Scene中的render方法

/**
 * Update and render the scene. It is usually not necessary to call this function
 * directly because {@link CesiumWidget} or {@link Viewer} do it automatically.
 * @param {JulianDate} [time] The simulation time at which to render.
 */
Scene.prototype.render = function (time) {
  /**
   *
   * Pre passes update. Execute any pass invariant code that should run before the passes here.
   *
   */
  this._preUpdate.raiseEvent(this, time);

  var frameState = this._frameState;
  frameState.newFrame = false;

  if (!defined(time)) {
    time = JulianDate.now();
  }

  // Determine if shouldRender
  var cameraChanged = this._view.checkForCameraUpdates(this);
  var shouldRender =
    !this.requestRenderMode ||
    this._renderRequested ||
    cameraChanged ||
    this._logDepthBufferDirty ||
    this._hdrDirty ||
    this.mode === SceneMode.MORPHING;
  if (
    !shouldRender &&
    defined(this.maximumRenderTimeChange) &&
    defined(this._lastRenderTime)
  ) {
    var difference = Math.abs(
      JulianDate.secondsDifference(this._lastRenderTime, time)
    );
    shouldRender = shouldRender || difference > this.maximumRenderTimeChange;
  }

  if (shouldRender) {
    this._lastRenderTime = JulianDate.clone(time, this._lastRenderTime);
    this._renderRequested = false;
    this._logDepthBufferDirty = false;
    this._hdrDirty = false;

    var frameNumber = CesiumMath.incrementWrap(
      frameState.frameNumber,
      15000000.0,
      1.0
    );
    updateFrameNumber(this, frameNumber, time);
    frameState.newFrame = true;
  }

  tryAndCatchError(this, prePassesUpdate);

  /**
   *
   * Passes update. Add any passes here
   *
   */
  if (this.primitives.show) {
    tryAndCatchError(this, updateMostDetailedRayPicks);
    tryAndCatchError(this, updatePreloadPass);
    tryAndCatchError(this, updatePreloadFlightPass);
    if (!shouldRender) {
      tryAndCatchError(this, updateRequestRenderModeDeferCheckPass);
    }
  }

  this._postUpdate.raiseEvent(this, time);

  if (shouldRender) {
    this._preRender.raiseEvent(this, time);
    frameState.creditDisplay.beginFrame();
    tryAndCatchError(this, render);
  }

  /**
   *
   * Post passes update. Execute any pass invariant code that should run after the passes here.
   *
   */
  updateDebugShowFramesPerSecond(this, shouldRender);
  tryAndCatchError(this, postPassesUpdate);

  // Often used to trigger events (so don't want in trycatch) that the user might be subscribed to. Things like the tile load events, ready promises, etc.
  // We don't want those events to resolve during the render loop because the events might add new primitives
  callAfterRenderFunctions(this);

  if (shouldRender) {
    this._postRender.raiseEvent(this, time);
    frameState.creditDisplay.endFrame();
  }
};