1、前言
在技術論壇中看到一則很有意思的KVC案例：
@interface Person : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end
Person *person = [Person new];
person.name = @"Tom";
person.age = 10;
[person setValue:@"100" forKey:@"age"];//此處賦值為字符串，類中屬性為Integer
第一反應是崩潰，因為OC是類型敏感的。可是自己實現并打印后的結果出于意料,沒有崩潰且賦值成功。所以有了深入了解KVC的內部實現的想法！
2、什么是KVC
key-value-coding:鍵值編碼，一種可以通過鍵名間接訪問和賦值對象屬性的機制
KVC是通過NSObject、NSArray、NSDictionary等的類別來實現的
主要方法包括一下幾個：
- (nullable id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;
- (void)setNilValueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;
- (nullable id)valueForUndefinedKey:(NSString *)key;
3、KVC執行分析
那么上面的案例中的- (void)setValue:(nullable id)value forKey:(NSString *)key;是怎樣的執行過程呢？借助反匯編工具獲得Foundation.framework部分源碼(為了解決和系統API沖突問題增加前綴_d,NS替換為DS)，以此分析KVC執行過程。（流程中的邊界判斷等已經忽略，如想了解可以參考源碼，本文只探究主流程。）
3.1 設置屬性
3.1.1 查找訪問器方法或成員變量
+ (DSKeyValueSetter *)_d_createValueSetterWithContainerClassID:(id)containerClassID key:(NSString *)key {
DSKeyValueSetter *setter = nil;
char key_cstr_upfirst[key_cstr_len + 1];
key_cstr[key_cstr_len + 1];
...
Method method = NULL;
//按順序尋找set<Key>,_set<Key>,setIs<Key>。找到后則生成對應的seter
if ((method = DSKeyValueMethodForPattern(self, "set%s:", key_cstr_upfirst)) ||
(method = DSKeyValueMethodForPattern(self, "_set%s:", key_cstr_upfirst)) ||
(method = DSKeyValueMethodForPattern(self, "setIs%s:", key_cstr_upfirst))
) { //生成Method：包含selector，IMP。返回和參數類型字符串
setter = [[DSKeyValueMethodSetter alloc] initWithContainerClassID:containerClassID key:key method:method];
} else if ([self accessInstanceVariablesDirectly]) {//如果沒有找到對應的訪問器方且工廠方法accessInstanceVariablesDirectly == ture ，則按照順序查找查找成員變量_<key>,_is<Key>,<key>,is<Key>(注意key的首字母大小寫，查找到則生成對應的setter)
Ivar ivar = NULL;
if ((ivar = DSKeyValueIvarForPattern(self, "_%s", key_cstr)) ||
(ivar = DSKeyValueIvarForPattern(self, "_is%s", key_cstr_upfirst)) ||
(ivar = DSKeyValueIvarForPattern(self, "%s", key_cstr)) ||
(ivar = DSKeyValueIvarForPattern(self, "is%s", key_cstr_upfirst))
) {
setter = [[DSKeyValueIvarSetter alloc] initWithContainerClassID:containerClassID key:key containerIsa:self ivar:ivar];
}
}
...
return setter;
}查找順序如下：
- 查找訪問器方法：set,_set,setIs
- 如果步驟1中沒找到對應的方法且 accessInstanceVariablesDirectly == YES
則查找順序如下：_,_is,,is
查找不到則調用valueForUndefinedKey并拋出異常
3.1.2 生成setter
+ (DSKeyValueSetter *)_d_createOtherValueSetterWithContainerClassID:(id)containerClassID key:(NSString *)key {
return [[DSKeyValueUndefinedSetter alloc] initWithContainerClassID:containerClassID key:key containerIsa:self];
}
//構造方法確定方法編號 d_setValue:forUndefinedKey: 和方法指針IMP _DSSetValueAndNotifyForUndefinedKey
- (id)initWithContainerClassID:(id)containerClassID key:(NSString *)key containerIsa:(Class)containerIsa {
...
