基于fabric.js的图片编辑器, 画布背景实现原理

2024-01-12 09:12 由 haixin-fang 发表于 #前端开发

图片上传

使用了element-plus提供的图片上传el-upload组件

<el-upload :show-file-list="false" :auto-upload="false" :on-change="(e) => uploadImage(e, 'background')" >
   <button class="right-canvas-resize-btn">上传图片</button>
</el-upload>Ï

图片上传支持两种元素,普通图片元素和背景图片元素,所以定义属性type进行区分

type为Image为普通图片, background为背景

背景也是一张图片,使用fabric.Image创建图片元素

因为没有图片服务器所以把上传的文件转换了base64图片,作为fabric图片的链接

需要区分上传类型type, 不同类型的图片走不同逻辑

fileUpload = async (file: File, name: string, type: string) => {
    const src = await this.handler.utils.fileToBase64(file);
    if (src) {
      const image = new Image();
      image.src = src;
      const options: any = {
        name,
        type,
        src,
      };
      await new Promise((resolve) => {
        image.onload = () => {
          options.width = image.width;
          options.height = image.height;
          resolve(true);
        };
      });
      let marterialObject;
      if (type == "background") {
        marterialObject = this.handler.workareaHandler.setBgImage(options);
      } else {
        marterialObject = this.handler.add(options);
      }
      return marterialObject;
    }
 }

背景设置原理

背景图片和普通图片的区别

不能选中,不能移动,不能修改,没有操作控件

给背景元素添加以下属性

        hasControls: false,
        hasBorders: false,
        selectable: false,
        lockMovementX: true,
        lockMovementY: true,
        lockScalingX: true,
        lockScalingY: true

背景需要自适应画布

计算规则如下

获取宽高比例最大值作为元素缩放值,目的是图片元素保持宽高比例不变的情况下覆盖画布
根据缩放值计算图片最新宽高,需要基于画布居中展示
水平居中规则如下,垂直居中同理

 _getBgPosition(bgObject: FabricImage | any) {
    const { width, height } = this.workspace as any;
    let scale = 1;
    if (width > bgObject.width || height > bgObject.height) {
      if (width / bgObject.width > height / bgObject.height) {
        scale = width / bgObject.width;
      } else {
        scale = height / bgObject.height;
      }
    }
    // 居中
    const bgHeight = bgObject.height * scale;
    const bgWidth = bgObject.width * scale;
    const bgLeft = width / 2 - bgWidth / 2;
    const bgTop = height / 2 - bgHeight / 2;
    return {
      left: bgLeft,
      top: bgTop,
      scaleX: scale,
      scaleY: scale,
    }

背景图片支持修改,所以上传时需要把画布中的背景元素给移除

 // 获取背景元素
  getBgObject() {
    return this.handler.canvas.getObjects().find((item: any) => {
      if (item.type == "background") {
        return item;
      }
    });
  }
  
  // 去重, 防止出现多个背景元素
   const bgObject = this.getBgObject();
   if (bgObject && bgObject.src !== src) {
      this.handler.canvas.remove(bgObject);
   }

背景元素永远置底,但比画布高一层,所以先置底再上移一层

      this.canvas.sendToBack(this.bgObject);
      this.canvas.bringForward(this.bgObject);

画布中有元素层级的逻辑,所以当我们选中某个元素的时候需要保持原有层级,但是fabric.js默认是对象在选中时不保持在当前堆栈位置

所以我们需要在初始化画布时指定保留层级

preserveObjectStacking: true

完整代码如下

async setBgImage(options: WorkareaOption) {
    const { src } = options || {};
    const editable = false;
    const option = {
      editable,
      hasControls: editable,
      hasBorders: editable,
      selectable: editable,
      lockMovementX: !editable, 
      lockMovementY: !editable,
      lockScalingX: !editable,
      lockScalingY: !editable,
      hoverCursor: "default",
      name: "背景图片",
      type: "background",
      src,
    };
    // 去重, 防止出现多个背景元素
    const bgObject = this.getBgObject();
    if (bgObject && bgObject.src !== src) {
      this.handler.canvas.remove(bgObject);
    }
    const poiOptions = this._getBgPosition(options);
    const newOptions = Object.assign({}, option, poiOptions);
    this.bgObject = await this.handler.add(newOptions, false);
    if (this.bgObject) {
      this.canvas.add(this.bgObject);
      this.canvas.sendToBack(this.bgObject);
      this.canvas.bringForward(this.bgObject);
    }
    this.canvas.requestRenderAll();
    return this.bgObject;
  }