今天朋友问到我 iOS实现到计时器切换到后台无法执行，倒计时器不执行。于是查询了许多资料。最终实践解决了这个问题，我将核心代码粘贴到下方：
__block int timeout=60; //倒计时时间
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);
dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
dispatch_source_set_event_handler(_timer, ^{
if(timeout<=0){ //倒计时结束，关闭
dispatch_source_cancel(_timer);
dispatch_async(dispatch_get_main_queue(), ^{
//设置界面的按钮显示 根据自己需求设置
// [_sendButton setTitle:@"发送验证码" forState:UIControlStateNormal];
// _sendButton.userInteractionEnabled = YES;
});
}else{
int seconds = timeout % 60;
NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
dispatch_async(dispatch_get_main_queue(), ^{
//设置界面的按钮显示 根据自己需求设置
//[_sendButton setTitle:[NSString stringWithFormat:@"重新发送(%@)",strTime] forState:UIControlStateNormal];
//_sendButton.userInteractionEnabled = NO;
});
timeout--;
}
});
dispatch_resume(_timer);
}