return [super initWithContainerClassID:containerClassID key:key implementation:method_getImplementation(class_getInstanceMethod(containerIsa, @selector(d_setValue:forUndefinedKey:))) selector:@selector(d_setValue:forUndefinedKey:) extraArguments:arguments count:1];
}
3.1.3 賦值
基本的訪問器方法、變量的查找和異常處理已經清楚的知道了。那么上面的例子是如何出現的呢？明明傳入的是字符串，最后賦值的時候轉變為訪問器方法所對應的類型？繼續刨根問底！
DSKeyValueSetter對象已經生成，即確定了發送消息的對象object、訪問器方法名SEL、訪問器函數指針IMP、以及使用KVC時傳入的Key和Value。下面進入方法調用階段：_DSSetUsingKeyValueSetter(self,setter, value);
IMP指針為_DSSetIntValueForKeyWithMethod其定義如下：之所以有文章開頭提到的效果就是這里起了作用，在IMP調用的時候做了[value valueGetSelectorName]，將對應的NSNumber轉換為簡單數據類型。這里是intValue。
void _DSSetIntValueForKeyWithMethod(id object, SEL selector,id value, NSString *key, Method method) {// object:person selector:setAge: value:@(100) key:age method:selector + IMP + 返回類型和參數類型 即_extraArgument2,其在第一步查找到訪問器方法后生成
__DSSetPrimitiveValueForKeyWithMethod(object, selector, value, key, method, int, intValue);
}
#define
if (value) {\
void (*imp)(id,SEL,valueType) = (void (*)(id,SEL,valueType))method_getImplementation(method);\
imp(object, method_getName(method), [value valueGetSelectorName]);\調用person的setAge:方法。參數為100
}\
else {\
[object setNilValueForKey:key];\
}\
}while(0)
//如果第一步中沒有找到訪問器方法只找到了成員變量則直接執行賦值操作
void _DSSetIntValueForKeyInIvar(id object, SEL selector, id value, NSString *key, Ivar ivar) {
if (value) {
*(int *)object_getIvarAddress(object, ivar) = [value intValue];
}
else {
[object setNilValueForKey:key];
}
}起始問題完美解決！執行流程如下：
3.2 取值
3.2.1 查找訪問器方法或成員變量
+ (DSKeyValueGetter *)_d_createValueGetterWithContainerClassID:(id)containerClassID key:(NSString *)key {
DSKeyValueGetter * getter = nil;
...
Method getMethod = NULL;
if((getMethod = DSKeyValueMethodForPattern(self,"get%s",keyCStrUpFirst)) ||
(getMethod = DSKeyValueMethodForPattern(self,"%s",keyCStr)) ||
(getMethod = DSKeyValueMethodForPattern(self,"is%s",keyCStrUpFirst)) ||
(getMethod = DSKeyValueMethodForPattern(self,"_get%s",keyCStrUpFirst)) ||
(getMethod = DSKeyValueMethodForPattern(self,"_%s",keyCStr))) {
getter = [[DSKeyValueMethodGetter alloc] initWithContainerClassID:containerClassID key:key method:getMethod];
}// 查找對應的訪問器方法
...
else if([self accessInstanceVariablesDirectly]) {//查找屬性
Ivar ivar = NULL;
if((ivar = DSKeyValueIvarForPattern(self, "_%s", keyCStr)) ||
(ivar = DSKeyValueIvarForPattern(self, "_is%s", keyCStrUpFirst)) ||
(ivar = DSKeyValueIvarForPattern(self, "%s", keyCStr)) ||
(ivar = DSKeyValueIvarForPattern(self, "is%s", keyCStrUpFirst))
) {
getter = [[DSKeyValueIvarGetter alloc] initWithContainerClassID:containerClassID key:key containerIsa:self ivar:ivar];
}
}
}
if(!getter) {
getter = [self _d_createValuePrimitiveGetterWithContainerClassID:containerClassID key:key];
}
return getter;
}- 按照get,,is,_的順序查找成員方法
- 如果1.沒有找到對應的方法且accessInstanceVariablesDirectly==YES，則繼續查找成員變量，查找順序為_,_is,,is
- 如果1，2沒有找到對應的方法和屬性則調用 valueForUndefinedKey:并拋出異常
3.2.2 如上步驟沒定位到訪問器方法或成員變量則走下面的流程生成對應的getter
訪問器方法生成IMP
- (id)initWithContainerClassID:(id)containerClassID key:(NSString *)key method:(Method)method {
NSUInteger methodArgumentsCount = method_getNumberOfArguments(method);
NSUInteger extraAtgumentCount = 1;
if(methodArgumentsCount == 2) {
char *returnType = method_copyReturnType(method);
IMP imp = NULL;
switch (returnType[0]) {
...
