The Dark Side of Objective-C
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The document discusses the Objective-C runtime and how it manages classes, methods, and memory behind the scenes in Objective-C and Swift applications. It provides an overview of common runtime functions like looking up classes and selectors, storing associated objects, accessing ivars and properties, enumerating classes and methods. It also discusses more advanced runtime services like adding and replacing methods, forwarding invocations, changing an object's class, and creating new classes. Examples are given of how often typical iOS apps use various runtime functions.
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The Dark Side of Objective-C
- 1. The Dark Side of Objective-C Martin Kiss | kish | @Tricertops PixelCut s. r. o.
- 2. Objective-C Runtime Creates illusion of objects and messages
- 3. @interface @implementation [self bracketSyntax] @property (strong) self.dotSyntax @selector() - (void)methodDeclarations:(id)weird The Light Side
- 5. What is Runtime? • C support library • Manages classes and methods • Handles method invocations • Memory management routines • Manages lifetime of weak references
- 6. Why should Swift care? • Swift is a new syntax for Objective-C • Frameworks are all in Objective-C • NSObject and @objc • Many of things are not possible, yet • Swift will need its own runtime
- 7. What needs runtime? • NSCoding • Storyboards & XIBs • Key-Value Coding • Key-Value Observing • Core Animation • Testing & Mocking • Core Data • NSProxy • UIAppearance • NSUndoManager • Cocoa Bindings • UIResponder
- 10. Look up Class by Name High Level Low Level How It Works Used By NSClassFromString() objc_getClass() objc_lookUpClass() Runtime holds a table of all classes by their names as keys NSCoding, Storyboards, XIBs
- 11. Perform Selector by Name High Level Low Level How It Works Used By NSSelectorFromString() -performSelector: sel_registerName() objc_msgSend() Classes store method implementations in tables by selectors as keys Key-Value Coding, Storyboards, XIBs, UIControl, UIGestureRecognizer
- 12. Store Associated Objects Functions How It Works What Is It For Used By objc_getAssociatedObject() objc_setAssociatedObject() Runtime holds side-table for each object until it deallocates Adding storage to objects without adding ivars, for example rarely used values UIViews, Auto Layout, UIAppearance, many more
- 14. Access Ivars by Name Functions How It Works Used By class_getInstanceVariable() class_setInstanceVariable() Instance variables are accessed indirectly and classes contain lookup table Key-Value Coding, Key-Value Observing
- 15. Access Attributes of Properties Functions How It Works What Is It For Used By class_getProperty() property_getAttributes() Classes include all info about properties, like name, attributes, and backing ivar Check if property is weak or strong or if the property has ivar Custom CALayer animatable properties
- 16. Enumerate Ivars & Properties Functions How It Works What Is It For Used By class_copyIvarList() class_copyPropertyList() Classes include tables and metadata for ivars and properties Inspection of object contents CALayer animatable properties, Core Image Filters, automatic descriptions
- 17. Enumerate Methods Function How It Works What Is It For Used By class_copyMethodList() Classes include table of methods with their selectors and implementations Find methods that match a pattern Tests & Mocks, UIAppearance
- 18. Enumerate Classes Functions How It Works What Is It For objc_copyClassList() objc_copyClassNamesForImage() Runtime holds table of all classes loaded in your process, per executable Find classes that match a pattern, find all subclasses of a class ⚠ Warning: Accessing framework classes will trigger side effects!
- 19. Sending Messages Function Related How It Works What Is It For objc_msgSend() -methodForSelector: -methodSignatureForSelector: All [bracket calls] are translated to objc_msgSend() Perform selectors when standard method -performSelector: is not enough ⚠ Warning: Requires casting to work properly!
