iOS日常积累

iOS 自定义KVO

2017-10-18  本文已影响755人  mws100

自己实现kvo之前，需要知道iOS系统对kvo的实现。

系统实现kvo的原理

这依赖了OC强大的runtime特性。
在我们对某个Student的实例对象的name属性addObserver时，系统会动态创建一个继承自Student的类（NSKVONotifying_Student），并重写setName:方法。在这里获取新值、旧值、调用父类方法、并发通知给监听者。把对象的isa指针(指向该对象所属的类对象)指向该子类。

同时，系统还重写了class方法，使该方法返回父类名称，以使开发者不会察觉。

我们通过断点调试，验证下：

- (void)viewDidLoad {
    [super viewDidLoad];    
    _s = [[Student alloc] init];
    [_s addObserver:self forKeyPath:@"score" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
}

在addObserver处打断点。

监听之前isa指向.jpg
可以看到s的isa指针指向Student类。单步执行下： 监听之后isa指向.jpg

isa指针指向了NSKVONotifying_Student类。

自定义kvo

既然所有对象都能使用kvo的方法，那先建个NSObject的category。并添加addObserver方法：

//  NSObject+Extension.h
#import <Foundation/Foundation.h>

@interface NSObject (Extension)
- (void)ws_addObserver:(NSObject *_Nonnull)observer forKeyPath:(NSString *_Nonnull)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
@end

就是系统方法加了前缀。
仿照系统的方式进行实现。

接下来 一步步实现：

1.动态创建子类(WSKVONotifying_原类名)

NSString *oldClassName = NSStringFromClass([self class]);
NSString *newClassName = [NSString stringWithFormat:@"WSKVONotifying_%@", oldClassName];
    
Class class = objc_getClass(newClassName.UTF8String);
if (!class) {
    class = objc_allocateClassPair([self class], newClassName.UTF8String, 0);
    objc_registerClassPair(class);
}

2.重写被监听属性的set方法

//set方法首字母大写
NSString *keyPathChange = [[[keyPath substringToIndex:1] uppercaseString] stringByAppendingString:[keyPath substringFromIndex:1]];
NSString *setNameStr = [NSString stringWithFormat:@"set%@", keyPathChange];
SEL setSEL = NSSelectorFromString([setNameStr stringByAppendingString:@":"]);

//添加set方法
Method getMethod = class_getInstanceMethod([self class], @selector(keyPath));
const char *types = method_getTypeEncoding(getMethod);
class_addMethod(class, setSEL, (IMP)setMethod, types);


void setMethod(id self, SEL _cmd, id newValue) {
//获取get、set方法名
NSString *setNameStr = objc_getAssociatedObject(self, WSKVO_setter);
NSString *getNameStr = objc_getAssociatedObject(self, WSKVO_getter);
    
//保存子类类型
Class class = [self class];
    
//isa指向原类
object_setClass(self, class_getSuperclass(class));
    
//调用原类get方法，获取oldValue
id oldValue = objc_msgSend(self, NSSelectorFromString(getNameStr));
    
//调用原类set方法
objc_msgSend(self, NSSelectorFromString([setNameStr stringByAppendingString:@":"]), newValue);
    
//调用observer的observeValueForKeyPath: ofObject: change: context:方法
id observer = objc_getAssociatedObject(self, WSKVO_observer);
    
NSMutableDictionary *change = @{}.mutableCopy;
if (newValue) {
    change[NSKeyValueChangeNewKey] = newValue;
}
if (oldValue) {
    change[NSKeyValueChangeOldKey] = oldValue;
}
    
objc_msgSend(observer, @selector(observeValueForKeyPath: ofObject: change: context:), getNameStr, self, change, nil);
    
//isa改回子类类型
object_setClass(self, class);
}

3.改变isa指针的指向

object_setClass(self, class);

上面是分段写的，接下来附上完整的.m实现：

#import "NSObject+Extension.h"
#import <objc/message.h>

static const char *WSKVO_observer = "WSKVO_observer";
static const char *WSKVO_getter = "WSKVO_getter";
static const char *WSKVO_setter = "WSKVO_setter";

@implementation NSObject (Extension)

- (void)ws_addObserver:(NSObject *_Nonnull)observer forKeyPath:(NSString *_Nonnull)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context {
    //创建、注册子类
    NSString *oldClassName = NSStringFromClass([self class]);
    NSString *newClassName = [NSString stringWithFormat:@"WSKVONotifying_%@", oldClassName];
    
    Class class = objc_getClass(newClassName.UTF8String);
    if (!class) {
        class = objc_allocateClassPair([self class], newClassName.UTF8String, 0);
        objc_registerClassPair(class);
    }
    
    //set方法首字母大写
    NSString *keyPathChange = [[[keyPath substringToIndex:1] uppercaseString] stringByAppendingString:[keyPath substringFromIndex:1]];
    NSString *setNameStr = [NSString stringWithFormat:@"set%@", keyPathChange];
    SEL setSEL = NSSelectorFromString([setNameStr stringByAppendingString:@":"]);
    
    //添加set方法
    Method getMethod = class_getInstanceMethod([self class], @selector(keyPath));
    const char *types = method_getTypeEncoding(getMethod);
    class_addMethod(class, setSEL, (IMP)setMethod, types);
    
    //改变isa指针，指向子类
    object_setClass(self, class);
    
    //保存observer
    objc_setAssociatedObject(self, WSKVO_observer, observer, OBJC_ASSOCIATION_ASSIGN);
    
