Tuesday, November 27, 2012

selector objective C

Selector is another feature which supports ObjectiveC. A Selector is same as function pointers in C. So Using selector, we will get the reference of the method, later we will use that reference for calling the method. A SEL is a special type which holds a pointer to the symbolic name of a method.

@selector is a directive to get the pointer for the method. After getting the reference, using below two methods we can do the operations.
  • respondsToSelector : This used to check whether the method definition is available or not. It does not look for the declaration of the method, it just checks in implementation file for the method definition.
  • performSelector : This is used to call the method.
Syntax for the selector:
//both are same , any one is fine
SEL selectorName= @selector(methodName);
SEL selectorName = NSSelectorFromString(@”methodName");

Using Selectors to Call Methods: Below is the example for the usage of the selector and its methods.
//This is in .h file
@interface selectorDemo : NSObject
-(void) display;
-(void) output;

//below is in .m file
@implementation selectorDemo
-(void) display
    NSLog(@"this is display function");
-(void) output
    NSLog(@"this is output function");

//this is in main.m
int main(int argc, const char * argv[])

    selectorDemo *obj = [[selectorDemo alloc] init];
    // This selector declaration for display method
    SEL selDisplay = @selector(display);

    //checking whether method definition is available or not
    if([obj respondsToSelector:selDisplay])
        //calling method
        [obj performSelector:selDisplay];
    //another syntax for the selector declaration
    SEL selOutput = NSSelectorFromString(@"output");
    //calling method
    [obj performSelector:selOutput];
    return 0;

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