@dynamic属性及其用法？ [英] @dynamic properties and its usage?

问题描述

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

Can anyone give me clear picture about dynamic property and its usage? y not use the usual @property everywhere?

推荐答案

当你在编译时没有提供实现时，会使用动态属性，但要确保在运行时存在一个。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。

Dynamic properties are used when you don't provide an implementation at compile time, but ensure that one exists at runtime. Being a dynamic language, Objective-C can respond to messages at runtime, even if the class doesn't have an implementation at compile time.

这是一个人为的例子：让我们看看假设你有一个 Book 类，由 NSMutableDictionary 支持，其中包含键 title 和作者。但是，您希望预订以回复标题和作者同样，并将它们作为属性; title 和 author 将从字典中获取相应的值， setTitle：和 setAuthor：将更改存储在字典中的值。您可以使用此代码执行此操作：

Here's a contrived example: Let's say you have a Book class, backed by an NSMutableDictionary that contains the keys title and author. However, you want Book to respond to title and author as well, and have them as properties; title and author will grab the appropriate value from the dictionary, and setTitle: and setAuthor: will change the value stored in the dictionary. You could do so with this code:

#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;
}

注意方法是在运行时通过 forwardInvocation：;因此， title 和 author 是动态属性。

Note the the methods are "created" at runtime via forwardInvocation:; hence, title and author are dynamic properties.

^{（这不是最好的例子，但我认为它得到了重点。）}

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