iOS底層原理（二）KVO和KVC-有解無憂

KVO

KVO的全稱是Key-Value Observing，俗稱“鍵值監聽”，可以用于監聽某個物件屬性值的改變

KVO的使用

可以通過addObserver: forKeyPath:方法對屬性發起監聽，然后通過observeValueForKeyPath: ofObject: change:方法中對應進行監聽，見下面示例代碼

// 示例代碼
@interface Person : NSObject

@property (assign, nonatomic) int age;
@property (assign, nonatomic) int height;
@end

@implementation Person

@end

@interface ViewController ()

@property (strong, nonatomic) Person *person1;
@property (strong, nonatomic) Person *person2;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person1 = [[Person alloc] init];
    self.person1.age = 1;
    
    self.person2 = [[Person alloc] init];
    self.person2.age = 2;
    
    // 列印添加監聽之前person1和person2對應的isa指標指向的型別
    NSLog(@"person1添加KVO監聽之前 - %@ %@",
          object_getClass(self.person1),
          object_getClass(self.person2)); 
    // 列印結果：Person Person
    
    // 列印添加監聽之前person1和person2對應的setAge方法是否有改變
    NSLog(@"person1添加KVO監聽之前 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
    // 0x10b60c4b0 0x10b60c4b0
          
    // 給person1物件添加KVO監聽
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld;
    [self.person1 addObserver:self forKeyPath:@"age" options:options context:@"123"];
    
    // 列印添加監聽之后person1和person2對應的isa指標指向的型別
    NSLog(@"person1添加KVO監聽之后 - %@ %@",
          object_getClass(self.person1),
          object_getClass(self.person2)); 
	// 列印結果：NSKVONotifying_Person Person
	
	 // 列印添加監聽之后person1和person2對應的setAge方法是否有改變
    NSLog(@"person1添加KVO監聽之前 - %p %p",
          [self.person1 methodForSelector:@selector(setAge:)],
          [self.person2 methodForSelector:@selector(setAge:)]);
    // 0x7fff207b62b7 0x10b60c4b0
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    self.person1.age = 20;    
}

- (void)dealloc {
    [self.person1 removeObserver:self forKeyPath:@"age"];
}

// 當監聽物件的屬性值發生改變時，就會呼叫
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    NSLog(@"監聽到%@的%@屬性值改變了 - %@ - %@", object, keyPath, change, context);
}

@end

注意：監聽的物件銷毀之前要移除該監聽removeObserver: forKeyPath:

KVO的實作本質

1.通過上面示例代碼發現，函式在呼叫addObserver: forKeyPath:方法之后，person1的實體物件的isa指標指向了一個新的型別NSKVONotifying_Person，而沒有添加監聽的person2的isa指標還是指向了Person這個型別

2.我們發現通過object_getClass列印person1的類物件和元類物件都是新派生出來的NSKVONotifying_Person這個型別

NSLog(@"類物件 - %@ %@",
          object_getClass(self.person1), 
          object_getClass(self.person2)); 
// NSKVONotifying_Person Person

NSLog(@"元類物件 - %@ %@",
          object_getClass(object_getClass(self.person1)), 
          object_getClass(object_getClass(self.person2))); 
// NSKVONotifying_Person Person

3.我們發現通過object_getClass列印person1的superclass是Person這個型別，說明新派生出來的NSKVONotifying_Person是Person的子類

NSLog(@"父類 - %@ %@", 
		object_getClass(self.person1).superclass,
        object_getClass(self.person2).superclass);
        // Person NSObject

4.通過列印我們發現，person1呼叫的setAge方法的記憶體地址發生了改變，通過LLDB列印該地址的詳細資訊發現setAge方法的實作實際是Foundation框架中的_NSSetIntValueAndNotify這個函式

