可以在nsObject上添加类别以添加所需的功能。

//NSObject+DynamicCast.h
@interface NSObject (DynamicCast)
-(id)objectIfMemberOfClass:(Class)aClass;
@end

//NSObject+DynamicCast.m
@implementation NSObject (DynamicCast)
-(id)objectIfMemberOfClass:(Class)aClass;
{
  return [self isMemberOfClass:aClass] ? self : nil;
}
@end

然后你可以这样做:

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;
id generic_pointer;

// **The nice one-line cast below**
sub_pointer = [super objectIfMemberOfClass:[SubClass class]];
// Prints NO
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

sub_pointer = [sub objectIfMemberOfClass:[SubClass class]];
// Prints YES
printf("Is a subclass: %s\n", sub_pointer ? "YES" : "NO");

我使用宏:

#define DYNAMIC_CAST(x, cls)                                \
  ({                                                        \
    cls *inst_ = (cls *)(x);                                \
    [inst_ isKindOfClass:[cls class]] ? inst_ : nil;        \
  })

我稍微喜欢它使用nsObject上的类别,因为返回的对象是正确的类型(而不是ID),尽管我知道在大多数情况下,您只需将它分配给相同类型的变量即可。

如果允许将任何C++投掷到混合中,可以避免宏并使用模板例程获得正确的类型:

template <typename T, typename U>
inline T* objc_cast(U* instance)
{
    return [instance isMemberOfClass:[T class]] ?
               static_cast<T*>(instance) :
               nil;
}

然后,这个电话看起来很简单:

sub_pointer = objc_cast<SubClass>(super);

Xcode的最新版本提供 instancetype 并允许一个非常整洁的基于类别的解决方案。使用方法如下:

TypeIWant *const thingIWant = [TypeIWant tryCast: thingIWantToCast];

类别声明打开 NSObject

@interface NSObject (DynamicCast)
// Try a dynamic cast. Return nil if the class isn't compatible.
+(instancetype) tryCast: (id) toCast;

// Check a dynamic cast. Throw if the class isn't compatible.
+(instancetype) checkCast: (id) toCast;
@end

类别实施

@implementation NSObject (DynamicCast)

+(instancetype) tryCast: (id) toCast
{
    return [toCast isKindOfClass: self] ? toCast : nil;
}

+(instancetype) checkCast:(id)toCast
{
    const id casted = [self tryCast: toCast];
    if(!casted)
    {
        [NSException raise: NSInvalidArgumentException format: @"Can't cast %@ to be an %@", toCast, NSStringFromClass(self)];
    }
    return casted;
}

@end

使用

这是一个 类方法 可赎回的 任何 班有 NSO对象 作为一个超级类(基本上,任何类)。

你发送 tryCast: 方法 到 班 你希望投到 ,将要强制转换的对象作为参数。这样地 [ClassIWant tryCast: thingIWantCasted] .

使用实例

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    UIPopoverController *const popoverDestination =
    [UIStoryboardPopoverSegue tryCast: segue].popoverController;
    if(popoverDestination)
    {
        UITableViewCell *const tableViewSender =
        [UITableViewCell tryCast: sender];
        if(tableViewSender)
        {
            // Things you need to do in this case.
            ...

我突然想到,我可以用三元运算符把这些都放到一行上,但这仍然有点混乱:

SuperClass *super = [[SuperClass alloc] init];
SubClass *sub = [[SubClass alloc] init];
SubClass *sub_pointer;

// One line, but still a bit wordy
sub_pointer = [super isMemberOfClass:[SubClass class]] ? (SubClass*) super : nil;
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

sub_pointer = [sub isMemberOfClass:[SubClass class]] ? (SubClass*) sub : nil;
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

如果我要从函数中获取预期的变量,我必须将其缓存在一个ID变量中,以便这个版本工作。

SubClass *sub_pointer;
id generic_pointer;

// One line, but still a bit wordy
generic_pointer = (id) mySuperFunc();
sub_pointer = [generic_pointer isMemberOfClass:[SubClass class]] ? generic_pointer : nil;
// Logs NO
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

generic_pointer = (id) mySubFunc();
sub_pointer = [generic_pointer isMemberOfClass:[SubClass class]] ? generic_pointer : nil;
// Logs YES
NSLog(@"Is a subclass: %@", sub_pointer ? @"YES" : @"NO");

基本上…

id sub_pointer = [foo isMemberOfClass:AClass] ? foo : nil;
NSLog(@"Is a subclass: %i", sub_pointer!=nil ); 

似乎没有多大的词藻。