iOS KVO的使用以及原理
简介
KVO：(Key - Value - Observer) 键值观察者,是观察者设计模式的一种具体实现(C层和M层的通信)
KVO触发机制:
一个对象(观察者),检测另一个对象(被观察者)的某属性是否发生变化,若被监测的属性发生了更改,会触发观察者的一个方法(方法名固定,类似代理方法)
KVO的使用
//
// ViewController.m
// kvoTest
//
// Created by Admin on 2021/12/7.
//
#import "ViewController.h"
@interface Person:NSObject
@property (nonatomic,assign) int age;
@end
@implementation Person
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Person *p1 = [[Person alloc] init];
p1.age = 1;
// self 监听 p1的 age属性
NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[p1 addObserver:self forKeyPath:@"age" options:options context:nil];
p1.age = 10;
[p1 removeObserver:self forKeyPath:@"age"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
NSLog(@"监听到%@的%@改变了%@", object, keyPath,change);
}
@end
使用方法
- 注册观察者(为被观察这指定观察者以及被观察者属性)(
addObserver: forKeyPath:) - 实现回调方法(
observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context) - 触发回调方法(
通过点语法或者set方法又或者KVC赋值) - 移除观察者(
removeObserver: forKeyPath:)
<aside>
💡 常见Crash如下
- 观察者未实现监听方法(
观察者对象 observeValueForKeyPath:ofObject:change:context: 未实现) - 未及时移除观察者(
Thread 1: EXC_BAD_ACCESS (code=1, address=0x105e0fee02c0)) - 多次移除(
Cannot remove an observer for the key path “age” from because it is not registered as an observer)
</aside>
KVO的原理
当一个对象使用了
KVO监听，iOS系统会修改这个对象的isa指针，改为指向一个全新的通过Runtime动态创建的子类，子类拥有自己的set方法实现，set方法实现内部会顺序调用willChangeValueForKey方法、原来的setter方法实现、didChangeValueForKey方法，而didChangeValueForKey方法内部又会调用监听器的observeValueForKeyPath:ofObject:change:context:监听方法。
验证代码
//
// ViewController.m
// kvoTest
//
// Created by Admin on 2021/12/7.
//
#import "ViewController.h"
#import <objc/message.h>
@interface Person:NSObject
@property (nonatomic,assign) int age;
@end
@implementation Person
- (void)setAge:(int)age
{
NSLog(@"setAge:");
_age = age;
}
- (void)willChangeValueForKey:(NSString *)key
{
NSLog(@"willChangeValueForKey: - begin");
[super willChangeValueForKey:key];
NSLog(@"willChangeValueForKey: - end");
}
- (void)didChangeValueForKey:(NSString *)key
{
NSLog(@"didChangeValueForKey: - begin");
[super didChangeValueForKey:key];
NSLog(@"didChangeValueForKey: - end");
}
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Person *p1 = [[Person alloc] init];
Person *p2 = [[Person alloc] init];
p1.age = 1;
// self 监听 p1的 age属性
NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[p1 addObserver:self forKeyPath:@"age" options:options context:nil];
[self printMethods: object_getClass(p2)];
[self printMethods: object_getClass(p1)];
p1.age = 10;
[p1 removeObserver:self forKeyPath:@"age"];
}
- (void) printMethods:(Class)cls
{
unsigned int count ;
Method *methods = class_copyMethodList(cls, &count);
NSMutableString *methodNames = [NSMutableString string];
[methodNames appendFormat:@"%@ - ", cls];
for (int i = 0 ; i < count; i++) {
Method method = methods[i];
NSString *methodName = NSStringFromSelector(method_getName(method));
[methodNames appendString: methodName];
[methodNames appendString:@" "];
}
NSLog(@"%@",methodNames);
free(methods);
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
NSLog(@"监听到%@的%@改变了%@", object, keyPath,change);
}
@end
KVO如何手动触发
被监听的属性的值被修改时，就会自动触发KVO。如果想要手动触发KVO，则需要我们自己调用
willChangeValueForKey和didChangeValueForKey方法即可在不改变属性值的情况下手动触发KVO，并且这两个方法缺一不可
验证代码
- (void)viewDidLoad {
[super viewDidLoad];
Person *p1 = [[Person alloc] init];
Person *p2 = [[Person alloc] init];
p1.age = 1;
// self 监听 p1的 age属性
NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
[p1 addObserver:self forKeyPath:@"age" options:options context:nil];
[self printMethods: object_getClass(p2)];
[self printMethods: object_getClass(p1)];
// p1.age = 10;
[p1 willChangeValueForKey:@"age"];
[p1 didChangeValueForKey:@"age"];
[p1 removeObserver:self forKeyPath:@"age"];
}
💡 **willChangeValueForKey和didChangeValueForKey 它们分别做了什么？**
willChangeValueForKey是为了通过KVC方法取得旧值
didChangeValueForKey中会调用监听器的监听方法，最终来到监听者的observeValueForKeyPath方法中
