1、前言
在技術(shù)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(yīng)是崩潰，因為OC是類型敏感的?？墒亲约簩崿F(xiàn)并打印后的結(jié)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(nèi)部實現(xiàn)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(xiàn)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(zhí)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(zhí)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(tǒng)API沖突問題增加前綴_d,NS替換為DS)，以此分析KVC執(zhí)行過程。（流程中的邊界判斷等已經(jīng)忽略，如想了解可以參考源碼，本文只探究主流程。）
3.1 設(shè)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?qū)ふ襰et<Key>,_set<Key>,setIs<Key>。找到后則生成對應(yīng)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(shù)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(yīng)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(yīng)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(yīng)的方法且 accessInstanceVariablesDirectly == YES
則查找順序如下：_,_is,,is
查找不到則調(diào)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(gòu)造方法確定方法編號 d_setValue:forUndefinedKey: 和方法指針I(yè)MP _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(jīng)清楚的知道了。那么上面的例子是如何出現(xiàn)的呢？明明傳入的是字符串，最后賦值的時候轉(zhuǎn)變?yōu)樵L問器方法所對應(yīng)的類型？繼續(xù)刨根問底！
DSKeyValueSetter對象已經(jīng)生成，即確定了發(fā)送消息的對象object、訪問器方法名SEL、訪問器函數(shù)指針I(yè)MP、以及使用KVC時傳入的Key和Value。下面進入方法調(diào)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(diào)用的時候做了[value valueGetSelectorName]，將對應(yīng)的NSNumber轉(zhuǎn)換為簡單數(shù)據(jù)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(shù)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(diào)用person的setAge:方法。參數(shù)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(zhí)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(zhí)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(yīng)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(yīng)的方法且accessInstanceVariablesDirectly==YES，則繼續(xù)查找成員變量，查找順序為_,_is,,is
- 如果1，2沒有找到對應(yīng)的方法和屬性則調(diào)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(yīng)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(diào)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(diào)用如下：
4、簡單數(shù)據(jù)類型KVC包裝和拆裝關(guān)系
NSNunber:
NSValue
5、KVC高級
修改數(shù)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(shù)，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(shù)據(jù)篩選
經(jīng)過上面的分析可以明白KVC的真正執(zhí)行流程。下面結(jié)合日常工程中的實際應(yīng)用來優(yōu)雅的處理數(shù)據(jù)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(jié)如下：修改拒收數(shù)量時更新總妥投數(shù)和總拒收數(shù)、勾選明細更新總妥投數(shù)和總拒收數(shù)、全選、清空、反選。如果用通常的做法是每次操作都要循環(huán)去計算總數(shù)和記錄選擇狀態(tài)。下面是采用KVC的實現(xiàn)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(shù)
- (void)updateDeliveryInfo {
//總數(shù)
NSNumber *allDeliveryAmount = [self.orderDetailModel.deliveryGoodsDetailList valueForKeyPath:@"@sum.totalAmount"];
//妥投數(shù)
NSNumber *allRealDeliveryAmount = [self.orderDetailModel.deliveryGoodsDetailList valueForKeyPath:@"@sum.deliveryAmount"];
//拒收數(shù)
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(jié)
KVC在處理簡單數(shù)據(jù)類型時會經(jīng)過數(shù)據(jù)封裝和拆裝并轉(zhuǎn)換為對應(yīng)的數(shù)據(jù)類型。通過KVC的特性我們可以在日常使用中更加優(yōu)雅的對數(shù)據(jù)進行篩選和處理。優(yōu)點如下：可閱讀性更高，健壯性更好。
