SEL in Objective-c (reprint)

SEL

In Objective-c, the SEL is a type of selector (selector). A selector is a pointer to a method, and the reader can simply understand that running the program here will execute the specified method, so you can define a selector:

1. SEL action = [button action];

We use a selector like this, and the following selector is called Action:

1. [Foo Action]
2. [Bar Action]

In target-action mode (a common pattern in a Cocoa program): Target specifies a class, and the Action specifies a method. Setting an action on an object is done by the selector:

1. -(void) Settarget: (ID) target;
2. -(void) Setaction: (SEL) Action;

The following statement sets the action on a button object to "@selector (start:)", that is, it calls the Start method:

1. [Button setaction: @selector (start:)];

If you have two parameters on your method, such as:

1. -(void) SetName: (NSString *) name Age: (int.) Age;

So, your selector should write like this:

1. Sel sel = @selector (setname:age:);

If the method does not exist, the application calling the method may abort abnormally. Therefore, you need to use the Respondstoselector method to determine if the object has a corresponding method, and use the Performselector:withobject: method to invoke the selector:

1. Sel sel = @selector (start:);//Specify Action
2. if ([obj Respondstoselector:sel]) {//Determine if the object has a corresponding method
3. [obj Performselector:sel withobject:self]; Call Selector method
4. }

Let's look at an example of an app selector.

Example 2-16 Selector instance.

2. #import <foundation/foundation.h>
4. @interface Classa:nsobject {
6. }
8. -(void) print;
10. @end
12. @implementation ClassA
14. -(void) print{
16. NSLog (@ "I ' m ClassA.");
18. }
20. @end
22. int main (int argc, const char * argv[]) {
24. NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
26. Sel sel = @selector (print);
28. ClassA *ClassA = [[ClassA alloc]init];
30. [ClassA Performselector:sel Withobject:nil]; Invokes the method specified by the selector
32. [Pool drain];
34. return 0;
36. }

"Program Results"

1. I ' m ClassA.

The following explanation of this code, the reader may not understand, because so far did not tell the class-related knowledge. Readers only need to understand the use of selectors in the above examples, and the knowledge about classes will be elaborated in later chapters.

The code first creates a class named ClassA, which contains only one method, print. In the subsequent implementation file, we implemented this method:

1. -(void) print{
2. NSLog (@ "I ' m ClassA.");
3. }

The reader is not difficult to see, this method is only printed to the console a sentence. In the next main method, a selector sel is defined, which points to a method called print. We don't know which class this method is, because the specific information is automatically judged by the system during the run.

1. Sel sel = @selector (print);

Then an object is constructed (the reader does not have to stick to the syntax, we will elaborate in a later chapter) and call this object Performselector:withobject: the method.

1. ClassA *ClassA = [[ClassA alloc]init];
2. [ClassA Performselector:sel Withobject:nil];

At this time, the system will automatically call the ClassA object's Print method, and finally get the program running results.

The above article is reproduced in the csdn of a great God, follow-up I will be specific sample code to show

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SEL in Objective-c (reprint)