接上篇的业务，实现了基础的画图，选中与拖动的效果，还有点细节问题，多个元素重叠在一起，选中的不能在最上面显示，以及最后绘制的元素要能覆盖前面绘制的，
优化如下

优化后效果

• 在后面绘制的矩形要能覆盖前面的,这个比较简单，改变每个div的z-index就可以
  
• 代码调整

drawRect() {
    const div = document.createElement("div");
    div.className = "draw-rect";
    div.style = `width: ${this.disX}px;height: ${
      this.disY
    }px;border:1px solid #ccc;position: absolute;left: ${this.startX}px;top: ${
      this.startY
    }px;z-index:${this.zIndex++};background:greenyellow`;
    div.appendChild(this.addDeleteBtn());
    document.body.appendChild(div);
    this.allRect.push(div);
    this.setCurrentBorderColor(div);
  }

• 多个图形重叠在一起的时候，选择的元素在最顶层,也比较简单

changeBorderColor(target) {
  this.nowMoveTarget = target;
  this.setCurrentBorderColor(target);
  // 改变鼠标指针
  target.style.cursor = "move";
  target.style.zIndex = ++this.zIndex;
}

• 完整代码

class Draw {
  constructor() {
    this.x = 0;
    this.y = 0;
    this.disX = 0;
    this.disY = 0;
    this.startX = 0;
    this.startY = 0;
    this.offsetX = 0;
    this.offsetY = 0;
    this.nowMoveTarget = null;
    this.mouseDown = this.mouseDown.bind(this);
    this.mouseMove = this.mouseMove.bind(this);
    this.mouseUp = this.mouseUp.bind(this);
    this.handleRectMove = this.handleRectMove.bind(this);
    this.handleRectUp = this.handleRectUp.bind(this);
    this.zIndex = 0;
    this.allRect = [];
    this.shadowBox = document.createElement("div");
    this.init();
  }

  init() {
    this.draw();
  }
  draw() {
    document.addEventListener("mousedown", this.mouseDown, false);
  }
  mouseDown(e) {
    console.log("🚀 ~ Draw ~ mouseDown ~ e:", e);
    if (e.target.className == "delete-btn") return;
    // 校验点击的是不是画的的元素
    if (e.target.className == "draw-rect") {
      // 改变边框颜色
      this.changeBorderColor(e.target);
      this.handleRectDown(e);
      return false;
    } else {
      this.x = e.clientX;
      this.y = e.clientY;
      document.addEventListener("mousemove", this.mouseMove);
      document.addEventListener("mouseup", this.mouseUp);
    }
  }
  mouseMove(e) {
    // 不要选中文字
    e.preventDefault();
    // this.disX = e.clientX - this.x
    // this.disY = e.clientY - this.y
    // const startX = e.clientX < this.x ? e.clientX : this.x
    // const startY = e.clientY < this.y ? e.clientY : this.y
    // this.disX = e.clientX > this.x ? e.clientX - this.x : this.x - e.clientX
    // this.disY = e.clientY > this.y ? e.clientY - this.y : this.y - e.clientY
    this.startX = Math.min(e.clientX, this.x);
    this.startY = Math.min(e.clientY, this.y);
    this.disX = Math.abs(e.clientX - this.x);
    this.disY = Math.abs(e.clientY - this.y);

    // console.log('🚀 ~ Draw ~ mouseMove ~ e:', this.disX, this.disY)
    this.drawShadeRect();
  }
  mouseUp(e) {
    document.removeEventListener("mousemove", this.mouseMove);
    document.removeEventListener("mouseup", this.mouseUp);
    this.drawRect();
    this.shadowBox && this.shadowBox.remove();
  }
  drawShadeRect(startX, startY) {
    this.shadowBox.style = `width: ${this.disX}px;height: ${
      this.disY
    }px;border:1px solid red;background:rgba(94,243,243,.5);position: absolute;left: ${
      this.startX
    }px;top: ${this.startY}px;z-index:${this.zIndex++}`;
    document.body.appendChild(this.shadowBox);
  }
  drawRect() {
    const div = document.createElement("div");
    div.className = "draw-rect";
    div.style = `width: ${this.disX}px;height: ${
      this.disY
    }px;border:1px solid #ccc;position: absolute;left: ${this.startX}px;top: ${
      this.startY
    }px;z-index:${this.zIndex++};background:greenyellow`;
    div.appendChild(this.addDeleteBtn());
    document.body.appendChild(div);
    this.allRect.push(div);
    this.setCurrentBorderColor(div);
  }
  handleRectDown(e) {
    this.startX = e.clientX;
    this.startY = e.clientY;
    this.offsetX = e.clientX - this.nowMoveTarget.offsetLeft;
    this.offsetY = e.clientY - this.nowMoveTarget.offsetTop;
    document.addEventListener("mousemove", this.handleRectMove);
    document.addEventListener("mouseup", this.handleRectUp);
  }
  handleRectMove(e) {
    this.disX = e.clientX - this.offsetX;
    this.disY = e.clientY - this.offsetY;
    this.nowMoveTarget.style.left = `${this.disX}px`;
    this.nowMoveTarget.style.top = `${this.disY}px`;
  }
  handleRectUp() {
    document.removeEventListener("mousemove", this.handleRectMove);
    document.removeEventListener("mouseup", this.handleRectUp);
  }
  changeBorderColor(target) {
    this.nowMoveTarget = target;
    this.setCurrentBorderColor(target);
    // 改变鼠标指针
    target.style.cursor = "move";
    target.style.zIndex = ++this.zIndex;
  }
  // 动态添加一个删除按钮
  addDeleteBtn() {
    const btn = document.createElement("button");
    btn.innerHTML = "删除";
    btn.className = "delete-btn";
    btn.style = `position: absolute;right: 0px;bottom: -25px`;
    // 绑定事件
    btn.onclick = function () {
      this.parentElement.remove();
    };
    return btn;
  }

  setCurrentBorderColor(target) {
    // 改变边框颜色,当前选中的高亮
    this.allRect.forEach((item) => {
      if (item != target) {
        item.style.border = "1px solid #ccc";
      }
    });
    target.style.border = "1px solid blue";
  }
}

const d = new Draw();
d.init();

这样就对我们的画图的层级问题优化完成了。

总结

• 层级问题多半是css的z-index问题，需要我们手动去更新
• 还需要实现拖拉边框实现调整矩形大小的业务