ios 关联对象运用 objc_setAssociatedObject

点按钮的时候，给alertView添加一个关联对象（被点击这个按钮），

objc_setAssociatedObject(alert, &kRepresentedObject, 
                           sender,
                           OBJC_ASSOCIATION_RETAIN_NONATOMIC);

在UIAlertViewDelegate中取出关联对象（被点击的按钮）

UIButton *sender = objc_getAssociatedObject(alertView, 
                                              &kRepresentedObject);

&kRepresentedObject到底是什么东西，

kRepresentedObject从哪里来

static const char kRepresentedObject;

为什么会用char？？

主要代码如下 ，

#import "ViewController.h"
#import <objc/runtime.h>

@implementation ViewController

static const char kRepresentedObject;
#pragma mark - event response
/** 按钮事件 */
- (IBAction)doSomething:(id)sender {
  UIAlertView *alert = [[UIAlertView alloc]
                        initWithTitle:@"Alert" message:nil
                        delegate:self
                        cancelButtonTitle:@"OK"
                        otherButtonTitles:nil];
  objc_setAssociatedObject(alert, &kRepresentedObject, 
                           sender,
                           OBJC_ASSOCIATION_RETAIN_NONATOMIC);
  [alert show];
  
}
#pragma mark - UIAlertViewDelegate
/** UIAlertViewDelegate */
- (void)alertView:(UIAlertView *)alertView 
clickedButtonAtIndex:(NSInteger)buttonIndex {
  UIButton *sender = objc_getAssociatedObject(alertView, 
                                              &kRepresentedObject);
  self.buttonLabel.text = [[sender titleLabel] text];
}
@end

让我们再来看看其他关联对象是如何关联的

在分类中加入了几个属性

#import <Foundation/Foundation.h>
#import "XTableViewCellItem.h"

@interface NSArray (YDTableSectionView)

@property (nonatomic, strong) XTableViewCellItem *sectionHeaderItem;

@property (nonatomic, strong) XTableViewCellItem *sectionFooterItem;

@end

@interface YDtableSectionView : UIControl

@property (nonatomic, strong) XTableViewCellItem *sectionItem;

@end

m文件中这样实现

#import "NSArray+YDTableSectionView.h"
#import <objc/runtime.h>

@implementation NSArray (YDTableSectionView)

- (void)setSectionHeaderItem:(XTableViewCellItem *)sectionHeaderItem
{
    objc_setAssociatedObject(self, "sectionHeaderItem", sectionHeaderItem, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (XTableViewCellItem *)sectionHeaderItem
{
    return objc_getAssociatedObject(self, "sectionHeaderItem");
}

- (void)setSectionFooterItem:(XTableViewCellItem *)sectionFooterItem
{
    objc_setAssociatedObject(self, "sectionFooterItem", sectionFooterItem, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (XTableViewCellItem *)sectionFooterItem
{
    return objc_getAssociatedObject(self, "sectionFooterItem");
}

@end

用的是

"sectionFooterItem"

而非

@"sectionFooterItem"

最上面用的是

static const char kRepresentedObject;

char kRepresentedObject;说是有是“”而非@“”
static 局部，本文件访问
const 常量，不变