限制 Text Field 输入的内容类型：只允许输入数字

效果如下：

ViewController.h

 #import <UIKit/UIKit.h>

 @interface ViewController : UIViewController<UITextFieldDelegate>
 @property (strong, nonatomic) IBOutlet UITextField *txtLimitInput;

 @end

ViewController.m

 #import "ViewController.h"

 @interface ViewController ()
 - (void)layoutUI;
 - (BOOL)validateNumberByASCII:(NSString *)string;
 - (BOOL)validateNumberByRange:(NSString *)string;
 - (BOOL)validateNumberByRegExp:(NSString *)string;
 @end

 @implementation ViewController

 - (void)viewDidLoad {
     [super viewDidLoad];

     [self layoutUI];
 }

 - (void)didReceiveMemoryWarning {
     [super didReceiveMemoryWarning];
     // Dispose of any resources that can be recreated.
 }

 - (void)layoutUI {
     _txtLimitInput.placeholder = @"请输入数字";

     //第一种方式：设置键盘类型
     //_txtLimitInput.keyboardType = UIKeyboardTypeDecimalPad;

     //第二种方式：通过 UITextFieldDelegate 的 shouldChangeCharactersInRange: 方法
     _txtLimitInput.delegate = self;
 }

 #pragma mark - 三种判断字符串内容是否是有效数字的方式
 /**
  *  『ASCII码』判断字符串内容是否是有效数字
  *
  *  @param string 需要验证的字符串
  *
  *  @return 字符串内容是否是有效数字
  */
 - (BOOL)validateNumberByASCII:(NSString *)string {
     BOOL isValid = YES;
     NSUInteger len = string.length;
     if (len > ) {
         for (NSUInteger i=; i<len; i++) {
             NSUInteger asciiCode = [string characterAtIndex:i];
             if (asciiCode <  || asciiCode > ) {
                 isValid = NO;
                 break;
             }
         }
     }
     return isValid;
 }

 /**
  *  『字符范围』判断字符串内容是否是有效数字
  *
  *  @param string 需要验证的字符串
  *
  *  @return 字符串内容是否是有效数字
  */
 - (BOOL)validateNumberByRange:(NSString *)string {
     BOOL isValid = YES;
     NSUInteger len = string.length;
     if (len > ) {
         NSCharacterSet *validNumberCS = [NSCharacterSet characterSetWithCharactersInString:@""];
         NSUInteger singleStrIndex = ;
         do {
             NSString *singleStr = [string substringWithRange:NSMakeRange(singleStrIndex, )];
             NSRange range = [singleStr rangeOfCharacterFromSet:validNumberCS];
             if (range.length == ) {
                 isValid = NO;
                 break;
             }
             singleStrIndex++;
         } while (singleStrIndex < len);
     }
     return isValid;
 }

 /**
  *  『正则表达式；推荐使用，不用循环遍历，控制更灵活』判断字符串内容是否是有效数字
  *
  *  @param string 需要验证的字符串
  *
  *  @return 字符串内容是否是有效数字
  */
 - (BOOL)validateNumberByRegExp:(NSString *)string {
     BOOL isValid = YES;
     NSUInteger len = string.length;
     if (len > ) {
         NSString *numberRegex = @"^[0-9]*$";
         NSPredicate *numberPredicate = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", numberRegex];
         isValid = [numberPredicate evaluateWithObject:string];
     }
     return isValid;
 }

 #pragma mark - UITextFieldDelegate
 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
     //return [self validateNumberByASCII:string];
     //return [self validateNumberByRange:string];
     return [self validateNumberByRegExp:string]; //推荐方式
 }

 @end

Main.storyboard

 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="7706" systemVersion="14E46" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" initialViewController="vXZ-lx-hvc">
     <dependencies>
         <deployment identifier="iOS"/>
         <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="7703"/>
     </dependencies>
     <scenes>
         <!--View Controller-->
         <scene sceneID="ufC-wZ-h7g">
             <objects>
                 <viewController id="vXZ-lx-hvc" customClass="ViewController" sceneMemberID="viewController">
                     <layoutGuides>
                         <viewControllerLayoutGuide type="top" id="jyV-Pf-zRb"/>
                         <viewControllerLayoutGuide type="bottom" id="2fi-mo-0CV"/>
                     </layoutGuides>
                     <view key="view" contentMode="scaleToFill" id="kh9-bI-dsS">
                         <rect key="frame" x="0.0" y="0.0" width="600" height="600"/>
                         <autoresizingMask key="autoresizingMask" flexibleMaxX="YES" flexibleMaxY="YES"/>
                         <subviews>
                             <textField opaque="NO" clipsSubviews="YES" contentMode="scaleToFill" contentHorizontalAlignment="left" contentVerticalAlignment="center" borderStyle="roundedRect" minimumFontSize="17" translatesAutoresizingMaskIntoConstraints="NO" id="mb3-C5-UpN">
                                 <rect key="frame" x="200" y="285" width="200" height="30"/>
                                 <constraints>
                                     <constraint firstAttribute="width" constant="200" id="8dl-R0-OzV"/>
                                 </constraints>
                                 <fontDescription key="fontDescription" type="system" pointSize="14"/>
                                 <textInputTraits key="textInputTraits"/>
                             </textField>
                             <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="限制 Text Field 输入的内容类型" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="8zf-7E-ija">
                                 <rect key="frame" x="200" y="255" width="196" height="17"/>
                                 <fontDescription key="fontDescription" type="system" pointSize="14"/>
                                 <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="calibratedRGB"/>
                                 <nil key="highlightedColor"/>
                             </label>
                         </subviews>
                         <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite"/>
                         <constraints>
                             <constraint firstItem="mb3-C5-UpN" firstAttribute="centerX" secondItem="kh9-bI-dsS" secondAttribute="centerX" id="Dh5-bw-2LN"/>
                             <constraint firstItem="mb3-C5-UpN" firstAttribute="centerY" secondItem="kh9-bI-dsS" secondAttribute="centerY" id="Lby-hq-mya"/>
                             <constraint firstItem="mb3-C5-UpN" firstAttribute="leading" secondItem="8zf-7E-ija" secondAttribute="leading" id="SyI-SC-iNN"/>
                             <constraint firstItem="mb3-C5-UpN" firstAttribute="top" secondItem="8zf-7E-ija" secondAttribute="bottom" constant="13" id="yuO-kC-OCZ"/>
                         </constraints>
                     </view>
                     <connections>
                         <outlet property="txtLimitInput" destination="mb3-C5-UpN" id="cYi-Ep-sWq"/>
                     </connections>
                 </viewController>
                 <placeholder placeholderIdentifier="IBFirstResponder" id="x5A-6p-PRh" sceneMemberID="firstResponder"/>
             </objects>
         </scene>
     </scenes>
 </document>