iOS：KVO
2019-11-09 本文已影响0人
码小菜
风景
目录
一，基本知识
二，NSKVONotifying_Person
三，NSKVONotifying_Person实例方法
四，setAge:方法
五，class，dealloc，_isKVOA
六，手动发送通知
七，使用进阶
八，移除观察者
一，基本知识
1，概念
KVO是Key-Value-Observing的缩写，意思是键值观察，作用是监听对象某个属性的变化
2，使用
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
self.person.age = 1;
// 添加监听
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
self.person.age = 2;
}
// 监听到变化的回调
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context {
if (context == @"111") {
NSLog(@"%@---%@---%@", object, keyPath, change);
}
}
- (void)dealloc {
// 移除监听
[self.person removeObserver:self
forKeyPath:@"age"
context:@"111"];
}
// 打印
<Person: 0x600003ae8ea0>---age---{
kind = 1;
new = 2;
old = 1;
}
二，NSKVONotifying_Person
当self.person添加监听后，系统会自动生成Person的子类NSKVONotifying_Person，并让self.person的isa指针指向它的类对象
self.person = [Person new];
NSLog(@"添加监听前：%@", object_getClass(self.person));
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
NSLog(@"添加监听后：%@", object_getClass(self.person));
// 打印
添加监听前：Person
添加监听后：NSKVONotifying_Person
添加监听前
添加监听后
三，NSKVONotifying_Person实例方法
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
[self logMethodNamesWithClass:[Person class]];
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
[self logMethodNamesWithClass:[Person class]];
}
// 打印类对象或元类对象中的方法名
- (void)logMethodNamesWithClass:(Class)cls {
unsigned int count;
Method *methodList = class_copyMethodList(cls, &count);
NSMutableString *methodNames = [NSMutableString string];
for (int i = 0; i < count; i++) {
Method method = methodList[i];
SEL selector = method_getName(method);
NSString *methodName = NSStringFromSelector(selector);
[methodNames appendString:methodName];
[methodNames appendString:@", "];
}
free(methodList);
NSLog(@"%@---%@", cls, methodNames);
}
// 打印
Person---setAge:, age,
NSKVONotifying_Person---setAge:, class, dealloc, _isKVOA,
实例方法
四，setAge:方法
1，在NSKVONotifying_Person类中的实现
- (void)setAge:(NSInteger)age {
[self willChangeValueForKey:@"age"];
[super setAge:age];
[self didChangeValueForKey:@"age"];
}
运行下面代码来验证一下
@implementation Person
- (void)setAge:(NSInteger)age {
_age = age;
NSLog(@"%@:setAge", object_getClass(self));
}
- (void)willChangeValueForKey:(NSString *)key {
[super willChangeValueForKey:key];
NSLog(@"%@:willChangeValueForKey", object_getClass(self));
}
- (void)didChangeValueForKey:(NSString *)key {
[super didChangeValueForKey:key];
NSLog(@"%@:didChangeValueForKey", object_getClass(self));
}
@end
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
NSLog(@"添加监听前");
self.person.age = 1;
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
NSLog(@"添加监听后");
self.person.age = 2;
}
// 打印
添加监听前
Person:setAge
添加监听后
NSKVONotifying_Person:willChangeValueForKey
NSKVONotifying_Person:setAge
NSKVONotifying_Person:didChangeValueForKey
2，didChangeValueForKey:方法
- (void)didChangeValueForKey:(NSString *)key {
NSLog(@"%@:didChangeValueForKey - begin", object_getClass(self));
[super didChangeValueForKey:key];
NSLog(@"%@:didChangeValueForKey - end", object_getClass(self));
}
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context {
if (context == @"111") {
NSLog(@"%@---%@---%@", object_getClass(object), keyPath, change);
}
}
// 打印
NSKVONotifying_Person:didChangeValueForKey - begin
NSKVONotifying_Person---age---{
kind = 1;
new = 2;
old = 1;
}
NSKVONotifying_Person:didChangeValueForKey - end
由上可以看出是在[super didChangeValueForKey:key]中触发observeValueForKeyPath:方法的，所以在NSObject中此方法的实现如下
- (void)didChangeValueForKey:(NSString *)key {
// 参数是在添加监听时保存的
[observer observeValueForKeyPath:keyPath
ofObject:self
change:change
context:context];
}
五，class，dealloc，_isKVOA
1，class
问题：self.person本质是NSKVONotifying_Person，为什么class方法返回Person？
答：因为NSKVONotifying_Person重写了class方法，返回了父类Person，这么做是为了隐藏底层实现细节，如果返回NSKVONotifying_Person，class方法的调用者会感到莫名其妙
self.person = [Person new];
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
NSLog(@"%@", [self.person class]);
NSLog(@"%@", object_getClass(self.person));
// 打印
Person
NSKVONotifying_Person
2，dealloc
因为系统会自动生成NSKVONotifying_Person，所以需要重写dealloc方法，以便在销毁时清理系统产生的垃圾
3，_isKVOA
标识NSKVONotifying_Person是KVO产生的类
- (BOOL)_isKVOA {
return YES;
}
六，手动发送通知
1，automaticallyNotifiesObserversForKey:方法是一个开关，如果返回YES，表示允许自动发送通知，修改属性值会收到回调；如果返回NO，表示不允许自动发送通知，修改属性值不会收到回调，必须手动发送通知才会收到回调；此方法默认返回YES
2，手动发送通知必须同时调用willChangeValueForKey:方法和didChangeValueForKey:方法
@implementation Person
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
return NO;
}
@end
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
self.person.age = 1;
[self.person addObserver:self
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
// 手动通知
[self.person willChangeValueForKey:@"age"];
