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KVO的使用与底层探索

KVO的使用与底层探索

作者: 收纳箱 | 来源:发表于2020-04-13 17:16 被阅读0次

    1. 使用

    1.1 自动通知

    // 调用set方法
[account setName:@"Savings"];

// 使用KVC forKey或forKeyPath
[account setValue:@"Savings" forKey:@"name"];
[document setValue:@"Savings" forKeyPath:@"account.name"];

// 使用 mutableArrayValueForKey: 检索关系代理对象
Transaction *newTransaction = <#Create a new transaction for the account#>;
NSMutableArray *transactions = [account mutableArrayValueForKey:@"transactions"];
[transactions addObject:newTransaction];

    示例

    @interface ViewController ()
@property (nonatomic, strong) Person *person;
@property (nonatomic, strong) NSMutableArray<Person *> *people;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // 非集合
    self.person = [[Person alloc] init];
    [self.person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    self.person.name = @"Tom";
    [self.person setValue:@"Jerry" forKey:@"name"];
    [self setValue:@"Tom" forKeyPath:@"person.name"];
    // 集合
    self.people = [NSMutableArray array];
    Person *person0 = [[Person alloc] init];
    person0.name = @"Tom";
    [self.people addObject:person0];
    Person *person1 = [[Person alloc] init];
    person1.name = @"Jerry";
    [self.people addObject:person1];
    NSString *key = @"people";
    [self addObserver:self forKeyPath:key options:NSKeyValueObservingOptionNew context:nil];
    Person *person2 = [[Person alloc] init];
    person2.name = @"Frank";
    NSMutableArray *people = [self mutableArrayValueForKey:key];
    [people addObject:person2];
    NSLog(@"People: \n%@", self.people);
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"name"]) {
        NSLog(@"new name: %@", change[NSKeyValueChangeNewKey]);
    } else if ([keyPath isEqualToString:@"people"]) {
        NSLog(@"new array: %@", change[NSKeyValueChangeNewKey]);
        NSArray<Person *> *people = change[NSKeyValueChangeNewKey];
        NSLog(@"new person: %@", people.firstObject.name);
    }
}
@end
// 输出
new name: Tom
new name: Jerry
new name: Tom
new array: (
    "<Person: 0x60000276cc20>"
)
new person: Frank
People: 
(
    "Person name: Tom",
    "Person name: Jerry",
    "Person name: Frank"
)

    1.2 手动通知

    手动通知提供了更自由的方式去决定什么时间，什么方式去通知观察者。想要使用手动通知必须实现automaticallyNotifiesObserversForKey: (或者automaticallyNotifiesObserversOf<Key>)方法。在一个类中同时使用自动和手动通知是可行的。对于想要手动通知的属性，可以根据它的keyPath返回NO，而其对于其他位置的keyPath，要返回父类的这个方法。

    + (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
    if ([key isEqualToString:@"name"]) {
        return NO;
    } else {
        return [super automaticallyNotifiesObserversForKey:key];
    }
}
// 或者
+ (BOOL)automaticallyNotifiesObserversOfName {
    return NO;
}
    一对一关系
    - (void)setOpeningBalance:(double)theBalance {
     if (theBalance != _openingBalance) {
        [self willChangeValueForKey:@"openingBalance"];
        _openingBalance = theBalance;
        [self didChangeValueForKey:@"openingBalance"];
     }
}

    如果一个操作会导致多个属性改变，需要嵌套通知：

    - (void)setOpeningBalance:(double)theBalance {
     [self willChangeValueForKey:@"openingBalance"];
     [self willChangeValueForKey:@"itemChanged"];
     _openingBalance = theBalance;
     _itemChanged = _itemChanged + 1;
     [self didChangeValueForKey:@"itemChanged"];
     [self didChangeValueForKey:@"openingBalance"];
}
    一对多的关系

    必须注意不仅仅是这个key改变了，还有它改变的类型以及索引。

    - (void)removeTransactionsAtIndexes:(NSIndexSet *)indexes {
     [self willChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:@"transactions"];
     // Remove the transaction objects at the specified indexes.
     [self didChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:@"transactions"];
}

