@dynamic属性及其用法？

Question

有人能给我清楚的动态属性及其用法吗？你不是到处都使用通常的@property吗？

Answer 1

如果在编译时未提供实现,但在运行时确保存在实现,则使用动态属性.作为一种动态语言,Objective-C可以在运行时响应消息,即使该类在编译时没有实现.

这是一个人为的例子:假设你有一个Book类,由一个NSMutableDictionary包含密钥title和密钥的类支持author.但是,你要Book来回应title,并author为好,并将它们作为性能; title并将author从字典中获取适当的值setTitle:,setAuthor:并将更改存储在字典中的值.您可以使用以下代码执行此操作:

#import <Foundation/Foundation.h>

@interface Book : NSObject
{
    NSMutableDictionary *data;
}
@property (retain) NSString *title;
@property (retain) NSString *author;
@end

@implementation Book
@dynamic title, author;

- (id)init
{
    if ((self = [super init])) {
        data = [[NSMutableDictionary alloc] init];
        [data setObject:@"Tom Sawyer" forKey:@"title"];
        [data setObject:@"Mark Twain" forKey:@"author"];
    }
    return self;
}

- (void)dealloc
{
    [data release];
    [super dealloc];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    NSString *sel = NSStringFromSelector(selector);
    if ([sel rangeOfString:@"set"].location == 0) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:@"];
    } else {
        return [NSMethodSignature signatureWithObjCTypes:"@@:"];
    }
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    NSString *key = NSStringFromSelector([invocation selector]);
    if ([key rangeOfString:@"set"].location == 0) {
        key = [[key substringWithRange:NSMakeRange(3, [key length]-4)] lowercaseString];
        NSString *obj;
        [invocation getArgument:&obj atIndex:2];
        [data setObject:obj forKey:key];
    } else {
        NSString *obj = [data objectForKey:key];
        [invocation setReturnValue:&obj];
    }
}

@end

int main(int argc, char **argv)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    Book *book = [[Book alloc] init];
    printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);
    book.title = @"1984";
    book.author = @"George Orwell";
    printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);

    [book release];
    [pool release];
    return 0;
}

注意,这些方法是在运行时"创建"的forwardInvocation:; 因此,title和author是动态属性.

^{(这不是最好的例子,但我认为它得到了重点.)}