(lldb) p (IMP)0x7fff207b62b7
(IMP) $2 = 0x00007fff207b62b7 (Foundation`_NSSetIntValueAndNotify)
(lldb) p (IMP) 0x108801480
(IMP) $3 = 0x0000000108801480 (Interview01`-[Person setAge:] at Person.m:13)

5.我們手動創建這個派生型別NSKVONotifying_Person，并且在Person里面重寫setAge:、willChangeValueForKey:、didChangeValueForKey:這三個方法，運行程式并觀察呼叫情況

@interface NSKVONotifying_Person : Person

@end

@implementation NSKVONotifying_Person

@end


@interface Person : NSObject

@property (assign, nonatomic) int age;
@property (assign, nonatomic) int height;
@end

@implementation Person

- (void)setAge:(int)age
{
    _age = age;
    
    NSLog(@"setAge:");
}

- (void)willChangeValueForKey:(NSString *)key
{
    [super willChangeValueForKey:key];
    
    NSLog(@"willChangeValueForKey");
}

- (void)didChangeValueForKey:(NSString *)key
{
    NSLog(@"didChangeValueForKey - begin");
    
    [super didChangeValueForKey:key];
    
    NSLog(@"didChangeValueForKey - end");
}

@end

由此可見，當監聽的屬性發生改變，系統派生出的這個類NSKVONotifying_Person會對應的先后呼叫willChangeValueForKey:、setAge:、didChangeValueForKey:這三個方法，并在didChangeValueForKey:里呼叫觀察者的observeValueForKeyPath: ofObject: change:來通知值屬性值的變化

// 執行后列印
2021-01-19 13:42:02.071987+0800 Interview01[37119:19609444] willChangeValueForKey
2021-01-19 13:42:02.072192+0800 Interview01[37119:19609444] setAge:
2021-01-19 13:42:02.072332+0800 Interview01[37119:19609444] didChangeValueForKey - begin
2021-01-19 13:42:02.072662+0800 Interview01[37119:19609444] 監聽到<Person: 0x6000036ac2c0>的age屬性值改變了 - {
    kind = 1;
    new = 21;
    old = 1;
} - 123
2021-01-19 13:42:02.072817+0800 Interview01[37119:19609444] didChangeValueForKey - end

6.通過class方法列印person1的類發現還是Person這個型別，說明在派生出的這個類NSKVONotifying_Person內部重寫了class方法，并回傳的是Person這個型別，所以只能通過object_getClass才能獲取到真實的型別

NSLog(@"%@ %@",
          [self.person1 class], 
          [self.person2 class]); 
// Person Person

NSLog(@"%@ %@",
          object_getClass(self.person1), 
          object_getClass(self.person2)); 
// NSKVONotifying_Person Person

7.通過Runtime的class_copyMethodList函式查看NSKVONotifying_Person內部還動態生成了dealloc、_isKVOA這兩個函式

- (void)printMethodNamesOfClass:(Class)cls
{
    unsigned int count;
    // 獲得方法陣列
    Method *methodList = class_copyMethodList(cls, &count);
    
    // 存盤方法名
    NSMutableString *methodNames = [NSMutableString string];
    
    // 遍歷所有的方法
    for (int i = 0; i < count; i++) {
        // 獲得方法
        Method method = methodList[i];
        // 獲得方法名
        NSString *methodName = NSStringFromSelector(method_getName(method));
        // 拼接方法名
        [methodNames appendString:methodName];
        [methodNames appendString:@", "];
    }
    
    // 釋放
    free(methodList);
    
    // 列印方法名
    NSLog(@"%@ %@", cls, methodNames);
}

[self printMethodNamesOfClass:object_getClass(self.person1)];
[self printMethodNamesOfClass:object_getClass(self.person2)];

// 列印結果
2021-01-19 15:38:13.552990+0800 Interview01[41940:19730538] NSKVONotifying_MJPerson setAge:, class, dealloc, _isKVOA,
2021-01-19 15:38:13.553166+0800 Interview01[41940:19730538] MJPerson setAge:, age,

