reactjs - react useState 选项值落后于 onChange 选择 1 步

Question

size.value _updateData 时状态是后面的一个选项函数触发 <select> 的 onChange .例如，假设 <select> 的起始默认值选项是“全部”，locations为空...选项更改为“中”，locations是“全部”...选项更改为“大”，locations是“中等”...等等。最初我想要 _updateData选择“全部”运行 onload 的函数也不起作用，它会抛出错误 Cannot read property 'target' of undefined on setSize({value: event.target.value}) .我在这里做错了什么？谢谢您的帮助。

const Map = () => {
    const [viewport, setViewport] = useState({longitude: -98.58, latitude: 39.83, zoom: 3.5})
    const [locations, setLocations] = useState([])
    const [geojson, setGeojson] = useState(null)
    const [size, setSize] = useState({value: "All"})

    useEffect(() => {
        setLocations(geodata)
        _updateData()
    }, []);

    const _updateViewport = viewport => {
        setViewport(viewport)
    }

    const _updateData = event => {
        setSize({value: event.target.value})
        const tempLocations = [];
        locations.forEach(function(res) {
            if (size.value === "All") {
                tempLocations.push(res);
            } else if (res.Size === size.value) {
                tempLocations.push(res);
            }
        });
        var data = {
            ...
        };
        setGeojson(data);
    }

    return (
        <ReactMapGL
            {...viewport}
            onViewportChange={_updateViewport}
            width="100%"
            height="100%"
            mapStyle={mapStyle}
            mapboxApiAccessToken={TOKEN}>
            <Source id="my-data" type="geojson" data={geojson}>
                <Layer {...icon} />
            </Source>
            <div style={navStyle}>
                <NavigationControl onViewportChange={_updateViewport} />
                <select onChange={_updateData} defaultValue={size}>
                    <option value="All">All</option>
                    <option value="Large">Large</option>
                    <option value="Medium">Medium</option>
                    <option value="Small">Small</option>
                    <option value="Very Small">Very Small</option>
                </select>
            </div>
        </ReactMapGL>
    );
}

export default Map;

Answer 1

是的，您正在安装 useEffect Hook 中调用您选择的 onChange 处理程序，没有事件对象来取消引用 target 属性。我会分解出 updateData 代码的其余部分，以便您可以使用初始状态值调用它。这将允许您使用初始大小状态日期更新安装位置详细信息AND 选择的 onChange 将保持原样。 p>

注意:您应该注意状态更新直到下一个渲染周期才会生效，因此在您的代码中您使用新值调用 setSize 但继续处理位置当前大小值，所以你需要转发当前值。

const Map = () => {
    const [viewport, setViewport] = useState({longitude: -98.58, latitude: 39.83, zoom: 3.5})
    const [locations, setLocations] = useState([])
    const [geojson, setGeojson] = useState(null)
    const [size, setSize] = useState({value: "All"}) // initial size state here

    useEffect(() => {
        setLocations(geodata);
        updateLocationData(size.value); // call the location updater on mount with the initial size state value
    }, []);

    const _updateViewport = viewport => {
        setViewport(viewport)
    }

    const _updateData = event => {
        setSize({value: event.target.value})
        updateLocationData(event.target.value); // forward current size value
    }

    const updateLocationData = (sizeValue) => { // forwarded size value
        const tempLocations = [];
        locations.forEach(function(res) {
            if (sizeValue === "All") { // forwarded size value for comparison
                tempLocations.push(res);
            } else if (res.Size === sizeValue) { // forwarded size value for comparison
                tempLocations.push(res);
            }
        });
        var data = {
            ...
        };
        setGeojson(data);
    };

    return (
        <ReactMapGL
            {...viewport}
            onViewportChange={_updateViewport}
            width="100%"
            height="100%"
            mapStyle={mapStyle}
            mapboxApiAccessToken={TOKEN}>
            <Source id="my-data" type="geojson" data={geojson}>
                <Layer {...icon} />
            </Source>
            <div style={navStyle}>
                <NavigationControl onViewportChange={_updateViewport} />
                <select onChange={_updateData} defaultValue={size.value}> // need to unpack the actual size value
                    <option value="All">All</option>
                    <option value="Large">Large</option>
                    <option value="Medium">Medium</option>
                    <option value="Small">Small</option>
                    <option value="Very Small">Very Small</option>
                </select>
            </div>
        </ReactMapGL>
    );
}

export default Map;