使用 GCD 实现倒计时效果

效果如下：

ViewController.h

 #import <UIKit/UIKit.h>

 @interface ViewController : UIViewController
 @property (assign, nonatomic) NSInteger surplusSecond;

 @property (strong, nonatomic) IBOutlet UILabel *lblMessage;
 @property (strong, nonatomic) IBOutlet UIButton *btnSendCAPTCHA;

 @end

ViewController.m

 #import "ViewController.h"

 @interface ViewController ()
 - (void)layoutUI;
 - (void)countDown;
 @end

 @implementation ViewController
 #define kSurplusSecond 5

 - (void)viewDidLoad {
     [super viewDidLoad];

     [self layoutUI];
 }

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

 - (void)layoutUI {
     _surplusSecond = kSurplusSecond; //剩余秒数；这里指验证码发送完，间隔多少秒才能再次点击「验证」按钮进行发送验证码

     _btnSendCAPTCHA.tintColor = [UIColor darkGrayColor];
     _btnSendCAPTCHA.layer.masksToBounds = YES;
     _btnSendCAPTCHA.layer.cornerRadius = 10.0;
     _btnSendCAPTCHA.layer.borderColor = [UIColor grayColor].CGColor;
     _btnSendCAPTCHA.layer.borderWidth = 1.0;
 }

 /**
  *  倒计时
  */
 - (void)countDown {
     //全局并发队列
     dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );
     //主队列；属于串行队列
     dispatch_queue_t mainQueue = dispatch_get_main_queue();

     //定时循环执行事件
     //dispatch_source_set_timer 方法值得一提的是最后一个参数（leeway），他告诉系统我们需要计时器触发的精准程度。所有的计时器都不会保证100%精准，这个参数用来告诉系统你希望系统保证精准的努力程度。如果你希望一个计时器每5秒触发一次，并且越准越好，那么你传递0为参数。另外，如果是一个周期性任务，比如检查email，那么你会希望每10分钟检查一次，但是不用那么精准。所以你可以传入60，告诉系统60秒的误差是可接受的。他的意义在于降低资源消耗。
     dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, , , globalQueue);
     dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 0.0 * NSEC_PER_SEC);
     dispatch_source_set_event_handler(timer, ^{ //计时器事件处理器
         NSLog(@"Event Handler");
         if (_surplusSecond <= ) {
             dispatch_source_cancel(timer); //取消定时循环计时器；使得句柄被调用，即事件被执行
             dispatch_async(mainQueue, ^{
                 _btnSendCAPTCHA.enabled = YES;
                 [_btnSendCAPTCHA setTitle:@"验证" forState:UIControlStateNormal];

                 _lblMessage.text = @"使用 GCD 实现倒计时效果";
                 _surplusSecond = kSurplusSecond;
             });
         } else {
             _surplusSecond--;
             dispatch_async(mainQueue, ^{
                 NSString *btnInfo = [NSString stringWithFormat:@"%ld秒", (long)(_surplusSecond + )];
                 _btnSendCAPTCHA.enabled = NO;
                 [_btnSendCAPTCHA setTitle:btnInfo forState:UIControlStateDisabled];
             });
         }
     });
     dispatch_source_set_cancel_handler(timer, ^{ //计时器取消处理器；调用 dispatch_source_cancel 时执行
         NSLog(@"Cancel Handler");
     });
     dispatch_resume(timer);  //恢复定时循环计时器；Dispatch Source 创建完后默认状态是挂起的，需要主动恢复，否则事件不会被传递，也不会被执行
 }

 - (IBAction)sendCAPTCHA:(id)sender {
     _lblMessage.text = [NSString stringWithFormat:@"验证码发送成功，%d秒后可重新发送", kSurplusSecond];

     [self countDown];
 }

 @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>
                             <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="使用 GCD 实现倒计时效果" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="1Kh-pV-cfz">
                                 <rect key="frame" x="200" y="289.5" width="200" height="20.5"/>
                                 <fontDescription key="fontDescription" type="system" pointSize="17"/>
                                 <color key="textColor" cocoaTouchSystemColor="darkTextColor"/>
                                 <nil key="highlightedColor"/>
                             </label>
                             <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="rFe-Xb-ZSc">
                                 <rect key="frame" x="240" y="510" width="120" height="50"/>
                                 <constraints>
                                     <constraint firstAttribute="width" constant="120" id="gVH-aT-gen"/>
                                     <constraint firstAttribute="height" constant="50" id="jJP-Vc-fpy"/>
                                 </constraints>
                                 <state key="normal" title="验证">
                                     <color key="titleShadowColor" white="0.5" alpha="1" colorSpace="calibratedWhite"/>
                                 </state>
                                 <connections>
                                     <action selector="sendCAPTCHA:" destination="vXZ-lx-hvc" eventType="touchUpInside" id="I6T-s9-9H6"/>
                                 </connections>
                             </button>
                         </subviews>
                         <color key="backgroundColor" white="1" alpha="1" colorSpace="custom" customColorSpace="calibratedWhite"/>
                         <constraints>
                             <constraint firstAttribute="centerX" secondItem="rFe-Xb-ZSc" secondAttribute="centerX" id="CoE-CP-eDN"/>
                             <constraint firstAttribute="centerY" secondItem="1Kh-pV-cfz" secondAttribute="centerY" id="bX4-jS-0xm"/>
                             <constraint firstAttribute="centerX" secondItem="1Kh-pV-cfz" secondAttribute="centerX" id="mKH-Zw-Utb"/>
                             <constraint firstItem="2fi-mo-0CV" firstAttribute="top" secondItem="rFe-Xb-ZSc" secondAttribute="bottom" constant="40" id="y3Q-IA-qIO"/>
                         </constraints>
                     </view>
                     <connections>
                         <outlet property="btnSendCAPTCHA" destination="rFe-Xb-ZSc" id="UFG-TS-ImX"/>
                         <outlet property="lblMessage" destination="1Kh-pV-cfz" id="gzx-3T-euc"/>
                     </connections>
                 </viewController>
                 <placeholder placeholderIdentifier="IBFirstResponder" id="x5A-6p-PRh" sceneMemberID="firstResponder"/>
             </objects>
         </scene>
     </scenes>
 </document>

输出结果：

 -- ::02.083 KMCountDown[:] Event Handler
 -- ::03.087 KMCountDown[:] Event Handler
 -- ::04.084 KMCountDown[:] Event Handler
 -- ::05.086 KMCountDown[:] Event Handler
 -- ::06.085 KMCountDown[:] Event Handler
 -- ::07.085 KMCountDown[:] Event Handler
 -- ::07.085 KMCountDown[:] Cancel Handler