// constants won't change. They're used here to set pin numbers:
const int BUTTON_PIN = 7;        // the number of the pushbutton pin
const int DEBOUNCE_DELAY = 80;   // the debounce time; increase if the output flickers

// Variables will change:
int lastSteadyState = LOW;       // the previous steady state from the input pin
int lastFlickerableState = LOW;  // the previous flickerable state from the input pin
int currentState;                // the current reading from the input pin

// the following variables are unsigned longs because the time, measured in
// milliseconds, will quickly become a bigger number than can be stored in an int.
unsigned long lastDebounceTime = 0;  // the last time the output pin was toggled

void setup() {
  // initialize serial communication at 9600 bits per second:
  Serial.begin(9600);
  // initialize the pushbutton pin as an pull-up input
  // the pull-up input pin will be HIGH when the switch is open and LOW when the switch is closed.
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  // read the state of the switch/button:
  currentState = digitalRead(BUTTON_PIN);

  // check to see if you just pressed the button
  // (i.e. the input went from LOW to HIGH), and you've waited long enough
  // since the last press to ignore any noise:

  // If the switch/button changed, due to noise or pressing:
  if (currentState != lastFlickerableState) {
    // reset the debouncing timer
    lastDebounceTime = millis(); //始终显示开发板运行的时间，最开始lastDebounceTime为0
    // save the the last flickerable state
    lastFlickerableState = currentState;
  }
  if ((millis() - lastDebounceTime) > DEBOUNCE_DELAY) {
    //如果距离上一次Button值改变的时间过小，小于DEBOUNCE_DELAY，则视为抖动，跳过该if
    // whatever the reading is at, it's been there for longer than the debounce
    // delay, so take it as the actual current state:

    // if the button state has changed:
    if (lastSteadyState == HIGH && currentState == LOW)
      Serial.println("The button is pressed");
    else if (lastSteadyState == LOW && currentState == HIGH)
      Serial.println("The button is released");

    // save the the last steady state
    lastSteadyState = currentState;
  }
}