View / add a method to compute a valid zoom level

The `getValidZoomLevel` apply the current resolution constraint to return
a value that is guaranteed valid.

This is used for interactions & controls which need a target value to work:
the +/- buttons, the zoom clider, the dragbox zoom and the mouse wheel zoom.

src/ol/View.js

@@ -1217,6 +1217,23 @@ class View extends BaseObject {
   endInteraction() {
     this.setHint(ViewHint.INTERACTING, -1);
   }
+
+  /**
+   * Get a valid zoom level according to the current view constraints.
+   * @param {number|undefined} targetZoom Target resolution.
+   * @param {number=} opt_direction Direction. Default is `0`. Specify `-1` or `1` to return
+   * the available value respectively lower or greater than the target one. Leaving `0` will simply choose
+   * the nearest available value.
+   * @return {number|undefined} Valid zoom level.
+   * @api
+   */
+  getValidZoomLevel(targetZoom, opt_direction) {
+    const direction = opt_direction || 0;
+    const currentRes = this.getResolution();
+    const currentZoom = this.getZoom();
+    return this.getZoomForResolution(
+      this.constraints_.resolution(currentRes, targetZoom - currentZoom, direction));
+  }
 }
 
 

src/ol/control/Zoom.js

@@ -114,20 +114,20 @@ class Zoom extends Control {
       // upon it
       return;
     }
-    const currentResolution = view.getResolution();
-    if (currentResolution) {
-      const newResolution = view.constrainResolution(currentResolution, delta);
+    const currentZoom = view.getZoom();
+    if (currentZoom !== undefined) {
+      const newZoom = view.getValidZoomLevel(currentZoom + delta);
       if (this.duration_ > 0) {
         if (view.getAnimating()) {
           view.cancelAnimations();
         }
         view.animate({
-          resolution: newResolution,
+          zoom: newZoom,
           duration: this.duration_,
           easing: easeOut
         });
       } else {
-        view.setResolution(newResolution);
+        view.setZoom(newZoom);
       }
     }
   }

src/ol/control/ZoomSlider.js

@@ -218,9 +218,10 @@ class ZoomSlider extends Control {
       event.offsetY - this.thumbSize_[1] / 2);
 
     const resolution = this.getResolutionForPosition_(relativePosition);
+    const zoom = view.getValidZoomLevel(view.getZoomForResolution(resolution));
 
     view.animate({
-      resolution: view.constrainResolution(resolution),
+      zoom: zoom,
       duration: this.duration_,
       easing: easeOut
     });
@@ -281,8 +282,11 @@ class ZoomSlider extends Control {
       const view = this.getMap().getView();
       view.endInteraction();
 
+      const zoom = view.getValidZoomLevel(
+        view.getZoomForResolution(this.currentResolution_));
+
       view.animate({
-        resolution: view.constrainResolution(this.currentResolution_),
+        zoom: zoom,
         duration: this.duration_,
         easing: easeOut
       });

src/ol/interaction/DragZoom.js

@@ -80,14 +80,14 @@ function onBoxEnd() {
     extent = mapExtent;
   }
 
-  const resolution = view.constrainResolution(
-    view.getResolutionForExtent(extent, size));
+  const resolution = view.getResolutionForExtent(extent, size);
+  const zoom = view.getValidZoomLevel(view.getZoomForResolution(resolution));
 
   let center = getCenter(extent);
   center = view.constrainCenter(center);
 
   view.animate({
-    resolution: resolution,
+    zoom: zoom,
     center: center,
     duration: this.duration_,
     easing: easeOut

src/ol/interaction/Interaction.js

@@ -189,34 +189,49 @@ export function zoom(view, resolution, opt_anchor, opt_duration, opt_direction)
  * @param {number=} opt_duration Duration.
  */
 export function zoomByDelta(view, delta, opt_anchor, opt_duration) {
+  const currentZoom = view.getZoom();
   const currentResolution = view.getResolution();
-  let resolution = view.constrainResolution(currentResolution, delta, 0);
 
-  if (resolution !== undefined) {
-    const resolutions = view.getResolutions();
-    resolution = clamp(
-      resolution,
-      view.getMinResolution() || resolutions[resolutions.length - 1],
-      view.getMaxResolution() || resolutions[0]);
+  if (currentZoom === undefined) {
+    return;
   }
 
+  const newZoom = view.getValidZoomLevel(currentZoom + delta);
+  const newResolution = view.getResolutionForZoom(newZoom);
+
   // If we have a constraint on center, we need to change the anchor so that the
   // new center is within the extent. We first calculate the new center, apply
   // the constraint to it, and then calculate back the anchor
-  if (opt_anchor && resolution !== undefined && resolution !== currentResolution) {
+  if (opt_anchor) {
     const currentCenter = view.getCenter();
-    let center = view.calculateCenterZoom(resolution, opt_anchor);
+    let center = view.calculateCenterZoom(newResolution, opt_anchor);
     center = view.constrainCenter(center);
 
     opt_anchor = [
-      (resolution * currentCenter[0] - currentResolution * center[0]) /
-          (resolution - currentResolution),
-      (resolution * currentCenter[1] - currentResolution * center[1]) /
-          (resolution - currentResolution)
+      (newResolution * currentCenter[0] - currentResolution * center[0]) /
+      (newResolution - currentResolution),
+      (newResolution * currentCenter[1] - currentResolution * center[1]) /
+      (newResolution - currentResolution)
     ];
   }
 
-  zoomWithoutConstraints(view, resolution, opt_anchor, opt_duration);
+  if (opt_duration > 0) {
+    if (view.getAnimating()) {
+      view.cancelAnimations();
+    }
+    view.animate({
+      resolution: newResolution,
+      anchor: opt_anchor,
+      duration: opt_duration,
+      easing: easeOut
+    });
+  } else {
+    if (opt_anchor) {
+      const center = view.calculateCenterZoom(newResolution, opt_anchor);
+      view.setCenter(center);
+    }
+    view.setResolution(newResolution);
+  }
 }
 
 

src/ol/interaction/MouseWheelZoom.js

@@ -239,8 +239,10 @@ class MouseWheelZoom extends Interaction {
       view.setResolution(resolution);
 
       if (rebound === 0 && this.constrainResolution_) {
+        const zoomDelta = delta > 0 ? -1 : 1;
+        const newZoom = view.getValidZoomLevel(view.getZoom() + zoomDelta);
         view.animate({
-          resolution: view.constrainResolution(resolution, delta > 0 ? -1 : 1),
+          resolution: view.getResolutionForZoom(newZoom),
           easing: easeOut,
           anchor: this.lastAnchor_,
           duration: this.duration_