Restrict Pan outside WMS extent in OpenLayers3

问题:

I have rectangle WMS of small area and want to restrict panning outside WMS extends, so there aren't white or black area outside the map visible at all. Adding extent to View does not work for me and in documentation about this option is written

The extent that constrains the center, in other words, center cannot be set outside this extent.

But as I understand this if center is in the area of extent, but on the very corner, it will show white area outside this extent, but I don't want to see white area at all.

Is it possible to achieve this with OL3?

回答1:

Here's my solution. I wrote it just now, and so it is not extensively tested. It would probably break if you start rotating the map, for example, and it may be glitchy if you zoom out too far.

var constrainPan = function() {
    var visible = view.calculateExtent(map.getSize());
    var centre = view.getCenter();
    var delta;
    var adjust = false;
    if ((delta = extent[0] - visible[0]) > 0) {
        adjust = true;
        centre[0] += delta;
    } else if ((delta = extent[2] - visible[2]) < 0) {
        adjust = true;
        centre[0] += delta;
    }
    if ((delta = extent[1] - visible[1]) > 0) {
        adjust = true;
        centre[1] += delta;
    } else if ((delta = extent[3] - visible[3]) < 0) {
        adjust = true;
        centre[1] += delta;
    }
    if (adjust) {
        view.setCenter(centre);
    }
};
view.on('change:resolution', constrainPan);
view.on('change:center', constrainPan);

This expects the variables map, view (with obvious meanings) and extent (the xmin, ymin, xmax, ymax you want to be visible) to be available.

回答2:

Here's a more robust implementation that should work really well in any case. It's written in ES6, and requires isEqual method (from lodash or anything else ...)

const extent = [-357823.2365, 6037008.6939, 1313632.3628, 7230727.3772];
const view = this.olMap.getView();

const modifyValues = {};

// Trick to forbid panning outside extent
let constrainPan = (e) => {
  const type = e.type;
  const newValue = e.target.get(e.key);
  const oldValue = e.oldValue;

  if (isEqual(oldValue, newValue)) {
    // Do nothing when event doesn't change the value
    return;
  }

  if (isEqual(modifyValues[type], newValue)) {
    // Break possible infinite loop
    delete modifyValues[type];
    return;
  }

  if (type === 'change:resolution' && newValue < oldValue) {
    // Always allow zoom-in.
    return;
  }

  const visibleExtent = view.calculateExtent(this.olMap.getSize());
  const intersection = ol.extent.getIntersection(visibleExtent, extent);
  const modify = !isEqual(intersection, visibleExtent);

  if (modify) {
    if (type === 'change:center') {
      const newCenter = newValue.slice(0);

      if (ol.extent.getWidth(visibleExtent) !== ol.extent.getWidth(intersection)) {
        newCenter[0] = oldValue[0];
      }

      if (ol.extent.getHeight(visibleExtent) !== ol.extent.getHeight(intersection)) {
        newCenter[1] = oldValue[1];
      }

      modifyValues[type] = newCenter;
      view.setCenter(newCenter);
    } else if (type === 'change:resolution') {
      modifyValues[type] = oldValue;
      view.setResolution(oldValue);
    }
  }
};
view.on('change:resolution', constrainPan);
view.on('change:center', constrainPan);

回答3:

This is an extension to @tremby answer, but to long for a comment.

First of all, his solution works really well for me, but it was called way to often. Therefore I wrapped it in a debounce function.

So

view.on('change:resolution', constrainPan);
view.on('change:center', constrainPan);

becomes

var dConstrainPan = debounce(constrainPan);
view.on('change:resolution', dConstrainPan);
view.on('change:center', dConstrainPan);

This will result in a slight flicker, when moving outside the bounding box, bot zooming/ moving works without delay.

Still not perfect but a useful improvement from my point of view.

Debounce code:

// Returns a function, that, as long as it continues to be invoked, will not
// be triggered. The function will be called after it stops being called for
// N milliseconds. If `immediate` is passed, trigger the function on the
// leading edge, instead of the trailing.
function debounce(func, wait, immediate) {
    var timeout;
    return function() {
        var context = this, args = arguments;
        var later = function() {
            timeout = null;
            if (!immediate) func.apply(context, args);
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait);
        if (callNow) func.apply(context, args);
    };
};

Soruce: https://davidwalsh.name/javascript-debounce-function , in underscore.js