How to use the @angular/animations.AnimationMetadataType.Keyframes function in @angular/animations

visitAnimate(metadata: AnimationAnimateMetadata, context: AnimationAstBuilderContext):
      AnimateAst {
    const timingAst = constructTimingAst(metadata.timings, context.errors);
    context.currentAnimateTimings = timingAst;

    let styles: StyleAst|KeyframesAst;
    let styleMetadata: AnimationMetadata = metadata.styles ? metadata.styles : style({});
    if (styleMetadata.type == AnimationMetadataType.Keyframes) {
      styles = this.visitKeyframes(styleMetadata as AnimationKeyframesSequenceMetadata, context);
    } else {
      let styleMetadata = metadata.styles as AnimationStyleMetadata;
      let isEmpty = false;
      if (!styleMetadata) {
        isEmpty = true;
        const newStyleData: {[prop: string]: string | number} = {};
        if (timingAst.easing) {
          newStyleData['easing'] = timingAst.easing;
        }
        styleMetadata = style(newStyleData);
      }
      context.currentTime += timingAst.duration + timingAst.delay;
      const styleAst = this.visitStyle(styleMetadata, context);
      styleAst.isEmptyStep = isEmpty;
      styles = styleAst;