1.单次运行的定时器

源码：

factory Timer(Duration duration, void Function() callback) {
    if (Zone.current == Zone.root) {
      // No need to bind the callback. We know that the root's timer will
      // be invoked in the root zone.
      return Zone.current.createTimer(duration, callback);
    }
    return Zone.current
        .createTimer(duration, Zone.current.bindCallbackGuarded(callback));
  }

示例代码：

 void countDown() {
    //创建一个单次的定时器
    print("当前时间=======${DateTime.now()}");
    Timer timer = Timer(const Duration(seconds: 5), (){
      print("5秒后执行定时器======${DateTime.now()}");
      setState(() {
        _counter++;
      });
    });
  }

在这里插入图片描述

如果你立刻执行一个定时器，可以调用run方法：

/// Runs the given [callback] asynchronously as soon as possible.
  ///
  /// This function is equivalent to `new Timer(Duration.zero, callback)`.
  static void run(void Function() callback) {
    new Timer(Duration.zero, callback);
  }
  
static const Duration zero = Duration(seconds: 0);

run()：方法的实质是创建一个时间为0的定时器
创建定时器的时候，如果传入的时间是负数或则0，这两种情况都当成时间是0，会立刻执行定时器，这一点官方文档是有说明的

2. 重复运行定时器

源码：

factory Timer.periodic(Duration duration, void callback(Timer timer)) {
    if (Zone.current == Zone.root) {
      // No need to bind the callback. We know that the root's timer will
      // be invoked in the root zone.
      return Zone.current.createPeriodicTimer(duration, callback);
    }
    var boundCallback = Zone.current.bindUnaryCallbackGuarded<Timer>(callback);
    return Zone.current.createPeriodicTimer(duration, boundCallback);
  }

示例代码：

void countDown2() {
    //创建一个多次重复运行的定时器
    print("当前时间=======${DateTime.now()}");
    Timer timer = Timer.periodic(const Duration(seconds: 3), (timer) {
      setState(() {
        print("3秒后执行定时器======${DateTime.now()}");
        _counter ++;
      });

      if (_counter == 5) {
        //循环5次之后停止定时器
        timer.cancel();
        print("执行5次之后停止定时器");
      }
    });
  }