    1.3 键之间的依赖

    在很多种情况下一个属性的值依赖于在其他对象中的属性。如果一个依赖属性的值改变了，这个属性也需要被通知到。

    一对一关系
    @interface Person : NSObject
@property (nonatomic, strong, nullable) NSString *firstName;
@property (nonatomic, strong, nullable) NSString *lastName;
@property (nonatomic, strong, readonly) NSString *fullName;
@end

    可以重写 keyPathsForValuesAffectingValueForKey: 方法。也可以通过实现 keyPathsForValuesAffecting<Key> 方法来达到前面同样的效果，这里的<Key>就是属性名，不过第一个字母要大写。

    @implementation Person
- (NSString *)fullName {
    return [NSString stringWithFormat:@"%@ %@", self.firstName, self.lastName];
}

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    if ([key isEqualToString:@"fullName"]) {
        NSArray *affectingKeys = @[@"lastName", @"firstName"];
        keyPaths = [keyPaths setByAddingObjectsFromArray:affectingKeys];
    }
    return keyPaths;
}
// 或者
+ (NSSet *)keyPathsForValuesAffectingFullName {
    return [NSSet setWithObjects:@"lastName", @"firstName", nil];
}
@end
    一对多关系

    keyPathsForValuesAffectingValueForKey:方法不能支持一对多关系。

    举个例子，比如你有一个Department对象，和很多个Employee对象。而Employee有一个salary属性。你可能希望Department对象有一个totalSalary的属性，依赖于所有的Employee的salary 。

    注册Department成为所有Employee的观察者。当Employee被添加或者被移除时进行计算。

    - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if (context == totalSalaryContext) {
        [self setTotalSalary:[self valueForKeyPath:@"employees.@sum.salary"]];
    }
    else
    // deal with other observations and/or invoke super...
}
 
- (void)setTotalSalary:(NSNumber *)newTotalSalary {
    if (totalSalary != newTotalSalary) {
        [self willChangeValueForKey:@"totalSalary"];
        _totalSalary = newTotalSalary;
        [self didChangeValueForKey:@"totalSalary"];
    }
}
 
- (NSNumber *)totalSalary {
    return _totalSalary;
}

    2. 实现细节

    2.1 isa-swizzling

    KVO的实现用了一种叫 isa-swizzling 的技术。

    当一个对象的一个属性注册了观察者后，被观察对象的isa指针的就指向了一个系统为我们生成的中间类，而不是我们自己创建的类。在这个类中，系统为我们重写了被观察属性的setter方法。

    通过 object_getClass(id obj) 方法可以获得实例对象真实的类(isa指针的指向)。

    @interface Person : NSObject
@property (nonatomic, strong, nullable) NSString *name;
@end
@implementation Person
@end
  
@interface ViewController ()
@property (nonatomic, strong) Person *p1;
@property (nonatomic, strong) Person *p2;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.p1 = [[Person alloc] init];
    self.p2 = [[Person alloc] init];  
    self.p1.name = @"Tom";
    
    NSLog(@"before kvo --- p2: %s", object_getClassName(self.p2));
    [self.p2 addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    NSLog(@"after  kvo --- p2: %s", object_getClassName(self.p2));
    
    self.p2.name = @"Jerry";
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"name"]) {
        NSLog(@"new name: %@", change[NSKeyValueChangeNewKey]);
    }
}
@end
// 输出
before kvo --- p2: Person
after  kvo --- p2: NSKVONotifying_Person
new name: Jerry

    我们在p2实例对象被键值观察的前后打印其isa指针(实际使用的类)。

    从结果中我们可以看到isa指针指向了一个中间类NSKVONotifying_Person。

    苹果的KVO中间类的命名规则是在类名前添加NSKVONotifying_，如果我们的类叫SonKVO之后的中间类为NSKVONotifying_Son。

    2.2 IMP

    我们再看一下KVO前后的函数方法的地址是否一样。

    - (void)viewDidLoad {
    [super viewDidLoad];
    self.p1 = [[Person alloc] init];
    self.p2 = [[Person alloc] init];
    self.p1.name = @"Tom";
    