case 'i': {
imp = (IMP)_DSGetIntValueWithMethod;
} break;
...
free(returnType);
if(imp) {
void *arguments[3] = {0};
if(extraAtgumentCount > 0) {
arguments[0] = method;
}
return [super initWithContainerClassID:containerClassID key:key implementation:imp selector:method_getName(method) extraArguments:arguments count:extraAtgumentCount];
}
}
單步調試后可以看到具體的IMP類型
定義如下：
NSNumber * _DSGetIntValueWithMethod(id object, SEL selctor, Method method) {//
return [[[NSNumber alloc] initWithInt: ((int (*)(id,SEL))method_getImplementation(method))(object, method_getName(method))] autorelease];
}3.2.3 取值
取值調用如下：
4、簡單數據類型KVC包裝和拆裝關系
NSNunber:
NSValue
5、KVC高級
修改數組中對象的屬性[array valueForKeyPath:@”uppercaseString”]利用KVC可以批量修改屬性的成員變量值
求和，平均數，最大值，最小值NSNumbersum= [array valueForKeyPath:@”@sum.self”];NSNumberavg= [array valueForKeyPath:@”@avg.self”];NSNumbermax= [array valueForKeyPath:@”@max.self”];NSNumbermin= [array valueForKeyPath:@”@min.self”];
6、數據篩選
經過上面的分析可以明白KVC的真正執行流程。下面結合日常工程中的實際應用來優雅的處理數據篩選問題。使用KVC處理可以減少大量for的使用并增加代碼可讀性和健壯性。
如圖所示：
項目中的細節如下：修改拒收數量時更新總妥投數和總拒收數、勾選明細更新總妥投數和總拒收數、全選、清空、反選。如果用通常的做法是每次操作都要循環去計算總數和記錄選擇狀態。下面是采用KVC的實現過程。
模型涉及：
@property (nonatomic,copy)NSString* skuCode;
@property (nonatomic,copy)NSString* goodsName;
@property (nonatomic,assign)NSInteger totalAmount;
@property (nonatomic,assign)NSInteger rejectAmount;
@property (nonatomic,assign)NSInteger deliveryAmount;
///單選用
@property (nonatomic, assign) BOOL selected;
1）更新總數
- (void)updateDeliveryInfo {
//總數
NSNumber *allDeliveryAmount = [self.orderDetailModel.deliveryGoodsDetailList valueForKeyPath:@"@sum.totalAmount"];
//妥投數
NSNumber *allRealDeliveryAmount = [self.orderDetailModel.deliveryGoodsDetailList valueForKeyPath:@"@sum.deliveryAmount"];
//拒收數
NSNumber *allRejectAmount = [self.orderDetailModel.deliveryGoodsDetailList valueForKeyPath:@"@sum.rejectAmount"];
}2）全選
[self.orderDetailModel.deliveryGoodsDetailList setValue:@(YES) forKeyPath:@”selected”];
3）清空
[self.orderDetailModel.deliveryGoodsDetailList setValue:@(NO) forKeyPath:@”selected”];
4)反選
NSPredicate *selectedPredicate = [NSPredicate predicateWithFormat:@"selected == %@",@(YES)];
NSArray *selectedArray = [self.orderDetailModel.deliveryGoodsDetailList filteredArrayUsingPredicate:selectedPredicate];
NSPredicate *unSelectedPredicate = [NSPredicate predicateWithFormat:@"selected == %@",@(NO)];
NSArray *unSelectedArray = [self.orderDetailModel.deliveryGoodsDetailList filteredArrayUsingPredicate:unSelectedPredicate];
[selectedArray setValue:@(NO) forKeyPath:@"selected"];
[unSelectedArray setValue:@(YES) forKeyPath:@"selected"];
7、總結
KVC在處理簡單數據類型時會經過數據封裝和拆裝并轉換為對應的數據類型。通過KVC的特性我們可以在日常使用中更加優雅的對數據進行篩選和處理。優點如下：可閱讀性更高，健壯性更好。
