1. 去除字符串首尾空格

   //可以指定字符集
   [text stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
   

   常用字符集：

   @property (readonly, class, copy) NSCharacterSet *controlCharacterSet; //控制符
   @property (readonly, class, copy) NSCharacterSet *whitespaceCharacterSet;//空格
   @property (readonly, class, copy) NSCharacterSet *whitespaceAndNewlineCharacterSet;//空格和换行
   @property (readonly, class, copy) NSCharacterSet *decimalDigitCharacterSet;//数字
   @property (readonly, class, copy) NSCharacterSet *letterCharacterSet;//字母
   @property (readonly, class, copy) NSCharacterSet *lowercaseLetterCharacterSet;//小写字母
   @property (readonly, class, copy) NSCharacterSet *uppercaseLetterCharacterSet;//大写字母
   @property (readonly, class, copy) NSCharacterSet *nonBaseCharacterSet;//不知道
   @property (readonly, class, copy) NSCharacterSet *alphanumericCharacterSet;//字母数字
   @property (readonly, class, copy) NSCharacterSet *decomposableCharacterSet;//可分解字符集
   @property (readonly, class, copy) NSCharacterSet *illegalCharacterSet;//非法字符
   @property (readonly, class, copy) NSCharacterSet *punctuationCharacterSet;//标点符号
   @property (readonly, class, copy) NSCharacterSet *capitalizedLetterCharacterSet;//大写字符
   @property (readonly, class, copy) NSCharacterSet *symbolCharacterSet;//符号
   @property (readonly, class, copy) NSCharacterSet *newlineCharacterSet NS_AVAILABLE(10_5, 2_0);//换行符
   

2. 修改导航栏返回按钮

   • 自定义返回按钮

     UIImage * backButtonImage = [[UIImage imageNamed:@"back_bg"] resizableImageWithCapInsets:UIEdgeInsetsMake(0, 30, 0, 0)];
     [[UIBarButtonItem appearance] setBackButtonBackgroundImage:backButtonImage forState:UIControlStateNormal barMetrics:UIBarMetricsDefault];
     

   • 将返回按钮的文字position设置不在屏幕上显示

     [[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(NSIntegerMin, NSIntegerMin) forBarMetrics:UIBarMetricsDefault];
     

   • 用系统的返回按钮的时候 会发现显示的是蓝色的，想要改变成白色的添加下面一句话

     [[UINavigationBar appearance]setTintColor:[UIColor whiteColor]];
     

   • 去除导航栏的黑色分隔线
     `objc
     // 去除导航栏的黑色分隔线
     [self.navigationController.navigationBar setBackgroundImage:[UIImage imageWithColor:[UIColor whiteColor]] forBarMetrics:UIBarMetricsDefault];
     self.navigationController.navigationBar.shadowImage = [UIImage imageWithColor:[UIColor clearColor]];

3. 给UITextView设置placehold 
```objc

UILabel *placeHolderLabel = [[UILabel alloc] init];
placeHolderLabel.text = NSLocalizedString(@"edit_signature_alert", nil);
placeHolderLabel.numberOfLines = 0;
placeHolderLabel.textColor = UIColorFromRGBA(0xb4b4b4, 1);
[placeHolderLabel sizeToFit];
placeHolderLabel.textAlignment= NSTextAlignmentLeft;
[self.textView addSubview:placeHolderLabel];
// same font
placeHolderLabel.font = [UIFont systemFontOfSize:16.f];
[self.textView setValue:placeHolderLabel forKey:@"_placeholderLabel"];