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TransitionInterpolator Class

Base interpolator class that provides common functionality required to interpolate between two View States. This class is not intended to be used directly. See View State Transitions for backgrounds.

Example

import {TransitionInterpolator} from '@deck.gl/core';

// Interpolate between two values
function lerp(from, to, t) {
  return from + (to - from) * t;
}

/*
 * This interpolator moves the camera along a straight path on the Web Mercator map
 * Horizontally it moves in the direction that is shorter
 */
class SphericalLinearInterpolator extends TransitionInterpolator {
  constructor({speed = 100} = {}) {
    super(['longitude', 'latitude']);

    // degrees per second
    this.speed = speed;
  }

  getDuration(startViewState, endViewState) {
    const deltaLat = Math.abs(startViewState.latitude - endViewState.latitude);
    let deltaLng = Math.abs(startViewState.longitude - endViewState.longitude);
    // Transition to the destination longitude along the smaller half of the circle
    if (deltaLng > 180) deltaLng = 360 - deltaLng;
    return Math.max(deltaLng, deltaLat) / this.speed * 1000;
  }

  initializeProps(startViewState, endViewState) {
    const fromLng = startViewState.longitude;
    let toLng = endViewState.longitude;
    // Transition to the destination longitude along the smaller half of the latitude circle
    if (toLng > fromLng + 180) toLng -= 360;
    if (toLng < fromLng - 180) toLng += 360;

    return {
      start: {longitude: fromLng, latitude: startViewState.latitude},
      end: {longitude: toLng, latitude: endViewState.latitude}
    };
  }

  interpolateProps(start, end, t) {
    const latitude = lerp(start.latitude, end.latitude, t);
    let longitude = lerp(start.longitude, end.longitude, t);
    // Put longitude back into the [-180, 180] range
    if (longitude > 180) longitude -= 360;
    if (longitude < -180) longitude += 360;

    return {longitude, latitude};
  }
}

Constructor

Parameters:

  • opts (Object | Array)
    • If an object is provided, it should contain the following fields:
      • compare (Array) - prop names used in equality check. Transition is triggered if some of the compare props are not deeply equal.
      • extract (Array) - prop names needed for interpolation. See initializeProps below.
      • required (Array) - prop names that must be supplied. See initializeProps below.
    • If an array is provided, the same list of prop names is used for all three above.

Interface

getDuration

This method can be optionally implemented by a subclass. If implemented, the user can specify transitionDuration: 'auto' when using this interpolator, and the duration will be dynamically computed using this method.

Receives the following arguments:

  • startViewState (Object) - the view state that is transitioning from.
  • endViewState (Object) - the view state that is transitioning to.

Returns:

  • A number in milliseconds representing the target duration of the transition. If 0, transition is disabled.
initializeProps

Called when a transition is about to be triggered. This can be used to preprocess values for use in every transition frame by interpolateProps.

Receives the following arguments:

  • startViewState (Object) - the view state that is transitioning from.
  • endViewState (Object) - the view state that is transitioning to.

Returns:

  • An object in the shape of {start, end} that will be passed to interpolateProps.

The default implementation takes all values of the extract prop names from the start and end view states.

If some required prop names are missing from the view state, an error will be thrown.

interpolateProps

This method must be implemented by a subclasses.

Receives the following arguments:

  • start (Object) - descriptor of the state that is transitioning from, generated by initializeProps.
  • end (Object) - descriptor of the state that is transitioning to, generated by initializeProps.
  • t (Number) - current time into the transition, between 0 and 1.

Returns:

  • An object that contains the interpolated view state fields.

Source

modules/core/src/transitions/transition-interpolator.js