jQuery on()方法绑定动态元素的点击事件无响应的解

$('#check_all').on('click' , function(){
alert(1);
});
$("#yujinlist").append(html);
count++; 
} 

<div class="row">\
<div class="col-xs-12">\
<div class="control-group">\
<label class="control-label bolder blue">选择镇街</label>\
<div class="row">\
<div class="checkbox col-xs-1">\
<label>\
<input type="checkbox" class="checkbox" id="check_all" />\
<span class="lbl">全区</span>\
</label>\
</div>\
<div id="check_item">\
<div class="checkbox col-xs-1 ">\
<label>\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">西南街道</span>\
</label>\
</div>\
<div class="checkbox col-xs-1 ">\
<label>\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">云东海街道</span>\
</label>\
</div>\
<div class="checkbox col-xs-1">\
<label>\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">白坭镇</span>\
</label>\
</div>\
<div class="checkbox col-xs-1">\
<label class="block">\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">乐平镇</span>\
</label>\
</div>\
<div class="checkbox col-xs-1">\
<label class="block">\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">大塘镇</span>\
</label>\
</div>\
<div class="checkbox col-xs-1">\
<label class="block">\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">芦苞镇</span>\
</label>\
</div>\
<div class="checkbox col-xs-1">\
<label class="block">\
<input name="towm'+count+'" type="checkbox" class="checkbox" />\
<span class="lbl">南山镇</span>\
</label>\
</div>\
</div>\
</div>\
</div>\
</div>\
</div>\
<hr />'; 
$('#check_all').on('click' , function(){
var that = this;
$('#check_item').find('input:checkbox')
.each(function(){
alert(2);
this.checked = that.checked;
$(this).closest('.col-xs-1').toggleClass('selected');
});
});

<div id="test">
<div class="evt">evt1</div>
</div>

<script>
// 先绑定事件再添加div
$('#test .evt').on('click', function() {alert($(this).text())});
$('#test').append('<div class="evt">evt2</div>');
</script>

<script>
$('body').on('click', '#test .evt', function() {alert($(this).text())});
$('#test').append('<div class="evt">evt2</div>');
</script>