iOS KVO的使用以及原理

簡(jiǎn)介

KVO：(Key - Value - Observer) 鍵值觀察者,是觀察者設(shè)計(jì)模式的一種具體實(shí)現(xiàn)(C層和M層的通信)

KVO觸發(fā)機(jī)制:

一個(gè)對(duì)象(觀察者),檢測(cè)另一個(gè)對(duì)象(被觀察者)的某屬性是否發(fā)生變化,若被監(jiān)測(cè)的屬性發(fā)生了更改,會(huì)觸發(fā)觀察者的一個(gè)方法(方法名固定,類似代理方法)

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 監(jiān)聽(tīng) p1的 age屬性
    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(@"監(jiān)聽(tīng)到%@的%@改變了%@", object, keyPath,change);
}

@end

使用方法

• 注冊(cè)觀察者(為被觀察這指定觀察者以及被觀察者屬性)(addObserver: forKeyPath:)
• 實(shí)現(xiàn)回調(diào)方法(observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context)
• 觸發(fā)回調(diào)方法(通過(guò)點(diǎn)語(yǔ)法或者set方法又或者KVC賦值)
• 移除觀察者(removeObserver: forKeyPath:)

<aside>
?? 常見(jiàn)Crash如下

• 觀察者未實(shí)現(xiàn)監(jiān)聽(tīng)方法(觀察者對(duì)象 observeValueForKeyPath:ofObject:change:context: 未實(shí)現(xiàn))
• 未及時(shí)移除觀察者(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

當(dāng)一個(gè)對(duì)象使用了KVO監(jiān)聽(tīng)，iOS系統(tǒng)會(huì)修改這個(gè)對(duì)象的isa指針，改為指向一個(gè)全新的通過(guò)Runtime動(dòng)態(tài)創(chuàng)建的子類，子類擁有自己的set方法實(shí)現(xiàn)，set方法實(shí)現(xiàn)內(nèi)部會(huì)順序調(diào)用willChangeValueForKey方法、原來(lái)的setter方法實(shí)現(xiàn)、didChangeValueForKey方法，而didChangeValueForKey方法內(nèi)部又會(huì)調(diào)用監(jiān)聽(tīng)器的observeValueForKeyPath:ofObject:change:context:監(jiān)聽(tīng)方法。

驗(yàn)證代碼

//
//  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 監(jiān)聽(tīng) p1的 age屬性
    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(@"監(jiān)聽(tīng)到%@的%@改變了%@", object, keyPath,change);
}

@end

KVO如何手動(dòng)觸發(fā)

被監(jiān)聽(tīng)的屬性的值被修改時(shí)，就會(huì)自動(dòng)觸發(fā)KVO。如果想要手動(dòng)觸發(fā)KVO，則需要我們自己調(diào)用willChangeValueForKey和didChangeValueForKey方法即可在不改變屬性值的情況下手動(dòng)觸發(fā)KVO，并且這兩個(gè)方法缺一不可

驗(yàn)證代碼

- (void)viewDidLoad {
    [super viewDidLoad];
    Person *p1 = [[Person alloc] init];
    Person *p2 = [[Person alloc] init];
    p1.age = 1;
    // self 監(jiān)聽(tīng) p1的 age屬性
    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 它們分別做了什么？**
willChangeValueForKey是為了通過(guò)KVC方法取得舊值
didChangeValueForKey中會(huì)調(diào)用監(jiān)聽(tīng)器的監(jiān)聽(tīng)方法，最終來(lái)到監(jiān)聽(tīng)者的observeValueForKeyPath方法中