使用KVC分配给SEL类型的属性

Question

ARC已启用.

我有一个类型为SEL的属性的类: @property SEL mySelector;

它是合成的: @synthesize mySelector;

然后我尝试使用KVC为它分配一个值:

SEL someSelector = @selector(doSomething:)
NSValue* someSelectorValue = [NSValue value:someSelector withObjCType:@encode(SEL)];
[target setValue:someSelectorValue forKey:@"mySelector"];

我收到错误消息:

[<LACMyClass 0x101b04bc0> setValue:forUndefinedKey:]:
this class is not key value coding-compliant for the key mySelector.

这显然不正确 - 该类符合KVC,它只是不喜欢我传入的值.它似乎在我定义类型的属性void*而不是SEL,但这不符合我的要求.

除了用value:withObjCType:,我也试过valueWithBytes:objCType:和valueWithPointer:

任何人都可以向我解释

1. 发生了什么,和
2. 怎么做得好？

Answer 1

似乎只有默认setValue:forKey:实现的自动装箱/拆箱支持原始类型的某个子集.请参阅"键 - 值编码编程指南" 的"标量和结构支持"一章中的表1和表2 .这里的含义是,只有BOOL,char,double,float,int,long,long long,short,和它们对应的无符号的充分支持,随着structs可通过NSValue.其他类型(例如SEL和其他指针值)似乎不受支持.

考虑以下程序:

#import <Foundation/Foundation.h>

@interface MyObject : NSObject

@property (nonatomic) SEL mySelector;
@property (nonatomic) void *myVoid;
@property (nonatomic) int myInt;
@property (nonatomic,unsafe_unretained) id myObject;

@end

@implementation MyObject
@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        SEL selector = @selector(description);
        NSValue *selectorValue = [NSValue valueWithPointer:selector];
        NSValue *voidValue = [NSValue valueWithPointer:selector];
        NSValue *intValue = @1;
        __unsafe_unretained id obj = (__bridge id)(const void *)selector;
        MyObject *object = [[MyObject alloc] init];

        // The following two calls succeed:
        [object setValue:intValue forKey:@"myInt"];
        [object setValue:obj forKey:@"myObject"];

        // These two throw an exception:
        [object setValue:voidValue forUndefinedKey:@"myVoid"];
        [object setValue:selectorValue forKey:@"mySelector"];
    }
}

我们可以设置int和id属性就好 - 甚至使用__unsafe_unretained和桥接强制转换让我们传递选择器值.但是,不支持尝试设置两种指针类型中的任何一种.

我们如何从这里开始？我们可以,例如,覆盖valueForKey:并setValueForKey:在MyObject支持拆箱要么SEL类型或拦截特定密钥.后一种方法的一个例子:

@implementation MyObject

- (id)valueForKey:(NSString *)key
{
    if ([key isEqualToString:@"mySelector"]) {
        return [NSValue valueWithPointer:self.mySelector];
    }

    return [super valueForKey:key];
}

- (void)setValue:(id)value forKey:(NSString *)key
{
    if ([key isEqualToString:@"mySelector"]) {
        SEL toSet;
        [(NSValue *)value getValue:&toSet];
        self.mySelector = toSet;
    }
    else {
        [super setValue:value forUndefinedKey:key];
    }
}

@end

在使用中,我们发现它按预期工作:

[object setValue:selectorValue forKey:@"mySelector"];
NSString *string = NSStringFromSelector(object.mySelector);
NSLog(@"selector string = %@", string);

这会将"selector string = description"记录到控制台.

当然,这有可维护性问题,因为您现在必须在使用KVC设置选择器所需的每个类中实现这些方法,并且还必须与硬编码键进行比较.解决这个风险的方法之一是使用方法调配并NSObject用我们自己的方法替换替换的KVC方法的实现,它们可以处理SEL类型的装箱和拆箱.

