Objective-C Runtime

The dynamic heart of Objective-C: objects, classes and messages, resolved while your app runs.

Read the documentation WWDC sessions

Introduced

2001

Mac OS X 10.0

Kind

Framework

Runs on

Get it from

iOS

2.0+

iPadOS

2.0+

Mac Catalyst

13.0+

macOS

10.0+

tvOS

9.0+

visionOS

1.0+

watchOS

2.0+

Overview

The Objective-C Runtime is the library that makes Objective-C dynamic. Every method call is a message the runtime delivers while your app runs, looking up the right implementation at that moment. That's what lets you add methods, swap implementations and inspect classes on the fly.

It also defines NSObject, the root of most Cocoa classes. You rarely need to call it directly today, but it's still underneath UIKit and AppKit, and Swift uses it whenever you mark code with @objc.

Message sending

Every method call is a message, matched to its code at run time.

NSObject

The root class that gives objects their basic runtime behavior.

Method swizzling

Swap or add method implementations while the app is running.

Introspection

Ask any class for its methods, properties and protocols.

Greeter.h / Greeter.m

// Greeter.h
#import <Foundation/Foundation.h>

@interface Greeter : NSObject
@property (nonatomic, copy) NSString *name;
- (instancetype)initWithName:(NSString *)name;
- (NSString *)greeting;
@end


// Greeter.m
#import "Greeter.h"

@implementation Greeter

- (instancetype)initWithName:(NSString *)name {
    self = [super init];
    if (self) {
        _name = [name copy];
    }
    return self;
}

- (NSString *)greeting {
    NSInteger hour = [[NSCalendar currentCalendar] component:NSCalendarUnitHour
                                                    fromDate:[NSDate date]];
    NSString *part = hour < 12 ? @"Good morning"
                   : hour < 18 ? @"Good afternoon"
                   : @"Good evening";
    return [NSString stringWithFormat:@"%@, %@!", part, self.name];
}

@end

History