我有组件 TopPage 和 ImagePreview 和 FileUploader

1. 首先显示 FileUploader，

2. FileUploader 更改了 TopPage 的 fileObj 状态。

3. 然后，FileUploader消失了，ImagePreview出现了，同时调用了useEffect()，但是此时没有ImagePreviewRef。

调用useState时，组件重新渲染和useEffect都会被调用。

命令是useEffect ->重新渲染？

如果是这样，我该如何解决这个问题？

const TopPage = (props) =>{
    const [fileObj, setFileObj ] = useState();
    const imagePreviewRef = useRef();
    useEffect(() =>{
        console.log("useEffect() is called with fileObj:",fileObj);
        console.log("imagePrevireRef.current is ready?",imagePreviewRef.current);
        imagePreviewRef.current.loadPic(fileObj);
    },[fileObj]);

    if (fileObj){
      return <ImagePreview ref={imagePreviewRef}></ImagePreview>
    }
    else {
      return <FileUploader SetFileObj=setFileObj></FileUploader>
    }
}

const FileUploader = (props) => {
   // props.setFileObj() is called here.
}
const ImagePreview = (props) => {
   const loadPic = () =>{// loadpicture to canvas }   
}

临时解决方案，

目前我确实喜欢这样， 渲染 ImagePreview 和 FileUploader 组件从一开始就通过 display:none 进行切换。

看起来有点尴尬......但目前效果很好。

const TopPage = (props) =>{
    const [fileObj, setFileObj ] = useState();
    const imagePreviewRef = useRef();
    useEffect(() =>{
        console.log("useEffect() is called with fileObj:",fileObj);
        console.log("imagePrevireRef.current is ready?",imagePreviewRef.current);
        imagePreviewRef.current.loadPic(fileObj);
    },[fileObj]);

    if (fileObj){
      return 
<React.Fragment>
<div style={{display:"block"}}>
<ImagePreview ref={imagePreviewRef}></ImagePreview>
</div>
<div style={{display:"none"}}>
<FileUploader SetFileObj=setFileObj></FileUploader>
</div>
</React.Fragment>
    }
    else {
      return 
<React.Fragment>
<div style={{display:"none"}}>
<FileUploader SetFileObj=setFileObj></FileUploader>
</div>
<div style={{display:"block"}}>
<ImagePreview ref={imagePreviewRef}></ImagePreview>
</div>
</React.Fragment>
    }
}

const FileUploader = (props) => {
   // props.setFileObj() is called here.
}
const ImagePreview = (props) => {
   const loadPic = () =>{// loadpicture to canvas }   
}