如何通过z-index对这些canvas元素进行排序？

Question

我想将这些画布合并到顶部有最大z-index的画布上.

<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>

给定任意数量的这样的画布层我想按z-index排序,然后使用drawImage将它们组合成一个图像以便导出它.是否有一个简单的按z-index排序？

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>?
  ]

我究竟做错了什么？

再次编辑 - 哦,没关系.我需要包含一个"返回"声明.GAH!

Answer 1

假设所有canvas元素都有一个指定的样式值来设置它们的z-index,你可以得到一个由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,您可以通过遍历drawImage()每个调用的数组进行迭代.对于正确的z阶渲染,您将首先绘制后面的图像(最低的z-index项),按照上面的排序,这意味着您从数组的开头到结尾绘制.