Blood magic

BloodMagic
Custom property attributes
CocoaHeads, 2014

WHOAMI
Github:
AlexDenisov
IRC: AlexDenisov
1101_debian
Twitter:

Outline
• What BloodMagic is
• How to use it
• How to extend it
• How does it work
• Q & A

Problem
@interface ViewController : UIViewController
@property (nonatomic) NSMutableArray *resources;
@property (nonatomic) ProgressView *progressView;
@property (nonatomic) ErrorView *errorView;
@property (nonatomic) DataLoader *dataLoader;
@end

Lazy Initialization
@implementation ViewController
- (NSMutableArray *)resources
{
if (!_resources) {
_resources = [NSMutableArray new];
}
return _resources;
}
- (ProgressView *)progressView
{
if (!_progressView) {
_progressView = [ProgressView new];
}
return _progressView;
}
- (ErrorView *)errorView
{
if (!_errorView) {
_errorView = [ErrorView new];
}
return _errorView;
}
- (DataLoader *)dataLoader
{
if (!_dataLoader) {
_dataLoader = [DataLoader new];
}
return _dataLoader;
}
@end

- (PropertyClass *)propertyName
{
if (!_propertyName) {
_propertyName = [PropertyClass new];
}
return _propertyName;
}
A lot of Boilerplate Code

Just add ‘lazy’ attribute
@property (nonatomic, lazy) NSMutableArray *resources;
@property (nonatomic, lazy) CenterProgress *centerProgress;
@property (nonatomic, lazy) BottomProgress *bottomProgress;
@property (nonatomic, lazy) DataLoader *dataLoader;
@end

Just add ‘lazy’ attribute
@property (nonatomic, lazy) CenterProgress *centerProgress;
@property (nonatomic, lazy) BottomProgress *bottomProgress;
@property (nonatomic, lazy) DataLoader *dataLoader;
@end
/tmp/lazy_test ➜ make
…
error: unknown property attribute 'lazy'
^
…

BloodMagic
a mechanism for creating
custom property attributes
based on your code

Let’s add magic
#import <BloodMagic/Lazy.h>
<BMLazy>
@property (nonatomic) NSMutableArray *resources;
@property (nonatomic) ProgressView *progressView;
@property (nonatomic) ErrorView *errorView;
@property (nonatomic) DataLoader *dataLoader;
@end

Let’s add magic
@dynamic resources;
@dynamic progressView;
@dynamic errorView;
@dynamic dataLoader;
- (void)actOnSomething
{
[self.dataLoader loadNextPage];
// ^ new object created
}
@end

Magic Happened
@dynamic resources;
@dynamic progressView;
@dynamic errorView;
@end
BloodMagic
- (NSMutableArray *)resources
{
if (!_resources) {
_resources = [NSMutableArray new];
}
return _resources;
}
- (ProgressView *)progressView
{
if (!_progressView) {
_progressView = [ProgressView new];
}
return _progressView;
}
- (ErrorView *)errorView
{
if (!_errorView) {
_errorView = [ErrorView new];
}
return _errorView;
}
- (DataLoader *)dataLoader
{
if (!_dataLoader) {
_dataLoader = [DataLoader new];
}
return _dataLoader;
}
@end

Single Custom Attribute
<BMLazy>
@property (nonatomic, bm_lazy) DataLoader *dataLoader;
@end

#import "ViewController.h"
- (void)viewDidLoad
{
[super viewDidLoad];
}
@end

- (void)viewDidLoad
{
[super viewDidLoad];
// ^ new object created
}
@end

Multiple Custom Attributes
#import <BloodMagic/Partial.h>
<BMLazy,
BMPartial>
@property (nonatomic, bm_lazy) DataLoader *dataLoader;
@property (nonatomic, bm_partial) ErrorView *errorView;
@end

Multiple Custom Attributes
@lazy(dataLoader)
@partial(errorView)
- (void)viewDidLoad
{
[super viewDidLoaded];
[self.view addSubview:self.errorView];
}
@end

@property (nonatomic, bm_preference) NSString *cocoaHeads;

Public Protocol
@protocol BMPreference
<NSObject>
@end

Public Header
#import <BloodMagic/Sources/Modules/Core/Public/BMPublicCoreDefnitions.h>
#import <BloodMagic/Sources/Modules/Preference/Public/BMPreference.h>
#ifndef bm_preference
#define bm_preference
#endif
#ifndef preference
#define preference(property_name) register_module(BMPreference, property_name)
#endif

Private Module Loader
#import <Foundation/Foundation.h>
@interface BMPreferenceModuleLoader : NSObject
@end

Private Module Loader
#import "BMPreferenceModuleLoader.h"
#import "BMPreference.h"
#import "BMBloodMagicInjector.h"
@implementation BMPreferenceModuleLoader
+ (void)load
{
@autoreleasepool {
BMBloodMagicInjector *injector = [BMBloodMagicInjector new];
[injector injectBloodMagicInto:@protocol(BMPreference)];
}
}
@end

Private Hook
#import "BMHook.h"
#import "BMPreference.h"
@interface BMPreferenceHook : NSObject
<BMHook,
BMPreference>
@end

Private Hook
#import "BMPreferenceHook.h"
#import "BMProperty.h"
@implementation BMPreferenceHook
static inline NSUserDefaults *bm_defaults()
{
return [NSUserDefaults standardUserDefaults];
}
+ (void)accessorHook:(id *)value
withProperty:(const BMProperty *)property
sender:(__unused id)sender
{
*value = [bm_defaults() objectForKey:property.name];
}
+ (void)mutatorHook:(id *)value
{
[bm_defaults() setObject:*value forKey:property.name];
}
@end

Private Hook
Accessor
{
}
{
*value = [bm_defaults() objectForKey:property.name];
}

Private Hook
Mutator
{
}
{
[bm_defaults() setObject:*value forKey:property.name];
}

BMPreference Usage
#import <BloodMagic/Preference.h>
@interface Settings : NSObject
<BMPreference>
@property (nonatomic, bm_preference) NSString *name;
@property (nonatomic, bm_preference) NSUInteger age;
@end

BMPreference Usage
#import "Settings.h"
@implementation Settings
@dynamic name;
@dynamic age;
@end

BMPreference Usage
Settings *settings = [Settings new];
settings.name = @"AlexDenisov";
settings.age = 26;
// …
NSLog(@"%@", defaults);

BMPreference Usage
Settings *settings = [Settings new];
settings.name = @"AlexDenisov";
settings.age = 26;
// …
NSLog(@"%@", defaults);
{
…
age = 26;
name = AlexDenisov;
…
}

Core Module: Hooks
@protocol BMHook
<NSObject>
@optional
sender:(id)sender;
sender:(id)sender;
@end

Core Module: Injections
@interface BMBloodMagicInjector : NSObject
- (void)injectBloodMagicInto:(Protocol *)protocol;
@end

Core Module: Injections
@implementation BMBloodMagicInjector
// pseudocode
- (void)injectBloodMagicInto:(Protocol *)protocol
{
class_list_t classes = collectClasses(protocol);
property_list_t properties = collectDynamicProperties(classes);
for (Property *property in properties) {
Hook *hook = hookForProperty(property);
property.accessor = hook.accessor;
property.mutator = hook.mutator;
}
}
@end

Sources:
https://github.com/railsware/BloodMagic
Blog-post:
http://l.rw.rw/dibm
Slides:
https://speakerdeck.com/alexdenisov/bloodmagic
https://speakerdeck.com/0xc010d/dependency-injection-ftw

Blood magic

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