javascript - ASP.NET 中动态生成的 Canvas 元素

Question

我有多个 Canvas 元素，它们基于 VB 代码动态显示在我的页面上。

这就是我的 aspx 页面的样子:

<canvas id="canvasnhlrc" width="225" height="200" runat="server"></canvas>
<canvas id="canvascwl" width="225" height="200" runat="server"></canvas>
<canvas id="canvashslanguages" width="225" height="200" runat="server"></canvas>

这就是我的 aspx.vb 代码的样子:

canvasnhlrc.Visible = False
canvascwl.Visible = False
canvashslanguages.Visible = False

        If RouteData.Values("mediasubType") IsNot Nothing Then

            Dim qsubMedia As String = RouteData.Values("mediasubType")

            Select Case qsubMedia

                Case "nhlrc"

                    canvasnhlrc.Visible = True

                Case "cwl"

                    canvascwl.Visible = True

                Case "hslanguages"

                    canvashslanguages.Visible = True

            End Select

        End If

所以，如果我的页面是 example.com/nhlrc ，则 Canvas 生成为

<canvas id="ContentPlaceHolder2_canvasnhlrc" width="225" height="200"></canvas>

因此，我将 js 文件格式化如下:

function init() 
{ 
    var w1 = document.getElementById('ContentPlaceHolder2_canvasnhlrc')
    var w1x = w1.getContext('2d');
    w1x.shadowOffsetX = 0;
    w1x.shadowOffsetY = 0;
    w1x.shadowBlur = 20;
    w1x.shadowColor = "rgba(0, 0, 0, 0.75)";
    w1x.fillStyle = 'rgb(218, 233, 246)';
    w1x.beginPath();
    w1x.moveTo(25, 25);
    w1x.lineTo(175, 25);
    w1x.lineTo(175, 50);
    w1x.lineTo(200, 75);
    w1x.lineTo(175, 100);
    w1x.lineTo(175, 175);
    w1x.lineTo(25, 175);
    w1x.closePath();
    w1x.fill();

    var w2 = document.getElementById('ContentPlaceHolder2_canvascwl')
    var w2x = w2.getContext('2d');
    w2x.shadowOffsetX = 0;
    w2x.shadowOffsetY = 0;
    w2x.shadowBlur = 20;
    w2x.shadowColor = "rgba(0, 0, 0, 0.75)";
    w2x.fillStyle = 'rgb(12, 64, 114)';
    w2x.beginPath();
    w2x.moveTo(25, 25);
    w2x.lineTo(175, 25);
    w2x.lineTo(175, 50);
    w2x.lineTo(200, 75);
    w2x.lineTo(175, 100);
    w2x.lineTo(175, 175);
    w2x.lineTo(25, 175);
    w2x.closePath();
    w2x.fill();

    var w3 = document.getElementById('ContentPlaceHolder2_canvashslanguages')
    var w3x = w3.getContext('2d');
    w3x.shadowOffsetX = 0;
    w3x.shadowOffsetY = 0;
    w3x.shadowBlur = 20;
    w3x.shadowColor = "rgba(0, 0, 0, 0.75)";
    w3x.fillStyle = 'rgb(12, 64, 114)';
    w3x.beginPath();
    w3x.moveTo(25, 25);
    w3x.lineTo(175, 25);
    w3x.lineTo(175, 50);
    w3x.lineTo(200, 75);
    w3x.lineTo(175, 100);
    w3x.lineTo(175, 175);
    w3x.lineTo(25, 175);
    w3x.closePath();
    w3x.fill();
}
onload = init;

遗憾的是，我只能让第一个 Canvas 元素(NHLRC)出现，而后续 Canvas ( example.com/cwl 等)不会渲染。代码没有任何问题，因为当我注释掉 JS 代码中的其他两个元素时，它工作正常，所以我错过了什么，做错了什么，或者有什么冲突不允许每个 Canvas 元素在其相应的中渲染页面？

Answer 1

你的 init 函数有太多职责(查找、验证和绘制)* 3 。当其中任何一项职责失败时，即找不到元素，表演就会停止。在您的情况下 w1, w2, w3 将仅在各自的页面中定义，而不是每个页面。

您可以将其简化如下:

function draw(theCanvasEl, theFill) 
{ 
    var w1x = theCanvasEl.getContext('2d');
    w1x.shadowOffsetX = 0;
    w1x.shadowOffsetY = 0;
    w1x.shadowBlur = 20;
    w1x.shadowColor = "rgba(0, 0, 0, 0.75)";
    w1x.fillStyle = theFill;
    w1x.beginPath();
    w1x.moveTo(25, 25);
    w1x.lineTo(175, 25);
    w1x.lineTo(175, 50);
    w1x.lineTo(200, 75);
    w1x.lineTo(175, 100);
    w1x.lineTo(175, 175);
    w1x.lineTo(25, 175);
    w1x.closePath();
    w1x.fill();
}

function init()
{
  var canvases = [['ContentPlaceHolder2_canvasnhlrc', 'rgb(218, 233, 246)'],
     ['ContentPlaceHolder2_canvascwl', 'rgb(12, 64, 114)'],
     ['ContentPlaceHolder2_canvashslanguages', 'rgb(12, 64, 114)']];

   for(var i=0;i<canvases.length;i++)
   { 
       var theCanvas = document.getElementById(canvases[i][0]);
       if (theCanvas!=null) {
         draw(theCanvas, canvases[i][1]);
         break;              
       }
   }
}

onload = init;