Stack组件实现Swiper堆叠动画效果

问题现象
Swiper如何实现卡片堆叠样式：

上下堆叠：Swiper内容像卡片堆叠，在底部留部分空间显示下一页的内容，上下滑实现卡片切换。
左右堆叠：Swiper内容像卡片堆叠，在右侧留部分空间显示下一页的内容，左右滑实现卡片切换。
背景知识
Swiper组件提供滑动轮播显示的能力。Swiper本身是一个容器组件，当设置了多个子组件后，可以对这些子组件进行轮播显示。
Stack是堆叠容器，子组件按照顺序依次入栈，后一个子组件覆盖前一个子组件。
gesture可以为组件绑定手势方法进行相应处理，如滑动手势事件PanGesture。
animateTo指定由于闭包代码导致的状态变化插入过渡动效。
解决方案
上下堆叠实现。
自定义实现卡片堆叠的组件：使用Stack组件堆叠需要展示的图片，设置最上面的图片向上偏移部分距离，露出下一张图片的底部。为Stack绑定上下滑动的手势处理，实现切换图片逻辑，同时使用animateTo接口设置图片切换动画。

export class SwiperData {
imageSrc: Resource;

constructor(imageSrc: Resource) {
this.imageSrc = imageSrc;
}
}

@Component
export struct SwiperStackComponent {
@Link currentIndex: number;
@Prop swiperData: SwiperData[];
private halfCount: number = Math.floor(3 / 2);
private automaticSlidingDuration: number = 300;

aboutToAppear(): void {
this.currentIndex = 0;
}

// 修改堆叠方向系数计算
getImgCoefficients(index: number): number {
const coefficient = this.currentIndex - index;
const tempCoefficient = Math.abs(coefficient);
if (tempCoefficient <= this.halfCount) {
return coefficient;
}
const dataLength = this.swiperData.length;
let tempOffset = dataLength - tempCoefficient;
if (tempOffset <= this.halfCount) {
return coefficient > 0 ? -tempOffset : tempOffset;
}
return 0;
}

// 修改堆叠方向偏移量计算
getOffSet(index: number): number {
let offsetIndex = this.getImgCoefficients(index);
const tempOffset = Math.abs(offsetIndex);
let offset = 0;
if (tempOffset === 1) {
if (offsetIndex === 1) {
offsetIndex = -1;
}
offset = 50 * offsetIndex;
}
return -offset;
}

startAnimation(isLeft: boolean, duration: number): void {
this.getUIContext().animateTo({ duration: duration, }, () => {
const dataLength: number = this.swiperData.length;
const tempIndex: number = isLeft ? this.currentIndex + 1 : this.currentIndex - 1 + dataLength;
this.currentIndex = tempIndex % dataLength;
});
}

build() {
Stack() {
ForEach(this.swiperData, (item: SwiperData, index: number) => {
Stack({ alignContent: Alignment.Bottom }) {
Image(item.imageSrc)
.objectFit(ImageFit.Cover)
.width('100%')
.height('100%')
.borderRadius(8);
}
.offset({ x: 0, y: this.getOffSet(index) })
.shadow(ShadowStyle.OUTER_DEFAULT_SM)
.backgroundColor(Color.White)
.borderRadius(8)
.blur(index !== this.currentIndex ? 12 : 0)
// 通过animateTo实现动画并且同时改变currentIndex数据中间值来判断组件zIndex实现切换动画
.zIndex(index !== this.currentIndex && this.getImgCoefficients(index) === 0 ?
0 : 2 - Math.abs(this.getImgCoefficients(index)))
.width(310)
.height(index !== this.currentIndex ? 130 : 180);
});
}
.height(200)
.width('100%')
.gesture(
PanGesture({ direction: PanDirection.Vertical })
.onActionStart((event: GestureEvent) => {
this.startAnimation(event.offsetY < 0, this.automaticSlidingDuration);
})
)
.alignContent(Alignment.Center)
.padding({ left: 12, right: 12, top: 12 });
}
}
在页面中直接使用上面封装好的SwiperStackComponent即可，示例如下：传给SwiperStackComponent要堆叠的图片。
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