小编Sea*_*Liu的帖子

我试图让 useRef obj 为 null 或函数，这里是代码片段

import "./styles.css";
import { useState, useRef, useEffect } from "react";
export default function App() {
  const testRef = useRef<string | null>(null);
  const unblockRef = useRef<() => void | null>(null);

  useEffect(() => {
    const test = () => {
      console.log("hey");
    };

    testRef.current = "";

    // this would give the error code
    unblockRef.current = null;
    // unblockRef.current = "saddsa";
  }, []);
  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen!</h2>
    </div>
  ); …

我正在尝试使用react-map-gl 库在两点之间画一条线。我无法从官方文档中找到示例，因此我尝试从以下使用 Mapbox 库的代码片段中重现相同的行为

var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v11',
    center: [-122.486052, 37.830348],
    zoom: 15
});

map.on('load', function() {
    map.addSource('route', {
        'type': 'geojson',
        'data': {
            'type': 'Feature',
            'properties': {},
            'geometry': {
                'type': 'LineString',
                'coordinates': [
                    [-122.483696, 37.833818],

                    [-122.493782, 37.833683]
                ]
            }
        }
    });
    map.addLayer({
        'id': 'route',
        'type': 'line',
        'source': 'route',
        'layout': {
            'line-join': 'round',
            'line-cap': 'round'
        },
        'paint': {
            'line-color': '#888',
            'line-width': 8
        }
    });
});

这是沙箱，我在控制台上没有看到任何错误，但未显示该行：
https://codesandbox.io/s/draw-line- Between-two-point-v0mbc?file=/src/index。 js:214-226

我的问题是 justify-content: space- Between 似乎在我的代码中不起作用。

import React from 'react';
import {Button, Navbar, Nav, NavDropdown} from 'react-bootstrap';
import '../styles/headerStyle.scss';
const Header = () => (
  <React.Fragment>
    <Navbar collapseOnSelect expand='lg' bg='dark' variant='dark'>
      <Navbar.Brand href='#home'>React-Bootstrap</Navbar.Brand>
      <Navbar.Toggle aria-controls='responsive-navbar-nav' />
      <Navbar.Collapse id='responsive-navbar-nav'>
        <Nav>
          <div className='container'>
            <Nav.Link>Features</Nav.Link>
            <Nav.Link>Pricing</Nav.Link>
            <NavDropdown title='Dropdown' id='collasible-nav-dropdown'>
              <NavDropdown.Item href='#action/3.1'>Action</NavDropdown.Item>
              <NavDropdown.Item href='#action/3.2'>
                Another action
              </NavDropdown.Item>
              <NavDropdown.Item href='#action/3.3'>Something</NavDropdown.Item>
              <NavDropdown.Divider />
              <NavDropdown.Item href='#action/3.4'>
                Separated link
              </NavDropdown.Item>
            </NavDropdown>
            <Nav.Link>More deets</Nav.Link>
            <Nav.Link eventKey={2} href='#memes'>
              Dank memes
            </Nav.Link>
          </div>
        </Nav>
      </Navbar.Collapse>
    </Navbar>
  </React.Fragment>
);
export default Header;

这是样式表： …

Here is the codesandbox. I want to use my customized checkbox in the Datagrid. So I would like to change the first column checkbox to a round checkbox. I already have the wrapper round checkbox I want to use, but I do not know how to replace the default checkbox in datagrid. It looks like I should use slots Checkbox, but did not write it correctly,

https://codesandbox.io/s/datatable-material-demo-with-different-checkbox-mo715?file=/demo.js

我正在阅读 Typescript 手册，我很难理解为什么以下代码片段有错误消息：

function fn(x: string): void;
function fn(vo) {
  // ...
}
// Expected to be able to call with zero arguments
fn();

这是解释，但我无法理解，有人可以向我解释这里发生了什么吗？

从外部看不到实现的签名。当编写重载函数时，您应该始终在函数的实现之上有两个或多个签名。

这是我想在 Vue.js 中实现的行为这是我试图制作的 Js 小提琴示例： https: //jsfiddle.net/richardcwc/ukqhf54k/

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;

//Mousedown
$(canvas).on('mousedown', function(e) {
    last_mousex = parseInt(e.clientX-canvasx);
    last_mousey = parseInt(e.clientY-canvasy);
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup', function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove', function(e) {
    mousex = parseInt(e.clientX-canvasx);
    mousey = parseInt(e.clientY-canvasy);
    if(mousedown) {
        ctx.clearRect(0,0,canvas.width,canvas.height); //clear canvas
        ctx.beginPath();
        var …