    //保存set、get方法名
    objc_setAssociatedObject(self, WSKVO_setter, setNameStr, OBJC_ASSOCIATION_COPY_NONATOMIC);
    objc_setAssociatedObject(self, WSKVO_getter, keyPath, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

void setMethod(id self, SEL _cmd, id newValue) {
    //获取get、set方法名
    NSString *setNameStr = objc_getAssociatedObject(self, WSKVO_setter);
    NSString *getNameStr = objc_getAssociatedObject(self, WSKVO_getter);
    
    //保存子类类型
    Class class = [self class];
    
    //isa指向原类
    object_setClass(self, class_getSuperclass(class));
    
    //调用原类get方法，获取oldValue
    id oldValue = objc_msgSend(self, NSSelectorFromString(getNameStr));
    
    //调用原类set方法
    objc_msgSend(self, NSSelectorFromString([setNameStr stringByAppendingString:@":"]), newValue);
    
    //调用observer的observeValueForKeyPath: ofObject: change: context:方法
    id observer = objc_getAssociatedObject(self, WSKVO_observer);
    
    NSMutableDictionary *change = @{}.mutableCopy;
    if (newValue) {
        change[NSKeyValueChangeNewKey] = newValue;
    }
    if (oldValue) {
        change[NSKeyValueChangeOldKey] = oldValue;
    }
    
    objc_msgSend(observer, @selector(observeValueForKeyPath: ofObject: change: context:), getNameStr, self, change, nil);
    
    //isa改回子类类型
    object_setClass(self, class);
}

@end

测试下

测试代码如下：可以正常实现监听

- (void)viewDidLoad {
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor purpleColor];
    
    _s = [[Student alloc] init];
    
    //使用自定义方法实现kvo监听
    [_s ws_addObserver:self forKeyPath:@"score" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"old=%@ new=%@", change[NSKeyValueChangeOldKey], change[NSKeyValueChangeNewKey]);
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    static NSInteger num = 0;
    _s.score = @(num++);
}

Demo地址

update 2017.10.26

为kvo添加block回调

大部分代码没有变，只是添加了block回调。

//  NSObject+WSKVO.h
#import <Foundation/Foundation.h>

typedef void(^WSKVOBlock)(NSDictionary * _Nonnull change);

@interface NSObject (WSKVO)

- (void)ws_observerkeyPath:(NSString *_Nonnull)keyPath options:(NSKeyValueObservingOptions)options block:(WSKVOBlock _Nonnull )block;

@end

在方法实现中，使用关联对象保存block，在动态添加的setter方法中取出并执行block。

NSObject+WSKVO.m
#import "NSObject+WSKVO.h"
#import <objc/message.h>

static const char *WSKVO_getter = "WSKVO_getter";
static const char *WSKVO_setter = "WSKVO_setter";
static const char *WSKVO_block = "WSKVO_block";

@implementation NSObject (WSKVO)

- (void)ws_observerkeyPath:(NSString *_Nonnull)keyPath options:(NSKeyValueObservingOptions)options block:(WSKVOBlock _Nonnull )block {
    //创建、注册子类
    NSString *oldClassName = NSStringFromClass([self class]);
    NSString *newClassName = [NSString stringWithFormat:@"WSKVONotifying_%@", oldClassName];
    
    Class class = objc_getClass(newClassName.UTF8String);
    if (!class) {
        class = objc_allocateClassPair([self class], newClassName.UTF8String, 0);
        objc_registerClassPair(class);
    }
    
    //set方法首字母大写
    NSString *keyPathChange = [[[keyPath substringToIndex:1] uppercaseString] stringByAppendingString:[keyPath substringFromIndex:1]];
    NSString *setNameStr = [NSString stringWithFormat:@"set%@", keyPathChange];
    SEL setSEL = NSSelectorFromString([setNameStr stringByAppendingString:@":"]);
    
    //添加set方法
    Method getMethod = class_getInstanceMethod([self class], @selector(keyPath));
    const char *types = method_getTypeEncoding(getMethod);
    class_addMethod(class, setSEL, (IMP)setMethod, types);
    
    //改变isa指针，指向子类
    object_setClass(self, class);
        
    //保存set、get方法名
    objc_setAssociatedObject(self, WSKVO_setter, setNameStr, OBJC_ASSOCIATION_COPY_NONATOMIC);
    objc_setAssociatedObject(self, WSKVO_getter, keyPath, OBJC_ASSOCIATION_COPY_NONATOMIC);
    
    //保存block
    objc_setAssociatedObject(self, WSKVO_block, block, OBJC_ASSOCIATION_COPY);
}

void setMethod(id self, SEL _cmd, id newValue) {
    //获取get、set方法名
    NSString *setNameStr = objc_getAssociatedObject(self, WSKVO_setter);
    NSString *getNameStr = objc_getAssociatedObject(self, WSKVO_getter);
    
    //保存子类类型
    Class class = [self class];
    
    //isa指向原类
    object_setClass(self, class_getSuperclass(class));
    
    //调用原类get方法，获取oldValue
    id oldValue = objc_msgSend(self, NSSelectorFromString(getNameStr));
    
    //调用原类set方法
    objc_msgSend(self, NSSelectorFromString([setNameStr stringByAppendingString:@":"]), newValue);
    
    NSMutableDictionary *change = @{}.mutableCopy;
    if (newValue) {
        change[NSKeyValueChangeNewKey] = newValue;
    }
    if (oldValue) {
        change[NSKeyValueChangeOldKey] = oldValue;
    }
    
    //取出block
    WSKVOBlock block = objc_getAssociatedObject(self, WSKVO_block);
    if (block) {
        block(change);
    }
    
    //isa改回子类类型
    object_setClass(self, class);
}

@end

使用block注意避免循环引用问题。

demo已更新，为方便查看，两种方式写在了两个分类中。Demo地址

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