问题描述

一个轮播图组件，展示网络图片，为了避免闪屏，先用本地默认图片作为占位符。

{previewUrl.length > 0 ? (<Swiper loop={true} height={windowWidth}>{previewUrl.map((item, index) => (<Imagekey={index}style={DetailStyle.image}resizeMode="contain"source={{uri: item}}/>))}</Swiper>
) : (<Swiper loop={true} height={windowWidth}><Imagestyle={DetailStyle.image}resizeMode="contain"source={require("@/static/defaultAvator.jpeg")}/></Swiper>
)}

结论

占位符必须在2张以上，不然react-native-swipper会认为当前激活的index一直为0。

{previewUrl.length > 0 ? (<Swiper loop={true} height={windowWidth}>{previewUrl.map((item, index) => (<Imagekey={index}style={DetailStyle.image}resizeMode="contain"source={{uri: item}}/>))}</Swiper>
) : (<Swiper loop={true} height={windowWidth}><Imagestyle={DetailStyle.image}resizeMode="contain"source={require("@/static/defaultAvator.jpeg")}/><Imagestyle={DetailStyle.image}resizeMode="contain"source={require("@/static/defaultAvator.jpeg")}/></Swiper>
)}

image外层也不能包裹fragment，不然react-native-swipper仍然会认为只有一个子节点。

分析过程

布局结束后触发onLayout，如果子节点数大于1就会更新this.internals.offset[this.state.dir]。

  onLayout = event => {const { width, height } = event.nativeEvent.layoutconst offset = (this.internals.offset = {})const state = { width, height }if (this.state.total > 1) {let setup = this.state.indexif (this.props.loop) {setup++}offset[this.state.dir] =this.state.dir === 'y' ? height * setup : width * setup}// ...}

滑动结束会调用updateIndex，根据diff判断是否需要更新子节点。

  updateIndex = (offset, dir, cb) => {const state = this.statelet index = state.indexif (!this.internals.offset)this.internals.offset = {}// 如果子节点数<2，this.internals.offset[dir]会是undefined，diff为NaNconst diff = offset[dir] - this.internals.offset[dir]const step = dir === 'x' ? state.width : state.heightlet loopJump = false// diff为NaN这里就会返回，导致index和this.internals.offset均无法正常更新if (!diff) return// ...const newState = {}newState.index = indexnewState.loopJump = loopJumpthis.internals.offset = offset// ...this.setState(newState, cb)}