beginForVisit() {
    // calculate whether gradient color map is needed this frame
    if (this._gradientMapDirty) {
      const pgw = this.getPhaseGradientColorMapWhiteColor();
      const pgb = this.getPhaseGradientColorMapBlackColor();
      this._needsGradientColorMap = (pgw.r + pgw.g + pgw.b + pgw.a + pgb.r + pgb.g + pgb.b + pgb.a) > 0.0;
      if (this.getIsCachingScreen()) {
        this.setScreenCacheDirty();
      }
      if (this.getIsCachingSurface()) {
        this.setSurfaceCacheDirty();
      }
    }

    // update whether caching screen
    if (!this.getScreenCacheRequested() || this._cachingScreenDirty) {
      this._stopCachingScreen();
    }

    // update whether caching surface
    if (!this.getSurfaceCacheRequested() || this._cachingSurfaceDirty) {
      this._stopCachingSurface();
    }

    if (!this.getIsCachingScreen() && !this.getIsCachingSurface()) {
      if (this.getScreenCacheRequested()) {
        this.getEventBus().trigger(EVENTS.caching_screen_setup, { type: EVENTS.caching_screen_setup });
      }
      if (this.getSurfaceCacheRequested()) {
        this.getEventBus().trigger(EVENTS.caching_surface_setup, { type: EVENTS.caching_surface_setup });
      }
    }
  }
Begins visit phase of fx.
NOTE: This should be called automatically at the beginning of the frame visit phase.