- 20. Sending Messages How it works? - (void)updateTitle:(NSString *)title { title = title.trimmedString; self.title = title; self.navigationBar.title = title; }
- 21. Sending Messages Components of a method updateTitle: void, id, SEL, NSString * { title = title.trimmedString; self.title = title; self.navigationBar.title = title; }
- 22. Selector Type Signature Implementation Sending Messages Components of a method updateTitle: void, id, SEL, id void func(id self, SEL _cmd, id title) { title = title.trimmedString; self.title = title; self.navigationBar.title = title; } SEL IMP "v@:@"
- 23. Sending Messages [object updateTitle:@"Hello"]; let message = void(*)(id,SEL,id)&objc_msgSend; SEL selector = @selector(updateTitle:); message(object, selector, @"Hello"); =
- 24. Expert Runtime Services learn t|he powER of t|he dark Side!
- 26. Add Methods to Classes Function Override What Is It For Used By class_addMethod() +resolveInstanceMethod: +resolveClassMethod: Implement methods on demand, for example getters of @dynamic properties Core Data, CALayer animatable properties, UIAppearance, KVO
- 27. Replace Implementations Functions Override What Is It For Used By class_replaceMethod() method_setImplementation() method_exchangeImplementations() +load in categories Extend existing framework classes with new behaviors UIAppearance, Key-Value Observing, Mocking frameworks ⚠ Warning: It is not trivial to implement method swizzling properly!
- 28. Forward Invocations Override How It Works What Is It For Used By -forwardInvocation: Unrecognized selectors are passed into this method before they throw exception Simulate multiple inheritance, forward methods to internal components, modify arguments NSProxy subclasses, NSUndoManager
- 29. Change Class of Object Functions How It Works What Is It For Used By object_setClass() Object has ivar isa, which points to the class and is used for method lookup Extend behavior of a single object, not all objects of that class Key-Value Observing, Core Data ⚠ Warning: New class should handle all methods of original class!
- 30. Create New Classes Functions How It Works What Is It For Used By objc_allocateClassPair() objc_registerClassPair() Allocate, add methods, ivars, properties, and then register in runtime Customize class methods or for using with object_setClass() Key-Value Observing, Core Data
- 31. self MyClass lookup message What is Metaclass? Implementations are looked up in class Instance Methods
- 32. self MyClass MyMetaClass lookup lookup message message Instance Methods Class Methods What is Metaclass? Class is also an object, so it has a class
- 34. How much is it used? Measured using symbolic breakpoints
- 35. Minimal Swift App import UIKit~~~~~~~~~~~~~~~ @UIApplicationMain~~~~~~~~~~~~~~~ class AppDelegate: NSObject,~~~~~~~~~~~~~~~ UIApplicationDelegate {} • Initializes UIApplication & Main Run Loop • No launch Storyboard or XIB
- 36. Minimal Swift App Look up Class 608× Look up Selector 718× Get / Set Associated Object 254× / 179× Copy Method List 378× Add / Change Method 13× / 1× Create Class 13×
- 37. iOS Dash App • Documentation viewer • Standard UI components • UIKit & WebKit • 20k LOC of Objective-C
- 38. iOS Dash App Look up Class 1357× Look up Selector 1283× Get / Set Associated Object 2803× / 1317× Copy Method List 515× Add / Change Method 87× / 14× Create Class 18×
- 39. iOS Kickstarter App • Crowdfunding app • Customized UI • 120k LOC of mixed code
- 40. iOS Kickstarter App Look up Class 2813× Look up Selector 1455× Get / Set Associated Object 4375× / 1966× Copy Method List 496× Add / Change Method 190× / 10× Create Class 20×
- 41. PaintCode • Vector drawing Mac app • Custom complex UI • 300k LOC of Objective-C • XIBs with Cocoa Bindings
- 42. PaintCode Look up Class 41189× Look up Selector 9615× Get / Set Associated Object 45230× / 2045× Copy Method List 370× Add / Change Method 664× / 1× Create Class 112×
- 43. Ask me anything Thank You Martin Kiss | kish | @Tricertops PixelCut s. r. o.