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, 0);
    //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, 0, 0, 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 <= 0) {
            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 + 1)];
                _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：

2015-08-31 14:40:02.083 KMCountDown[5102:103949] Event Handler
2015-08-31 14:40:03.087 KMCountDown[5102:103949] Event Handler
2015-08-31 14:40:04.084 KMCountDown[5102:103949] Event Handler
2015-08-31 14:40:05.086 KMCountDown[5102:103971] Event Handler
2015-08-31 14:40:06.085 KMCountDown[5102:103949] Event Handler
2015-08-31 14:40:07.085 KMCountDown[5102:103949] Event Handler
2015-08-31 14:40:07.085 KMCountDown[5102:103949] Cancel Handler