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
/* variable length structure */
struct objc_method method_list[1] OBJC2_UNAVAILABLE;

About Message
• Add method runtime

Add method runtime
struct objc_method {
  SEL method_name;
  char *method_types;
  IMP method_imp;
};
struct objc_method_list {
  struct objc_method_list *obsolete;
  int method_count;
  struct objc_method method_list[1];
};

Add method#import <objc/objc-class.h>
// create a class with no methods
@interface EmptyClass : NSObject { }
@end
@implementation EmptyClass
@end
// define the function to add as a method
id sayHello ( id self, SEL _cmd,... )
{
  NSLog (@"Hello");
}
void addMethod ()
{
  // create the method
  struct objc_method myMethod;
  myMethod.method_name = sel_registerName("sayHello");
  myMethod.method_imp  = sayHello;

  // build the method list.
  // this memory needs to stick around as long as the
  // methods belong to the class.
  struct objc_method_list * myMethodList;
  myMethodList = malloc (sizeof(struct objc_method_list));
  myMethodList->method_count = 1;
  myMethodList->method_list[0] = myMethod;

  // add method to the class
  class_addMethods ( [EmptyClass class], myMethodList );

  // try it out
  EmptyClass * instance = [[EmptyClass alloc] init];
  [instance sayHello];
  [instance release];
}

Method under the hood
MyClass * aObj;
int para = 5;
[aObj setCount:para];
objc_msgSend( aObj, @selector( setCount: ), para );
selector function
setCount: ...
count ...
經由 isa 找到 Class
structure
從table 找到相對應的 function

Demo
• DemoNCCU13 - MyCObject

Dynamic feature
• Dot operation
• KVC

Convenient Way
- Since Objective-C 2.0
- Dot Syntax
- Cascade
NSString *name = person.name;
// name = [person name]
person.name = @”Michael”;
// [person setName:@”Michael”]
person.bill.name = @”Andy”;
// [[person bill] setName:@”Andy”]
person.bill.name ;
// [[person bill] name]

Property means method
@interface Person : NSObject
@property int age;
@property (strong) NSString * name;
@end
@implementation Person
@end
Person * person = [[Person alloc] init];
person.name = @”Michael”;
person.age = 35;

Key-Value Coding

Another access method
• Read
• person.name;
• [person name];
• [person valueForKey:@”name”];
• Write
• person.name = @”michael”;
• [person setName:@”michael”];
• [person setValue:@”michael” forKey:@”name”];

• Find the action -name or -setName
• Find real variable _name and then name
Finding rule
forKey:@”name”

• attribute
• scalar // auto-boxing for KVC
• NSString
• Boolean
• Immutable object : NSColor, NSNumber
• to-many relationship - objects in NSArray, NSSet
Accessible Types

Recap
@interface MyObject : NSObject {
! NSString * name;
! NSString * _name;
}
@property (retain, readonly) NSString * _name;
@property (retain, readonly) NSString * name;
@end
MyObject * myo = [MyObject new];
[myo setValue:@"michael" forKey:@"name"];
NSLog(@" kvc %@",[myo valueForKey:@"name"]);
Result : kvc (null) ? 0. no setter
1. ﬁnd out _name = @”michael”
2. ﬁnd out name’s getter, return name
@implementation MyObject
@synthesize name, _name;
@end
下底線開頭的變數要⼩小⼼心使⽤用

[myCar valueForKeyPath: @"engine.vendor"]
KeyPath
@interface MyCar : NSObject {
! Engine * engine;
}
@end
@interface Engine : NSObject {
! NSString * vendor;
}
@end

NSNumber * count;
count = [myCars valueForKeyPath: @"@avg.price"]
Array Operation
@interface MyCar : NSObject {
! int price;
}
@end
@sum.xxx
@min.xxx
@max.xxx
myCar1 myCar2 myCar3myCars =

Recap
// 計算 engines array裡平均的price
NSArray * engines = [???:myengine1, myengine2, myengine3, nil];
NSLog(@" average price is %@",[engines valueForKeyPath:@"???"]);
@interface Engine : NSObject {
! int price;
}
@end

nil for scalar value ?
[engine setValue: nil forKey:@”price”]
error !!
- (void) setNilValueForKey: (NSString *) key {
if ([key isEqualToString: @"price"]) {
price = 0;
} else {
[super setNilValueForKey: key];
}
}
overwrite -setNilValueForKey:

Handle UndeﬁnedKey
- (void) setValue: (id) value forUndefinedKey: (NSString *) key {
// do something
}
- (id) valueForUndefinedKey:(NSString *)key {
// do something
}

Recap
實作
- (void) setValue: (id) value forUndeﬁnedKey: (NSString *) key;
- (id) valueForUndeﬁnedKey:(NSString *)key;

Reference
Introduction to Key-Value Coding Programming Guide
http://goo.gl/rrfmy

實例 - 解析 RSS
網路上免費資源 - 多⽤用來呈現新聞

With XML - RSS-like
<car>
<item>
<price> 99 </price>
<year> 1922 </year>
</item>
<item>
<price> 30 </price>
<year> 1990 </year>
</item>
<item>
<price> 82 </price>
<year> 1980 </year>
</item>
<item>
<price> 75 </price>
<year> 1920 </year>
</item>
<item>
<price> 60 </price>
<year> 1910 </year>
</item>
</car>
Array
Dictionary
key value
price 99
year 1922

Array and KVC
<car>
<item>
<price> 99 </price>
<year> 1922 </year>
</item>
<item>
<price> 30 </price>
<year> 1990 </year>
</item>
<item>
<price> 82 </price>
<year> 1980 </year>
</item>
<item>
<price> 75 </price>
<year> 1920 </year>
</item>
<item>
<price> 60 </price>
<year> 1910 </year>
</item>
</car>
Array
Dictionary
[array valueForKey:@”price”]
(
99,
30,
82,
75,
60,
)

Demo
ValueAndPredicate

Question

Objc under the_hood_2013

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