Is there a way to determine the scroll direction using IntersectionObserver?

Question

Is there a way to determine the scroll direction using IntersectionObserver?

Is there a way to determine the scroll direction when an event is triggered?

I've considered using the boundingClientRect in the returned object to store the last scroll position, but I'm concerned about potential performance issues.

Could the intersection event be utilized to detect the scroll direction (up / down)?

I have included a basic snippet below and would greatly appreciate any assistance.

Snippet:

var options = {
  rootMargin: '0px',
  threshold: 1.0
}

function callback(entries, observer) { 
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      console.log('entry', entry);
    }
  });
};

var elementToObserve = document.querySelector('#element');
var observer = new IntersectionObserver(callback, options);

observer.observe(elementToObserve);

#element {
  margin: 1500px auto;
  width: 150px;
  height: 150px;
  background: #ccc;
  color: white;
  font-family: sans-serif;
  font-weight: 100;
  font-size: 25px;
  text-align: center;
  line-height: 150px;
}

<div id="element">Observed</div>

I am looking for a solution to implement on a fixed headers menu to show/hide it based on the scroll direction.

javascript html css intersection-observer

Answer 1

Answer №1

It's uncertain whether managing boundingClientRect will impact performance negatively.

The IntersectionObserver does not operate on the main thread according to MDN:

This means websites no longer have to perform tasks on the main thread to monitor element intersection, allowing the browser to optimize intersection management as needed.

MDN, "Intersection Observer API"

We can determine scrolling direction by storing the value of

IntersectionObserverEntry.boundingClientRect.y

and comparing it to the previous value.

Execute the code snippet below for a demonstration:

const state = document.querySelector('.observer__state')
const target = document.querySelector('.observer__target')

const thresholdArray = steps => Array(steps + 1)
 .fill(0)
 .map((_, index) => index / steps || 0)

let previousY = 0
let previousRatio = 0

const handleIntersect = entries => {
  entries.forEach(entry => {
    const currentY = entry.boundingClientRect.y
    const currentRatio = entry.intersectionRatio
    const isIntersecting = entry.isIntersecting

    // Determining scroll direction
    if (currentY < previousY) {
      if (currentRatio > previousRatio && isIntersecting) {
        state.textContent ="Scrolling down enter"
      } else {
        state.textContent ="Scrolling down leave"
      }
    } else if (currentY > previousY && isIntersecting) {
      if (currentRatio < previousRatio) {
        state.textContent ="Scrolling up leave"
      } else {
        state.textContent ="Scrolling up enter"
      }
    }

    previousY = currentY
    previousRatio = currentRatio
  })
}

const observer = new IntersectionObserver(handleIntersect, {
  threshold: thresholdArray(20),
})

observer.observe(target)

html,
body {
  margin: 0;
}

.observer__target {
  position: relative;
  width: 100%;
  height: 350px;
  margin: 1500px 0;
  background: rebeccapurple;
}

.observer__state {
  position: fixed;
  top: 1em;
  left: 1em;
  color: #111;
  font: 400 1.125em/1.5 sans-serif;
  background: #fff;
}

<div class="observer__target"></div>
<span class="observer__state"></span>

If you find the thresholdArray helper function perplexing, it generates an array from 0.0 to 1.0 in increments determined by the specified number of steps. Using 5 will produce [0.0, 0.2, 0.4, 0.6, 0.8, 1.0].

Answer 2

It's uncertain whether managing boundingClientRect will impact performance negatively.

The IntersectionObserver does not operate on the main thread according to MDN:

This means websites no longer have to perform tasks on the main thread to monitor element intersection, allowing the browser to optimize intersection management as needed.

MDN, "Intersection Observer API"

We can determine scrolling direction by storing the value of

IntersectionObserverEntry.boundingClientRect.y

and comparing it to the previous value.

