Document: scroll事件

scroll 事件會在捲動文件的可視區域時觸發。元件(Element)捲動相關請參見Element: scroll事件 (en-US)

Bubbles
Cancelable
介面 Event
事件處理常式的屬性 onscroll (en-US)

**註:**iOS的UIWebViews中,scroll事件不會在捲動時觸發,而只在捲動結束後觸發。請參見Bootstrap issue #16202。Safari和WKWebViews不受本bug影響。

範例

限制捲動事件的觸發次數

Since scroll events can fire at a high rate, the event handler shouldn't execute computationally expensive operations such as DOM modifications. Instead, it is recommended to throttle the event using requestAnimationFrame(), setTimeout(), or a CustomEvent, as follows.

Note, however, that input events and animation frames are fired at about the same rate, and therefore the optimization below is often unnecessary. This example optimizes thescroll event for requestAnimationFrame.

有鑒於scroll事件可能會以很高的速率觸發,事件處理常式不應執行計算密集的操作(如DOM修改)。所以,建議改用requestAnimationFrame()setTimeout() (en-US)CustomEvent以調控事件的觸發次數,如下。

不過需要注意,輸入事件和動畫影格的觸發速率大致相同,因此通常沒有必要進行下述的最佳化。本範例最佳化requestAnimationFramescroll事件。

// 參考來源:http://www.html5rocks.com/en/tutorials/speed/animations/

let lastKnownScrollPosition = 0;
let ticking = false;

function doSomething(scrollPos) {
  // 依據捲動位置進行某些操作
}

document.addEventListener('scroll', function(e) {
  lastKnownScrollPosition = window.scrollY;

  if (!ticking) {
    window.requestAnimationFrame(function() {
      doSomething(lastKnownScrollPosition);
      ticking = false;
    });

    ticking = true;
  }
});

更多類似範例可見resize (en-US)事件頁面。

規格

Specification
Unknown specification
# eventdef-document-scroll
HTML Standard
# handler-onscroll

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