給excanvas添加fillText方法

excanvas使得IE能通過VML支持Canvas標籤,但是在一些細節方面還是不盡人意.
比如
1.動態生成的Canvas對象將不支持getContext方法,
2.drawImage方法不能用Canvas對象作爲第一個參數,
3.不支持fillText等方法.

本文針對以上三個問題給出改進方案.
1.要使IE中的Canvas對象支持getContext等方法需要調用excanvas中的G_vmlCanvasManager_.initElement方法,由於最後的

G_vmlCanvasManager = G_vmlCanvasManager_;

使得G_vmlCanvasManager的作用域是當前window,所以在需要初始化Canvas的時候可以採用如下的方法:

var cav = document.createElement("canvas");
if(!cav.getContext) {
  G_vmlCanvasManager.initElement(cav);
}
var ctx = cav.getContext("2d");
...

2.excanvas中的drawImage方法只接受Image對象爲第一個參數,爲了使其支持Canvas對象的繪製,可修改如下代碼(在excanvas.js中檢索到"contextPrototype.drawImage"所在行):

contextPrototype.drawImage = function(image, var_args) {
    if (image.getContext) { // draw canvas
      this.element_.innerHTML += image.getContext("2d").element_.innerHTML;
    }
  var dx, dy, dw, dh, sx, sy, sw, sh;
.

3.fillText,measureText也已成爲CanvasRenderingContext2D對象的標準方法,爲了使IE也能支持,在excanvas.js中添加以下代碼:

contextPrototype.measureText = function(textToDraw) {
  var hiddenSpan = document.createElement('span');
  hiddenSpan.style.font = this.font;
  hiddenSpan.innerHTML = textToDraw;
  var bodyNode = document.getElementsByTagName("body")[0];
  bodyNode.appendChild(hiddenSpan);
  var width = hiddenSpan.offsetWidth;
  bodyNode.removeChild(hiddenSpan);
  return {"width" : width + 1};
}

contextPrototype.fillText = function(textToDraw, x, y) {
  var vmlStr = [];
  var textHeightStr = this.font.split("px")[0].replace(/(^\s+)|(\s+$)/g, "");
  var textHeight = /^\d+$/.test(textHeightStr) ? parseInt(textHeightStr) : 0;
  vmlStr.push('<g_vml_:shape style="font:' + this.font + ';',
                  ' color:' + this.fillStyle + ';',
                  ' position:absolute;',
                  ' left:' + x + 'px;',
                  ' top:' + (y - textHeight) + 'px;',
                  ' width:' + this.measureText(textToDraw).width + 'px;',
                  ' height:' + textHeight + 'px;"',
                  ' ><g_vml_:textbox inset="0,0,0,0">' + textToDraw,
                  ' </g_vml_:textbox>',
                  '</g_vml_:shape>');

  this.element_.insertAdjacentHTML('BeforeEnd', vmlStr.join(''));
}

其中measureText是通過在頁面上添加一個隱藏的臨時文字容器並獲得其寬度來實現.
另外在屬性聲明的地方要添加默認字體:

    // Canvas context properties
    this.strokeStyle = '#000';
    this.fillStyle = '#000';
    this.font = '12px sans-serif';
...

如果還有其他的什麼需求或是想法 歡迎與我聯繫 [email protected]

轉自:http://ck-2036.iteye.com/blog/800897