以第一个例子为基础的以下程序源自Mike Ash的精彩"let's build KVC"文章,并且还使用了这个答案中的Swizzle()函数.请注意,我为了演示而偷工减料,并且此代码仅适用于具有适当命名的getter和setter的属性,并且不会直接检查实例变量,这与默认的KVC实现不同.SEL

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface MyObject : NSObject

@property (nonatomic) SEL mySelector;
@property (nonatomic) int myInt;

@end

@implementation MyObject
@end

@interface NSObject (ShadyCategory)
@end

@implementation NSObject (ShadyCategory)

// Implementations of shadyValueForKey: and shadySetValue:forKey: Adapted from Mike Ash's "Let's Build KVC" article
// http://www.mikeash.com/pyblog/friday-qa-2013-02-08-lets-build-key-value-coding.html
// Original MAObject implementation on github at https://github.com/mikeash/MAObject
- (id)shadyValueForKey:(NSString *)key
{
    SEL getterSEL = NSSelectorFromString(key);
    if ([self respondsToSelector: getterSEL]) {
        NSMethodSignature *sig = [self methodSignatureForSelector: getterSEL];
        char type = [sig methodReturnType][0];
        IMP imp = [self methodForSelector: getterSEL];
        if (type == @encode(SEL)[0]) {
            return [NSValue valueWithPointer:((SEL (*)(id, SEL))imp)(self, getterSEL)];
        }
    }
    // We will have swapped implementations here, so this call's NSObject's valueForKey: method
    return [self shadyValueForKey:key];
}

- (void)shadySetValue:(id)value forKey:(NSString *)key
{
    NSString *capitalizedKey = [[[key substringToIndex:1] uppercaseString] stringByAppendingString:[key substringFromIndex:1]];
    NSString *setterName = [NSString stringWithFormat: @"set%@:", capitalizedKey];
    SEL setterSEL = NSSelectorFromString(setterName);
    if ([self respondsToSelector: setterSEL]) {
        NSMethodSignature *sig = [self methodSignatureForSelector: setterSEL];
        char type = [sig getArgumentTypeAtIndex: 2][0];
        IMP imp = [self methodForSelector: setterSEL];
        if (type == @encode(SEL)[0]) {
            SEL toSet;
            [(NSValue *)value getValue:&toSet];
            ((void (*)(id, SEL, SEL))imp)(self, setterSEL, toSet);
            return;
        }
    }

    [self shadySetValue:value forKey:key];
}

@end

// Copied from: https://stackoverflow.com/a/1638940/475052
void Swizzle(Class c, SEL orig, SEL new)
{
    Method origMethod = class_getInstanceMethod(c, orig);
    Method newMethod = class_getInstanceMethod(c, new);
    if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    else
        method_exchangeImplementations(origMethod, newMethod);
}

int main(int argc, char *argv[]) {
    @autoreleasepool {
        Swizzle([NSObject class], @selector(valueForKey:), @selector(shadyValueForKey:));
        Swizzle([NSObject class], @selector(setValue:forKey:), @selector(shadySetValue:forKey:));

        SEL selector = @selector(description);
        MyObject *object = [[MyObject alloc] init];
        object.mySelector = selector;
        SEL fromProperty = object.mySelector;
        NSString *fromPropertyString = NSStringFromSelector(fromProperty);
        NSValue *fromKVCValue = [object valueForKey:@"mySelector"];
        SEL fromKVC;
        [fromKVCValue getValue:&fromKVC];
        NSString *fromKVCString = NSStringFromSelector(fromKVC);

        NSLog(@"fromProperty = %@ fromKVC = %@", fromPropertyString, fromKVCString);

        object.myInt = 1;
        NSNumber *myIntFromKVCNumber = [object valueForKey:@"myInt"];
        int myIntFromKVC = [myIntFromKVCNumber intValue];
        int myIntFromProperty = object.myInt;
        NSLog(@"int from kvc = %d from propety = %d", myIntFromKVC, myIntFromProperty);

        selector = @selector(class);
        NSValue *selectorValue = [NSValue valueWithPointer:selector];
        [object setValue:selectorValue forKey:@"mySelector"];
        SEL afterSettingWithKVC = object.mySelector;
        NSLog(@"after setting the selector with KVC: %@", NSStringFromSelector(afterSettingWithKVC));

        [object setValue:@42 forKey:@"myInt"];
        int myIntAfterSettingWithKVC = object.myInt;
        NSLog(@"after setting the int with KVC: %d", myIntAfterSettingWithKVC);
    }
}

该程序的输出演示了其装箱和拆箱功能:

2013-08-30 19:37:14.287 KVCSelector[69452:303] fromProperty = description fromKVC = description
2013-08-30 19:37:14.288 KVCSelector[69452:303] int from kvc = 1 from propety = 1
2013-08-30 19:37:14.289 KVCSelector[69452:303] after setting the selector with KVC: class
2013-08-30 19:37:14.289 KVCSelector[69452:303] after setting the int with KVC: 42

调酒当然不是没有风险,所以要小心!