画布绘制文字，垂直居中

Canvas(this.context)  
  .width('100%')  
  .height('100%')  
  .backgroundColor('#ffff00')  
  .onReady(() =>{  
    this.context.fillStyle = '#0000ff'  
    this.context.font = '18vp'  
    this.context.fillStyle = Color.Black  
    this.context.textAlign = 'center'  
    this.context.textBaseline = 'middle'  
    let str = (-10.3456).toFixed(2)  
    let metrics = this.context.measureText(str)  
    this.context.fillStyle = Color.Green  
    //这里把文字显示的区域绘制出来  
    this.context.fillRect(10, (this.context.height - metrics.height)/2, metrics.width, metrics.height)  
    this.context.fillStyle = Color.Black  
    //这里把文字绘制出来  
    this.context.fillText(str,10+(metrics.width/2), (this.context.height)/2)  
  })
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