message: perf(ImagePreview): improve render perf
diff: @@ -98,9 +98,12 @@ export default createComponent({
  watch: {
  value(val) {
  + if (val) {
  this.setActive(this.startPosition);
  -
  - if (!val) {
  + this.$nextTick(() => {
  + this.$refs.swipe.swipeTo(this.startPosition, { immediate: true });
  + });
  + } else {
  this.$emit('close', {
  index: this.active,
  url: this.images[this.active]
  @@ -310,13 +313,13 @@ export default createComponent({
  },
  render() {
  - if (!this.value) {
  + if (!this.shouldRender) {
  return;
  }
  return (
  <transition name="van-fade">
  - <div class={[bem(), this.className]}>
  + <div vShow={this.value} class={[bem(), this.className]}>
  {this.genImages()}
  {this.genIndex()}
  {this.genCover()}
  