ios擴充機制objc_setAssociatedObject，objc_getAssociatedObject

使用例子：

首先匯入標頭檔：#import <objc/runtime.h>

設定靜態常量：static char alertinfokey;

- (IBAction)showAlertAction:(id)sender {

    UIAlertView *alert = [[UIAlertView
alloc] initWithTitle:@"title"
message:@"warn"
delegate:self
cancelButtonTitle:@"cancel"
otherButtonTitles:@"ok",
nil];

    objc_setAssociatedObject(alert, &alertinfokey,
@"test value", OBJC_ASSOCIATION_RETAIN);

    [alert show];

    [alert release];

}

-(void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{

    if (buttonIndex == 0) {

        NSLog(@"== : %@",objc_getAssociatedObject(alertView, &alertinfokey));

    }

}

輸出結果：== : test value