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在上一個項目中有使用到驗證碼倒計時功能，之前同事的做法是直接使用NSTimer，比較嚴重的bug是app計入后臺之后倒計時就暫停，重新進入前臺才有倒計時，在網上看到使用GCD，完美解決bug，自己嘗試做了一個demo，主要代碼如下：

#import "ViewController.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIButton *codeBtn;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
}
// 開啟倒計時效果
-(void)open{
    
    //倒計時時間(秒)
    __block NSInteger time = 20;
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    //每秒執行
    dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0);
    
    __weak typeof(self) weakself = self;
    dispatch_source_set_event_handler(_timer, ^{
        
        if(time <= 0){ //倒計時結束，關閉
            
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                [weakself.codeBtn setTitle:@"重新發送" forState:UIControlStateNormal];
                weakself.codeBtn.enabled = YES;
            });
            
        }else{
            
            int seconds = time % 60;
            dispatch_async(dispatch_get_main_queue(), ^{
                
                //設置按鈕顯示讀秒效果
                [weakself.codeBtn setTitle:[NSString stringWithFormat:@"%ds", seconds] forState:UIControlStateNormal];
                weakself.codeBtn.enabled = NO;
            });
            time--;
        }
    });
    dispatch_resume(_timer);
}

- (IBAction)click:(id)sender {
    [self open];
}