javascript - 如何按 z-index 对这些 Canvas 元素进行排序？

Question

我想将这些 Canvas 合并为一个最大的 z-index 在上面。

<div id="sketchpad_container">
  <canvas id="sketchypad_layer_0" style="z-index: 20; " width="800" height="600">Your browser does not support canvas</canvas>
  <canvas id="sketchypad_layer_1" style="z-index: 10; " width="800" height="600">Your browser does not support canvas</canvas>
  <canvas id="sketchypad_layer_2" style="z-index: 30; " width="800" height="600">Your browser does not support canvas</canvas>
</div>

给定任意数量的此类 Canvas 层，我想按 z-index 排序，然后使用 drawImage 将它们组合成一个图像以便导出。是否有按 z-index 排序的简单方式？

编辑 - 我按照建议尝试了排序功能，但似乎不起作用。

这是我的控制台输出:

$("#sketchpad_container canvas"); //see what's in the collection
//console output gives back (you'll notice z-indices are 40, 20, 10, 30)
[
  <canvas id=​"sketchypad_interactive_layer" class=​"sketchypad_sketch_layer" style=​"z-index:​40" width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​, 
  <canvas id=​"sketchypad_layer_0" class=​"sketchypad_sketch_layer" style=​"z-index:​ 20;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​,
  <canvas class=​"sketchypad_sketch_layer" width=​"800" height=​"600">​,
  <canvas id=​"sketchypad_layer_1" class=​"sketchypad_sketch_layer" style=​"z-index:​ 10;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​,
  <canvas id=​"sketchypad_layer_2" class=​"sketchypad_sketch_layer" style=​"z-index:​ 30;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​
]

 //next try to sort the collection
 $("#sketchpad_container canvas").toArray().sort(function(a,b){a.style.zIndex - b.style.zIndex});
 //but array is still the same order of z-index 40, 20, 10, 30.
 //  I was expecting 40,30,20,10
 [
   <canvas id=​"sketchypad_interactive_layer" class=​"sketchypad_sketch_layer" style=​"z-index:​40" width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​,
   <canvas id=​"sketchypad_layer_0" class=​"sketchypad_sketch_layer" style=​"z-index:​ 20;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​,
   <canvas class=​"sketchypad_sketch_layer" width=​"800" height=​"600">​,
   <canvas id=​"sketchypad_layer_1" class=​"sketchypad_sketch_layer" style=​"z-index:​ 10;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​,
   <canvas id=​"sketchypad_layer_2" class=​"sketchypad_sketch_layer" style=​"z-index:​ 30;​ " width=​"800" height=​"600">​Your browser does not support canvas​</canvas>​
  ]

我做错了什么？

再次编辑——哦，没关系。我需要包括一个“返回”声明。啊!

Answer 1

假设您所有的 Canvas 元素都有一个指定的样式值来设置它们的 z-index，您可以获得一个按 z-index 排序的所有 Canvas 元素的数组，如下所示:

var items = $("#sketchpad_container canvas").toArray();
items.sort(function(a, b) {
    return(Number(a.style.zIndex) - Number(b.style.zIndex));
});

然后您大概可以遍历该数组，在每个数组上调用 drawImage()。为了正确的 z 顺序渲染，您需要首先绘制背面图像(最低 z-index 项目)，按照上面的排序，这意味着您从数组的开头绘制到结尾。