通過上面一系列操作可以匯總為：

利用RuntimeAPI動態生成一個子類，并且讓instance物件的isa指向這個全新的子類
全新的子類會重寫class這個函式，并回傳父型別別- 當修改instance物件的屬性時，會呼叫Foundation的_NSSetXXXValueAndNotify函式- 呼叫willChangeValueForKey:- 呼叫父類原來的setter- 呼叫didChangeValueForKey:- 內部會觸發監聽器（Oberser）的監聽方法 observeValueForKeyPath:ofObject:change:context:

KVO的應用場景

1.監聽ScrollView的偏移量，改變導航欄背景色

2.給TextView增加placeHolder，通過KVO監聽文本是否輸入對應隱藏展示placeHolder

KVC

KVC的全稱是Key-Value Coding，俗稱“鍵值編碼”，可以通過一個key來訪問某個屬性

KVC的使用

可以通過setValue: forKeyPath: 和setValue: forKey:來給屬性賦值，valueForKeyPath:和valueForKey:來獲取屬性值，

setValue: forKeyPath:可以根據keyPath找到更深層次的屬性來賦值，setValue: forKey:就只能找當前物件的屬性，見下面示例代碼

// 示例代碼
@interface Cat : NSObject

@property (assign, nonatomic) int weight;
@end

@interface Person : NSObject

@property (assign, nonatomic) int age;
@property (strong, nonatomic) Cat *cat;
@end

@implementation Cat

@end

@implementation Person

@end

Person *person = [[Person alloc] init];
[person setValue:@10 forKey:@"age"];    
    
person.cat = [[Cat alloc] init];
[person setValue:@80 forKeyPath:@"cat.weight"];
        
// NSLog(@"%d, %d", person.age, person.cat.weight);
NSLog(@"%@", [person valueForKey:@"age"]);
        NSLog(@"%@", [person valueForKeyPath:@"cat.weight"]);
// 輸出：10，80

注意：

如果person.cat沒有創建物件，那么setValue: forKeyPath:也不能給cat.weight屬性賦值
如果用setValue: forKey:方法來給cat.weight屬性賦值，那么會拋出例外[<Person 0x100510ec0> setValue:forUndefinedKey:]

KVC的實作本質

setValue: forKey: 的實作本質

1.在Person里分別添加和注釋setAge:、_setAge:兩個方法，然后運行程式發現，內部會按順序分別查找每個方法是否存在

@interface Person : NSObject

@end

@implementation Person

// 分別打開和注釋下面兩個方法

//- (void)setAge:(int)age
//{
//    NSLog(@"setAge: - %d", age);
//}

- (void)_setAge:(int)age
{
    NSLog(@"_setAge: - %d", age);
}

@end

2.注釋掉上面兩個方法后，重寫accessInstanceVariablesDirectly方法并對應回傳YES和NO，運行程式發現回傳NO會拋出例外，說明不會再去查找是否有對應的屬性，

accessInstanceVariablesDirectly默認的回傳值就是YES

// 默認的回傳值就是YES
+ (BOOL)accessInstanceVariablesDirectly
{
    //return YES;
    return NO;
}

3.最后我們在給Person物件分別添加和注釋_age、_isAge、age、isAge這幾個成員變數，運行程式發現，內部會按順序分別查找每個成員變數是否存在，如果都沒找到也會拋出例外

// 分別打開和注釋下面的每個成員變數

@interface Person : NSObject
{
    @public
//    int age;
//    int isAge;
//    int _isAge;
    int _age;
}

@end

通過上面一系列操作可以匯總為：

valueForKey: 的實作本質

1.在Person里分別添加和注釋getAge、age、isAge、_age幾個方法，然后運行程式發現，內部會按順序查找每個方法是否存在

@interface Person : NSObject

@end

@implementation MJPerson

// 分別打開和注釋下面兩個方法

- (int)getAge
{
    return 11;
}

//- (int)age
//{
//    return 12;
//}

//- (int)isAge
//{
//    return 13;
//}

//- (int)_age
//{
//    return 14;
//}

@end

2.同setValue: forKey:第二部操作一樣，如果回傳值為NO則拋出例外[<Person 0x105820160> valueForUndefinedKey:]

libc++abi.dylib: terminating with uncaught exception of type NSException
*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<Person 0x105820160> valueForUndefinedKey:]: this class is not key value coding-compliant for the key age.'