    NSLog(@"before kvo --- p1: %p p2: %p", [self.p1 methodForSelector:@selector(setName:)], [self.p2 methodForSelector:@selector(setName:)]);
    [self.p2 addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    NSLog(@" after  kvo --- p1: %p p2: %p", [self.p1 methodForSelector:@selector(setName:)], [self.p2 methodForSelector:@selector(setName:)]);
    
    self.p2.name = @"Jerry";
}
// 输出
before kvo --- p1: 0x10ccee670 p2: 0x10ccee670
after  kvo --- p1: 0x10ccee670 p2: 0x7fff258e454b

    我们看到监听之间两个实例对象的setName:方法的函数地址相同，KVO之后p2实例对象的setName:方法地址变了。

    我们可以查看一下这个方法地址：

    (lldb) image lookup -a 0x7fff258e454b
      Address: Foundation[0x000000000006954b] (Foundation.__TEXT.__text + 422667)
      Summary: Foundation`_NSSetObjectValueAndNotify

    这个是Foundation框架中的一个私有方法_NSSetObjectValueAndNotify。

    3. 自定义KVO

    下面我们根据KVO的实现细节，仿写一个简化版的KVO。

    NSString *ObserverKey = @"SetterMethodKey";
// 根据方法名获取Key
NSString *getKeyForSetter(NSString *setter) {
    NSRange range = NSMakeRange(3, setter.length - 4);
    NSString *key = [setter substringWithRange:range];
    NSString *letter = [[key substringToIndex:1] lowercaseString];
    key = [key stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:letter];
    return key;
}
// 实现一个setter和通知函数
void _MySetObjectValueAndNotify(id self, SEL selector, NSString *name) {
    // 1.调用父类的方法
    struct objc_super superClass = {
        self,
        class_getSuperclass([self class])
    };
    objc_msgSendSuper(&superClass, selector, name);
    // 2.通知观察者
    NSObject *observer = objc_getAssociatedObject(self, &ObserverKey);
    NSString *selectorName = NSStringFromSelector(selector);
    NSString *key = getKeyForSetter(selectorName);
    objc_msgSend(observer, @selector(observeValueForKeyPath:ofObject:change:context:), key, self, @{NSKeyValueChangeNewKey: name}, nil);
}

@implementation Person
- (void)snx_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context {
    // 1.创建一个子类
    NSString *oldName = NSStringFromClass([self class]);
    NSString *newName = [NSString stringWithFormat:@"CustomKVO_%@", oldName];
    Class customClass = objc_allocateClassPair([self class], newName.UTF8String, 0);
    objc_registerClassPair(customClass);
    // 2.修改修改isa指针
    object_setClass(self, customClass);
    // 3.重写set方法
    NSString *selectorName = [NSString stringWithFormat:@"set%@:", keyPath.capitalizedString];
    SEL sel = NSSelectorFromString(selectorName);
    class_addMethod(customClass, sel, (IMP)_MySetObjectValueAndNotify, "v@:@");
    // 4.绑定观察者
    objc_setAssociatedObject(self, &ObserverKey, observer, OBJC_ASSOCIATION_ASSIGN);
}
@end

    重要

    使用objc_msgSendSuper时，可能编译器会报错：

    Too many arguments to function call, expected 0, have 3

    解决办法：在Build Setting修改Enable Strict Checking of objc_msgSend Calls为No。

    - (void)viewDidLoad {
    [super viewDidLoad];
    self.p1 = [[Person alloc] init];
    self.p2 = [[Person alloc] init];
    self.p1.name = @"Tom";
    
    NSLog(@"before kvo --- p2: %s", object_getClassName(self.p2));
    NSLog(@"before kvo --- p1: %p p2: %p", [self.p1 methodForSelector:@selector(setName:)], [self.p2 methodForSelector:@selector(setName:)]);
//    [self.p2 addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    [self.p2 snx_addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew context:nil];
    NSLog(@"after  kvo --- p2: %s", object_getClassName(self.p2));
    NSLog(@"after  kvo --- p1: %p p2: %p", [self.p1 methodForSelector:@selector(setName:)], [self.p2 methodForSelector:@selector(setName:)]);
    
    self.p2.name = @"Jerry";
}
// 输出
before kvo --- p2: Person
before kvo --- p1: 0x103514460 p2: 0x103514460
after  kvo --- p2: CustomKVO_Person
after  kvo --- p1: 0x103514460 p2: 0x103513f90
new name: Jerry

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