这个方法也是在网上找的。具体如下：

.h文件

#import <UIKit/UIKit.h>

@interface UITextField (LimitLength)
/**
 *  使用时只要调用此方法，加上一个长度(int)，就可以实现了字数限制,汉字不可以
 *
 *  @param length
 */
- (void)limitTextLength:(int)length;
/**
 *  uitextField 抖动效果
 */
- (void)shake;
@end

#import "UITextField+LimitLength.h"
#import <objc/runtime.h>

@implementation UITextField (LimitLength)
static NSString *kLimitTextLengthKey = @"kLimitTextLengthKey";
- (void)limitTextLength:(int)length
{
    objc_setAssociatedObject(self, (__bridge const void *)(kLimitTextLengthKey), [NSNumber numberWithInt:length], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    [self addTarget:self action:@selector(textFieldTextLengthLimit:) forControlEvents:UIControlEventEditingChanged];
    
}
- (void)textFieldTextLengthLimit:(id)sender
{
    NSNumber *lengthNumber = objc_getAssociatedObject(self, (__bridge const void *)(kLimitTextLengthKey));
    
    int length = [lengthNumber intValue];
    
    if(self.text.length > length){
        
        self.text = [self.text substringToIndex:length];
    }
}

- (void)shake
{
    CAKeyframeAnimation *keyAn = [CAKeyframeAnimation animationWithKeyPath:@"position"];
    [keyAn setDuration:0.5f];
    NSArray *array = [[NSArray alloc] initWithObjects:
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x-5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x+5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x-5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x+5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x-5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x+5, self.center.y)],
                      [NSValue valueWithCGPoint:CGPointMake(self.center.x, self.center.y)],
                      nil];
    [keyAn setValues:array];
    
    NSArray *times = [[NSArray alloc] initWithObjects:
                      [NSNumber numberWithFloat:0.1f],
                      [NSNumber numberWithFloat:0.2f],
                      [NSNumber numberWithFloat:0.3f],
                      [NSNumber numberWithFloat:0.4f],
                      [NSNumber numberWithFloat:0.5f],
                      [NSNumber numberWithFloat:0.6f],
                      [NSNumber numberWithFloat:0.7f],
                      [NSNumber numberWithFloat:0.8f],
                      [NSNumber numberWithFloat:0.9f],
                      [NSNumber numberWithFloat:1.0f],
                      nil];
    [keyAn setKeyTimes:times];
    
    [self.layer addAnimation:keyAn forKey:@"TextAnim"];
}
@end

<span style="font-size: 14px; font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">在调用的地方如下：</span>

[m_userNamelimitTextLength:8];

这个类有个问题，就是不能对中文做限制，也会crash . 在使用时要注意，可以对textField设置键盘类型。

下个整理点：category与associative作为objective-c的扩展机制的两个特性，category即类型，可以通过它来扩展方法；associative，可以通过它来扩展属性；