self.person.age = 2;
[self.person didChangeValueForKey:@"age"];
}
// 打印
<Person: 0x6000020d72f0>---age---{
kind = 1;
new = 2;
old = 1;
}
七，使用进阶
1，属性依赖
一个对象的值取决于它内部所有属性的值，这种依赖关系可以通过keyPathsForValuesAffectingValueForKey:或keyPathsForValuesAffecting<#DependentKey#>方法进行设置，一旦给该对象添加了监听，它内部任何属性的改变都会通知观察者
// Dog
@interface Dog : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end
@implementation Dog
- (NSString *)description {
return [NSString stringWithFormat:@"name: %@, age: %zd", _name, _age];
}
@end
// Person
@interface Person : NSObject
@property (nonatomic, strong) Dog *dog;
@end
@implementation Person
- (instancetype)init {
if (self = [super init]) {
_dog = [Dog new];
}
return self;
}
//+ (NSSet<NSString *> *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
// if ([key isEqualToString:@"dog"]) {
// return [NSSet setWithObjects:@"_dog.name", @"_dog.age", nil];
// } else {
// return [super keyPathsForValuesAffectingValueForKey:key];
// }
//}
+ (NSSet<NSString *> *)keyPathsForValuesAffectingDog {
return [NSSet setWithObjects:@"_dog.name", @"_dog.age", nil];
}
@end
// 使用Person
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
self.person.dog.name = @"xiaoHuang";
self.person.dog.age = 1;
[self.person addObserver:self
forKeyPath:@"dog"
options:NSKeyValueObservingOptionNew
context:@"111"];
self.person.dog.name = @"xiaoHei";
}
// 打印
<Person: 0x6000004783d0>---dog---{
kind = 1;
new = "name: xiaoHei, age: 1";
}
2，监听容器对象中数据的变化
不能直接拿容器对象进行操作，必须通过mutableArrayValueForKey:方法来获取容器对象，这样才能触发回调
@interface Person : NSObject
@property (nonatomic, strong) NSMutableArray *friends;
@end
- (void)viewDidLoad {
[super viewDidLoad];
self.person = [Person new];
self.person.friends = @[@"zhangSan"].mutableCopy;
[self.person addObserver:self
forKeyPath:@"friends"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
// [self.person.friends addObject:@"liSi"];
[[self.person mutableArrayValueForKey:@"friends"] addObject:@"liSi"];
}
// 打印
<Person: 0x600002d30040>---friends---{
indexes = "<_NSCachedIndexSet: 0x600002f669c0>[number of indexes: 1 (in 1 ranges), indexes: (1)]";
kind = 2;
new = (
liSi
);
}
3，监听容器对象中元素（对象）的属性变化
添加监听和移除监听用另外的方法
@interface ViewController ()
@property (nonatomic, strong) NSArray *pArray;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Person *p1 = [Person new];
p1.age = 1;
Person *p2 = [Person new];
p2.age = 2;
self.pArray = @[p1, p2];
NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, self.pArray.count)];
[self.pArray addObserver:self
toObjectsAtIndexes:indexSet
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
p1.age = 3;
}
- (void)dealloc {
NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, self.pArray.count)];
[self.pArray removeObserver:self
fromObjectsAtIndexes:indexSet
forKeyPath:@"age"
context:@"111"];
}
@end
// 打印
<Person: 0x6000036abc90>---age---{
kind = 1;
new = 3;
old = 1;
}
八，移除观察者
1，在观察者销毁时必须把它移除掉，否则在再次改变属性值时会crash
// Person
@implementation Person
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context {
if (context == @"111") {
NSLog(@"%@---%@---%@", object, keyPath, change);
}
}
- (void)dealloc {
NSLog(@"Person dealloc");
}
@end
// Dog
@interface Dog : NSObject
@property (nonatomic, assign) NSInteger age;
@end
// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
[self.dog addObserver:self.person
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
self.dog.age = 2;
self.person = nil;
self.dog.age = 3; // crash
// 打印
<Dog: 0x60000203c420>---age---{
kind = 1;
new = 2;
old = 1;
}
Person dealloc
2，在移除观察者时必须保证观察者没有销毁，否则会crash
// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
[self.dog addObserver:self.person
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
self.dog.age = 2;
self.person = nil;
[self.dog removeObserver:self.person
forKeyPath:@"age"
context:@"111"];
// 打印
<Dog: 0x600001310830>---age---{
kind = 1;
new = 2;
old = 1;
}
Person dealloc
*** Terminating app due to uncaught exception 'NSRangeException',
reason: 'Cannot remove an observer <(null) 0x0> for the key path "age" from
<Dog 0x600001310830> because it is not registered as an observer.'
3，移除观察者与添加观察者的次数必须保持一致，否则会crash
// 使用
self.person = [Person new];
self.dog = [Dog new];
self.dog.age = 1;
BOOL isAdd = NO;
if (isAdd) {
[self.dog addObserver:self.person
forKeyPath:@"age"
options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew
context:@"111"];
}
self.dog.age = 2;
[self.dog removeObserver:self.person
forKeyPath:@"age"
context:@"111"];
// 打印
*** Terminating app due to uncaught exception 'NSRangeException',
reason: 'Cannot remove an observer <Person 0x600003173560> for the key path "age" from
<Dog 0x600003173570> because it is not registered as an observer.'