3.同setValue: forKey:最后一步操作一樣，只不過找到了對應的對應的成員變數直接取值，找不到也會拋出上面的例外

通過上面一系列操作也可以匯總為：

KVC的應用場景

1.可以通過KVC獲取到私有成員變數，以及修改私有成員變數的值

iOS13之后蘋果不允許通過KVC獲取系統API的私有成員了，會crash

通過KVC訪問自定義型別的私有成員還是可以的

2.字典轉模式

面試題

1.如何手動觸發KVO？手動呼叫`willChangeValueForKey:`和`didChangeValueForKey:`

2.直接修改成員變數會觸發KVO么

不會觸發KVO

3.通過KVC修改屬性會觸發KVO么？

會觸發KVO

如示例代碼所示，我們給Person添加一個成員變數age和一個只讀屬性weight，然后都是通過KVC的方式分別給它們賦值，發現都會觸發KV0監聽，并呼叫了willChangeValueForKey和didChangeValueForKey方法

// Person.h
@interface Person : NSObject
{
    @public
    int age;
}

@property (assign, nonatomic, readonly) int weight;
@end

// Person.m
@implementation Person

- (void)willChangeValueForKey:(NSString *)key
{
    [super willChangeValueForKey:key];
    
    NSLog(@"willChangeValueForKey");
}

- (void)didChangeValueForKey:(NSString *)key
{
    NSLog(@"didChangeValueForKey - begin");
    
    [super didChangeValueForKey:key];
    
    NSLog(@"didChangeValueForKey - end");
}
@end

// ViewController.m
@interface ViewController ()

@property (strong, nonatomic) Person *person;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.person = [[Person alloc] init];
            
    //添加KVO監聽
    [self.person addObserver: self forKeyPath:@"age" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
    [self.person addObserver: self forKeyPath:@"weight" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
    
    [self.person setValue:@10 forKey:@"age"];
    [self.person setValue:@20 forKey:@"weight"];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"observeValueForKeyPath: %@",change);
}

- (void)dealloc {
    [self.person removeObserver:self forKeyPath:@"age"];
    [self.person removeObserver:self forKeyPath:@"weight"];
}
@end

// 輸出結果
//willChangeValueForKey
//didChangeValueForKey - begin
//observeValueForKeyPath: {
//    kind = 1;
//    new = 10;
//    old = 0;
//}
//didChangeValueForKey - end
//
//
//willChangeValueForKey
//didChangeValueForKey - begin
//observeValueForKeyPath: {
//    kind = 1;
//    new = 20;
//    old = 0;
//}
//didChangeValueForKey - end

4.怎么通過KVO監聽陣列的元素變化？

我們可以通過陣列的KVC方式添加元素，其底層會呼叫KVO觸發監聽器來監聽陣列元素變化

@interface ViewController ()

@property (nonatomic, strong) NSMutableArray *lines;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.lines = [NSMutableArray array];
    [self addObserver: self forKeyPath:@"lines" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:NULL];
    [[self mutableArrayValueForKey:@"lines"] addObject:@"1"];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    NSLog(@"observeValueForKeyPath: %@",change);
}

- (void)dealloc {
    [self removeObserver:self forKeyPath:@"lines"];
}
@end

// 列印：
observeValueForKeyPath: {
    indexes = "<_NSCachedIndexSet: 0x6000030afe60>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 2;
    new =     (
        1
    );
}

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iOS底層原理（二）KVO和KVC

KVO