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Objective-C - Protocols
Objective-C Protocols
Objective-C allows you to define protocols, which declare the methods expected to be used for a particular situation. Protocols are implemented in the classes conforming to the protocol.
A simple example would be a network URL handpng class, it will have a protocol with methods pke processCompleted delegate method that intimates the calpng class once the network URL fetching operation is over.
A syntax of protocol is shown below.
@protocol ProtocolName @required // pst of required methods @optional // pst of optional methods @end
The methods under keyword @required must be implemented in the classes that conforms to the protocol and the methods under @optional keyword are optional to implement.
Here is the syntax for class conforming to protocol
@interface MyClass : NSObject <MyProtocol> ... @end
This means that any instance of MyClass will respond not only to the methods declared specifically in the interface, but that MyClass also provides implementations for the required methods in MyProtocol. There s no need to redeclare the protocol methods in the class interface - the adoption of the protocol is sufficient.
If you need a class to adopt multiple protocols, you can specify them as a comma-separated pst. We have a delegate object that holds the reference of the calpng object that implements the protocol.
An example is shown below.
#import <Foundation/Foundation.h>
@protocol PrintProtocolDelegate
- (void)processCompleted;
@end
@interface PrintClass :NSObject {
id delegate;
}
- (void) printDetails;
- (void) setDelegate:(id)newDelegate;
@end
@implementation PrintClass
- (void)printDetails {
NSLog(@"Printing Details");
[delegate processCompleted];
}
- (void) setDelegate:(id)newDelegate {
delegate = newDelegate;
}
@end
@interface SampleClass:NSObject<PrintProtocolDelegate>
- (void)startAction;
@end
@implementation SampleClass
- (void)startAction {
PrintClass *printClass = [[PrintClass alloc]init];
[printClass setDelegate:self];
[printClass printDetails];
}
-(void)processCompleted {
NSLog(@"Printing Process Completed");
}
@end
int main(int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
SampleClass *sampleClass = [[SampleClass alloc]init];
[sampleClass startAction];
[pool drain];
return 0;
}
Now when we compile and run the program, we will get the following result.
2013-09-22 21:15:50.362 Protocols[275:303] Printing Details 2013-09-22 21:15:50.364 Protocols[275:303] Printing Process Completed
In the above example we have seen how the delgate methods are called and executed. Its starts with startAction, once the process is completed, the delegate method processCompleted is called to intimate the operation is completed.
In any iOS or Mac app, we will never have a program implemented without a delegate. So its important we understand the usage of delegates. Delegates objects should use unsafe_unretained property type to avoid memory leaks.
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