Execute the code snippet below for a demonstration:

const state = document.querySelector('.observer__state')
const target = document.querySelector('.observer__target')

const thresholdArray = steps => Array(steps + 1)
 .fill(0)
 .map((_, index) => index / steps || 0)

let previousY = 0
let previousRatio = 0

const handleIntersect = entries => {
  entries.forEach(entry => {
    const currentY = entry.boundingClientRect.y
    const currentRatio = entry.intersectionRatio
    const isIntersecting = entry.isIntersecting

    // Determining scroll direction
    if (currentY < previousY) {
      if (currentRatio > previousRatio && isIntersecting) {
        state.textContent ="Scrolling down enter"
      } else {
        state.textContent ="Scrolling down leave"
      }
    } else if (currentY > previousY && isIntersecting) {
      if (currentRatio < previousRatio) {
        state.textContent ="Scrolling up leave"
      } else {
        state.textContent ="Scrolling up enter"
      }
    }

    previousY = currentY
    previousRatio = currentRatio
  })
}

const observer = new IntersectionObserver(handleIntersect, {
  threshold: thresholdArray(20),
})

observer.observe(target)

html,
body {
  margin: 0;
}

.observer__target {
  position: relative;
  width: 100%;
  height: 350px;
  margin: 1500px 0;
  background: rebeccapurple;
}

.observer__state {
  position: fixed;
  top: 1em;
  left: 1em;
  color: #111;
  font: 400 1.125em/1.5 sans-serif;
  background: #fff;
}

<div class="observer__target"></div>
<span class="observer__state"></span>

If you find the thresholdArray helper function perplexing, it generates an array from 0.0 to 1.0 in increments determined by the specified number of steps. Using 5 will produce [0.0, 0.2, 0.4, 0.6, 0.8, 1.0].

Answer 3

Answer №2

This particular method does not rely on any external state, making it more straightforward compared to approaches that involve tracking extra variables:

const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.boundingClientRect.top < 0) {
          if (entry.isIntersecting) {
            // Viewport entered from the top, implying scroll direction is upwards
          } else {
            // Viewport exited from the top, indicating scroll direction is downwards
          }
        }
      },
      {
        root: rootElement,
      },
    );

Answer 4

This particular method does not rely on any external state, making it more straightforward compared to approaches that involve tracking extra variables:

const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.boundingClientRect.top < 0) {
          if (entry.isIntersecting) {
            // Viewport entered from the top, implying scroll direction is upwards
          } else {
            // Viewport exited from the top, indicating scroll direction is downwards
          }
        }
      },
      {
        root: rootElement,
      },
    );

Answer 5

Answer №3

By comparing the values of boundingClientRect and rootBounds within the context of entry, one can easily determine if a target is located above or below the viewport.

Within the callback() function, the variables isAbove/isBelow are checked, and ultimately stored in wasAbove/wasBelow for future reference. This allows for tracking whether a target moves into view from the top or bottom based on its previous position.

An example implementation could look something like this:

var wasAbove = false;

function callback(entries, observer) {
    entries.forEach(entry => {
        const isAbove = entry.boundingClientRect.y < entry.rootBounds.y;

        if (entry.isIntersecting) {
            if (wasAbove) {
                // Target is coming from the top
            }
        }

        wasAbove = isAbove;
    });
}

I hope this explanation proves helpful.

Answer 6

By comparing the values of boundingClientRect and rootBounds within the context of entry, one can easily determine if a target is located above or below the viewport.

Within the callback() function, the variables isAbove/isBelow are checked, and ultimately stored in wasAbove/wasBelow for future reference. This allows for tracking whether a target moves into view from the top or bottom based on its previous position.

An example implementation could look something like this:

var wasAbove = false;

function callback(entries, observer) {
    entries.forEach(entry => {
        const isAbove = entry.boundingClientRect.y < entry.rootBounds.y;

        if (entry.isIntersecting) {
            if (wasAbove) {
                // Target is coming from the top
            }
        }

        wasAbove = isAbove;
    });
}

I hope this explanation proves helpful.

Answer 7

Answer №4

I had a specific requirement for handling scroll behavior:

When scrolling up, no action should be taken
When scrolling down, I needed to determine if an element was starting to hide from the top of the screen

To achieve this, I relied on certain information provided by the IntersectionObserverEntry:

The intersectionRatio (which should decrease from 1.0)
The boundingClientRect's bottom property
The boundingClientRect's height property

This led me to create the following callback function:

intersectionObserver = new IntersectionObserver(function(entries) {
  const entry = entries[0]; // observing one element
  const currentRatio = intersectionRatio;
  const newRatio = entry.intersectionRatio;
  const boundingClientRect = entry.boundingClientRect;
  const scrollingDown = currentRatio !== undefined && 
    newRatio < currentRatio &&
    boundingClientRect.bottom < boundingClientRect.height;

  intersectionRatio = newRatio;

  if (scrollingDown) {
    // The user is scrolling down and the observed image has started to hide.
    // Perform some actions...
  }

  console.log(entry);
}, { threshold: [0, 0.25, 0.5, 0.75, 1] });

You can find the complete code in my post.

