你想反馈一个 Bug 还是 提一个需求？

Bug

如果是 Bug：

* Bug 表现是什么？预期表现是什么？

iOS 真机中调试面板中报错

undefined is not an object (evaluating 'e.canvasId')

* 如何复现？

在 Component 中使用 canvas 绘图，移除组件后报错。

* 提供一个最简复现 Demo

// canvas-id-error.js /**  * [@since](/user/since) 20180321 16:33  * [@author](/user/author) vivaxy  */   Page({   data: {     show: true,   },   handleToggle() {     this.setData({ show: !this.data.show });   }, });

// canvas-id-error.json {   "navigationBarTitleText": "canvasId error",   "usingComponents": {     "custom-component": "./custom-component/custom-component"   } }

<!-- canvas-id-error.wxml --> <block wx:if="{{show}}">   <custom-component></custom-component> </block> <button bindtap="handleToggle">Toggle canvas</button>

/* canvas-id-error.wxss */ custom-component {   width: 750rpx;   height: 750rpx;   display: block; }

// custom-component/custom-component.js /**  * [@since](/user/since) 20180321 16:35  * [@author](/user/author) vivaxy  */ Component({   attached() {     this.canvasContext = wx.createCanvasContext('my-canvas-id', this);     this._render();   },   detached() {     this.canvasContext = null;   },   methods: {     _render() {       if (!this.canvasContext) {         return;       }       const ctx = this.canvasContext;       ctx.moveTo(10, 10);       ctx.lineTo(100, 10);       ctx.lineTo(100, 100);       ctx.fill();       ctx.draw();     },   }, });

// custom-component/custom-component.json {   "component": true }

<!-- custom-component/custom-component.wxml --> <canvas canvas-id="my-canvas-id"></canvas>

/* custom-component/custom-component.wxss */ canvas {   width: 750rpx;   height: 750rpx;   display: block; }