[sel object of Objective-C language]

Objective-C Linguistic SEL object

One 、SEL What is the object ？

SEL Objects are used to store methods , It includes the following attributes ：
1. Class method or object method ,BOOL type ;
2. return type ,NSString * type ;
3. Method name ,NSString * type ;
4. Method parameter ,NSString * type ;
5. Method body ,NSString * type ;

Two 、 How to store methods ？

1. So let's create one SEL object

2. Let's put this SEL Object as Class object （ Class object ） Properties of

Be careful ,Class object （ Class object ） Is stored in code snippets , It is used to store a class , It has the following properties ：
1. Class name ,NSString * type ;
2. Attribute array ,NSString * An array type ;
3. Methods the array ,SEL An array type ;

3. How to get the storage method SEL object

1. To get this SEL object , You should use one first SEL Take the pointer ？
SEL c1 =

2. Why? c1 No addition *, because SEL This data type is a custom data type , It's a typedef, It is defined as follows ：
typedef struct objc_selector *SEL;

3. Because the pointer type of this structure is defined as SEL When these three words , Has added * 了 , So the statement SEL Objects are not added * 了 .

4. How to get storage - (void)sayHi; Methodical SEL What about objects? ？
1). One @ Symbol , Pick up a selector, Followed by a parenthesis , Write the name of the method in this parenthesis sayHi that will do ;
As shown below ：
SEL c1 = @selector(sayHi);
after ,c1 Just point to the storage sayHi Methodical SEL object ;
It can be used %p hold c1 Output the value of ：
NSLog(@" c1 = %p ",c1);
The output looks like this ：
c1 = 0x100003f71

5. The essence of calling methods ：
1）[p1 sayHi]; The internal principle ：
a. Get the storage first sayHi Methodical SEL object ;
That is to get the storage first sayHi Address of method , That is to take
To the storage sayHi Methodical SEL data , Or someone also put
This becomes SEL news .
b. Put this SEL Message sent to p1 object .
c.p1 Object received this SEL After message , I knew it was to
Calling method .
d. Based on the iSA The pointer , Find the class object that stores the class , Then look in the class object , Do you have this SEL data , If there is , Is executed , without , Just find the parent class , Always find NSObject class , If not , Just report a mistake ;

6. Manually send... To the object SEL news ：
1） Get the storage first sayHi Methodical SEL object , namely SEL news ;
SEL s1 = @selector(sayHi);
2） Put this SEL news , Send to p1 object ;
p1 The object has a way , be called performSelector:(SEL)
You can put SEL Message sent to p1 object ：
[p1 performSelector:s1];

7. If SEL What if the object has parameters ？
such as , There's a way ：- (void)eatWithFood:(NSString *)foodName;
- (void)eatWithFood:(NSString *)foodName
{
NSLog(@" You gave me %@, Really delicious ...);
}
How to manually give objects p1 send out SEL news ？
1）SEL c1 = @selector(eatWithFood:);
[p1 performSelector:c1];
That's not gonna work , Because you have to pass it parameters ！
2） How to give SEL Object parameters ？
Use p1 Object method of object ：
performSelector:(SEL) withObject:(id);
for example ：
[p1 performSelector:c1 withObject:@“ Braised Pork ”];

8. If SEL Object with two parameters ？
call p1 Another object method of the object ：
[p1 performSelector:(SEL) withObject:(id) withObject:(id)];

9. If SEL What if the object has multiple parameters ？
You can only encapsulate multiple parameters in one object , Pass to SEL object ：
1） Write a class first ：Params
2）Params The attribute of is all parameters , For example, there are four parameters ,NSString *_p1;NSString *_p2;NSString *_p3;NSString *_p4;
for example ：
@interface Params:NSObject
{
@public
NSString *_p1;
NSString *_p2;
NSString *_p3;
NSString *_p4;
}
@end
3）- (void)testWith:(Params *)ps
{
NSLog(@“p1 = %@,p2=%@,p3=%@,p4=%@”,ps->_p1,ps->_p2,ps->_p3,ps->_p4);
}
3） Create a Params object ：
Params *ps = [Params new];
ps->_p1 = @“p1”;
ps->_p2 = @“p2”;
ps->_p3 = @“p3”;
ps->_p4 = @“p4”;
4） Get SEL object ：
SEL c4 = @selector(testWith:);
5） hold SEL Object is sent to p1 object ：
[p1 performSelector:c4 withObject:ps];

summary

1. Call an object's method , There are two kinds of ：
1）[p1 sayHi];
2）SEL c1 = @selector(sayHi);
[p1 performSelector:c1];