fix: 修复H5端measureText报错的Bug askID:84858

src/core/service/api/context/canvas.js

@@ -258,6 +258,18 @@ var methods3 = ['setFillStyle', 'setTextAlign', 'setStrokeStyle', 'setGlobalAlph
   'setTextBaseline', 'setLineDash'
 ]
 
+var tempCanvas
+function getTempCanvas () {
+  if (!tempCanvas) {
+    tempCanvas = document.createElement('canvas')
+  }
+  return tempCanvas
+}
+
+function TextMetrics (width) {
+  this.width = width
+}
+
 export class CanvasContext {
   constructor (id, pageId) {
     this.id = id
@@ -313,8 +325,15 @@ export class CanvasContext {
     }
   }
   // TODO
-  // measureText (text) {
-  // }
+  measureText (text) {
+    if (typeof document === 'object') {
+      var c2d = getTempCanvas().getContext('2d')
+      c2d.font = this.state.font
+      return new TextMetrics(c2d.measureText(text).width || 0)
+    } else {
+      return new TextMetrics(0)
+    }
+  }
   save () {
     this.actions.push({
       method: 'save',