我们如何在 canvas 上加载本地图像？

Question

您好，感谢您花时间阅读这个问题：

我正在尝试做一个学习 React 的练习，因为我最近在 JS 中做了一个代码，从本地加载一个文件，一个图像，到 canvas，然后当用户点击绘制圆圈；我尝试在 React 中做同样的事情。

这是 JS 练习：https://codereview.stackexchange.com/questions/186526/read-an-image-file-into-a-canvas-and-display-click-coordinates

我在 Canvas' 组件的 render() 中创建了一个 canvas，然后调用 make_base 函数获取上下文并将图像加载到其中。

代码如下：

import React from 'react';


class Canvas extends React.Component {
    //A canvas to display images with a title


    constructor(props) {
        super(props);

        this.state = {x: 0, y: 0, inside: ''};

        this.handleClick = this.handleClick.bind(this);
        this.make_base = this.make_base.bind(this);
    }

    _onMouseMove(e) {
        this.setState({x: e.nativeEvent.offsetX, y: e.nativeEvent.offsetY});
    }

    componentDidMount() {
        document.addEventListener('click', this.handleClick);
    }

    componentWillUnmount() {
        document.removeEventListener('click', this.handleClick);
    }

    handleClick(e) {
        e.stopPropagation();
        console.log('INSIDE');
        this.setState({inside: 'inside'});
    }

    make_base(image) {
        const context = this.refs.canvas.getContext('2d');
        let base_image = new Image();
        base_image.src = image;
        base_image.onload = function () {
            context.drawImage(base_image, 0, 0);
        }
    }


    render() {

        const {x, y, inside} = this.state;

        return (
            <div className="previewComponent">
                <div className="imgPreview">
                    {this.props.title}

                    <canvas ref="canvas" width={300} height={300}></canvas>
                    {this.make_base(this.props.image)}

                    <img src={this.props.image} alt="" onMouseMove={this._onMouseMove.bind(this)}
                         onClick={this.handleClick.bind(this)}/>

                    <h1>Mouse coordinates: {x} {y}</h1>
                    <h2>Inside?: {inside}</h2>
                </div>
            </div>
        )
    }
}

export {Canvas};

控制台告诉我们：

TypeError: Cannot read property 'getContext' of undefined
Canvas.make_base
C:/Users/YonePC/WebstormProjects/prototipo/src/components/animals/Canvas.js:36
  33 |    }
  34 | 
  35 |    make_base(image) {
> 36 |        const context = this.refs.canvas.getContext('2d');
  37 |        let base_image = new Image();
  38 |        base_image.src = image;
  39 |        base_image.onload = function () {
View compiled
Canvas.render
C:/Users/YonePC/WebstormProjects/prototipo/src/components/animals/Canvas.js:55
  52 |                    {this.props.title}
  53 | 
  54 |                    <canvas ref="canvas" width={300} height={300}></canvas>
> 55 |                    {this.make_base(this.props.image)}
  56 | 
  57 |                    <img src={this.props.image} alt="" onMouseMove={this._onMouseMove.bind(this)}
  58 |                         onClick={this.handleClick.bind(this)}/>

如果假设我们已经在 render() 方法中创建了 canvas，我不明白为什么 getContext() 会变得未定义。

另外我研究过：

https://blog.lavrton.com/using-react-with-html5-canvas-871d07d8d753

How to add image to canvas

编辑：

我已经尝试了建议的答案，控制台仍然告诉我们：

TypeError: Cannot read property 'getContext' of undefined
Canvas.make_base
C:/Users/YonePC/WebstormProjects/prototipo/src/components/animals/Canvas.js:36
  33 |    }
  34 | 
  35 |    make_base(image) {
> 36 |        const context = this.canvas.getContext('2d');
  37 |        let base_image = new Image();
  38 |        base_image.src = image;
  39 |        base_image.onload = function () {
View compiled
Canvas.render
C:/Users/YonePC/WebstormProjects/prototipo/src/components/animals/Canvas.js:55
  52 |                    {this.props.title}
  53 | 
  54 |                    <canvas ref={(canvas) => this.canvas = canvas} width={300} height={300}></canvas>
> 55 |                    {this.make_base(this.props.image)}
  56 | 
  57 |                    <img src={this.props.image} alt="" onMouseMove={this._onMouseMove.bind(this)}
  58 |                         onClick={this.handleClick.bind(this)}/>
View compiled

如果我们注释正在写入上下文的行，我们会看到两个空 canvas:

也许 canvas 不应该在 render 方法中创建，因为它仍然不能被外部方法访问？

EDIT2：给定的答案很有效：

但是我们想在光标所在的地方而不是按钮上绘制鼠标点击，所以这就是问题，我们如何获取坐标并将它们关联起来以在 canvas?

感谢您的帮助。

Answer 1

您使用的引用有误。

您必须将 ref 设置为 class 变量，如 this.canvas

class App extends React.Component {
  constructor(props) {
    super(props);
    this.makeBase = this.makeBase.bind(this);
    this.recordMouse = this.recordMouse.bind(this);
  }
  recordMouse(event) {
    const ctx = this.canvas.getContext('2d');
    ctx.beginPath();
    ctx.fillRect(event.nativeEvent.offsetX,event.nativeEvent.offsetY,3,3);
    ctx.stroke();
  }
  makeBase() {
    const ctx = this.canvas.getContext('2d');
    ctx.beginPath();
    ctx.arc(95,50,40,0,2*Math.PI);
    ctx.stroke();
  }
  render() {
    return (
      <div>
        <canvas ref={(canvas)=>this.canvas=canvas} width="200" height="100" onMouseMove={this.recordMouse} />
        <button onClick={this.makeBase}>Make Base</button>
      </div>
    );
  }
}
ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="root"></div>

https://reactjs.org/docs/refs-and-the-dom.html

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