Answer 8

I had a specific requirement for handling scroll behavior:

When scrolling up, no action should be taken
When scrolling down, I needed to determine if an element was starting to hide from the top of the screen

To achieve this, I relied on certain information provided by the IntersectionObserverEntry:

The intersectionRatio (which should decrease from 1.0)
The boundingClientRect's bottom property
The boundingClientRect's height property

This led me to create the following callback function:

intersectionObserver = new IntersectionObserver(function(entries) {
  const entry = entries[0]; // observing one element
  const currentRatio = intersectionRatio;
  const newRatio = entry.intersectionRatio;
  const boundingClientRect = entry.boundingClientRect;
  const scrollingDown = currentRatio !== undefined && 
    newRatio < currentRatio &&
    boundingClientRect.bottom < boundingClientRect.height;

  intersectionRatio = newRatio;

  if (scrollingDown) {
    // The user is scrolling down and the observed image has started to hide.
    // Perform some actions...
  }

  console.log(entry);
}, { threshold: [0, 0.25, 0.5, 0.75, 1] });

You can find the complete code in my post.

Answer 9

Answer №5

:)

It might not be feasible to achieve this using just one threshold value. One alternative could be to monitor the intersectionRatio, which is typically less than 1 when the container exits the viewport (due to the asynchronous nature of the intersection observer). However, it could also reach 1 if the browser updates quickly enough (although I haven't tested this personally).

Another option could be observing two thresholds by specifying multiple values:

threshold: [0.9, 1.0]

Receiving an event for 0.9 first would indicate that the container has entered the viewport...

I hope this suggestion proves helpful. :)

Answer 10

:)

It might not be feasible to achieve this using just one threshold value. One alternative could be to monitor the intersectionRatio, which is typically less than 1 when the container exits the viewport (due to the asynchronous nature of the intersection observer). However, it could also reach 1 if the browser updates quickly enough (although I haven't tested this personally).

Another option could be observing two thresholds by specifying multiple values:

threshold: [0.9, 1.0]

Receiving an event for 0.9 first would indicate that the container has entered the viewport...

I hope this suggestion proves helpful. :)

Answer 11

Answer №6

To simplify this process, consider setting a threshold of 0.5. This will make the center of the element a single trigger point to monitor the intersectionRect.y property.

When observing the element, if the value of intersectionRect.y is 0, it means the element passes the top of the bounding box. If the value is greater than 0, then the element passes the bottom of the bounding box.

It is important to keep track of the previous crossing status (using outside) to determine whether the element is inside or outside the bounding box. In case the element exits the bounding box, the boolean value of intersectionRect.y needs to be inverted.

const
  observer = new IntersectionObserver(observedArr => {
    let
      direction = outside ? observedArr[0].intersectionRect.x : !observedArr[0].intersectionRect.y
    if (direction) // intersection towards bottom
    else // intersection towards top
    outside = !outside
  }, { threshold: .5 })

let
  outside = false

Answer 12

To simplify this process, consider setting a threshold of 0.5. This will make the center of the element a single trigger point to monitor the intersectionRect.y property.

When observing the element, if the value of intersectionRect.y is 0, it means the element passes the top of the bounding box. If the value is greater than 0, then the element passes the bottom of the bounding box.

It is important to keep track of the previous crossing status (using outside) to determine whether the element is inside or outside the bounding box. In case the element exits the bounding box, the boolean value of intersectionRect.y needs to be inverted.

const
  observer = new IntersectionObserver(observedArr => {
    let
      direction = outside ? observedArr[0].intersectionRect.x : !observedArr[0].intersectionRect.y
    if (direction) // intersection towards bottom
    else // intersection towards top
    outside = !outside
  }, { threshold: .5 })

let
  outside = false

Is there a way to determine the scroll direction using IntersectionObserver?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

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