倾城之链

前端时间为移动游戏做一个网页活动需求(9宫格的刮奖),遇到一个很诡异的问题:Android端OK,就是在Ios设备上,点击非第一块区域,显示却是第一块区域被刮开咯,查看后端返回数据缺失OK的【前端采用backbone写法,遍历返回数据用的是underscore.js中的each】。经过一番查证,原来如此。

参考水木社区这里的叙述,知晓原来这个属于:IOS 8 Safari JIT bug 影响 jQuery 和 underscore,致使: $.each/.each,看到 .length 后,会把一个 object 当作数组来遍历。这个bug 会造成莫名其妙的结果。而我那里的结果就是:object数据对象,会在.each遍历之后被乱序了。然后就造成以上所描述的问题。

于此,当时的解决办法是:将后端返回的object对象数据,做了转数组处理后,set给定义的模板,交付给_.each遍历就没有问题了。

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var arrList = [];
for(var i=0; i<9; i++){
truearrList.push(data.datainit.list[i]);
};
this.model.set('arrList',arrList);

好吧,可以看到这里for循环用的是这种传统的方式(还有一个魔数9,额の)。一开始,这里用for..in方式进行遍历的,代码如下:

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for (k in data.datainit.list){
arrList.push(data.datainit.list[k]);
}

惊奇的结果是:这样搞,不行的;好奇怪的样子(得更努力的深入Js才行哇~)?在chrome的console下比较两者转化的数组结果一模一样。然后以ipod机器(系统版本Ios8.4.1)alert数组的长度,得到的结果是一样的,都是9。额,奇怪啊。就在即将放弃的时候alert了下转化数组结果,发现采用for..in 方式遍历object对象数据,在ios机器上(没试过所有,拿了ipad,iphone相对高版本系统)会打乱原有的顺序(原来以为是以value从大到小呢,后来多次尝试,不是如此,也未发现规律);可见,Ios8 Safari上,以for..in方式遍历object对象会造成乱序;但是为何会造成这样子呢?按照Jquery方面对此问题的修复时说法,此时该Object对象the highest property is 10,且是从data.datainit.list[1]开始轮询的~😄。

JQuery方面在 https://github.com/jquery/jquery/issues/2145 有过对此问题的叙述:

There is a timing bug in iOS8 that causes mobile Safari to incorrectly report a ‘length’ on objects that don’t have one.

To the best of my knowledge, this happens on iOS8+, possibly only on 64-bit systems. The bug is triggered for objects that have only numeric properties. For example:

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foo = { 1: 'a', 2: 'b', 3: 'c' }

In this case, if you query foo.length then mobile Safari will sometimes return 4 (the highest property + 1).

This causes functions like $.each() to treat objects such as foo above as arrays instead of objects, and when it tries to iterate them as such it fails since there is no foo[0].

The problem can be fixed in the function isArrayLike(). Instead of just checking for

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typeof length === "number"

you also need to check for

obj.hasOwnProperty(‘length’)
The latter check is immune to the iOS bug.

I realize this is a fix just for one browser, but it’s a browser with a very large user base.


当然:jQuery 1.11.3 和 underscore 1.8.3修正了这个问题。事实上,对于此问题:jquery方面在Github这里$.each fails intermittently on iOS due to Safari bug#2145有过相关的论述;有兴趣一探究竟,可以点进去看下。

来对比下underscore 1.7.0和underscore1.8.3中_.each的代码差异,来窥测下其修正手法:

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// The cornerstone, an `each` implementation, aka `forEach`. Handles raw objects in addition to array-likes. Treats all sparse array-likes as if they were dense. _.each = _.forEach = function(obj, iteratee, context) {
if (obj == null) return obj;
iteratee = createCallback(iteratee, context);
var i, length = obj.length;
if (length === +length) {
for (i = 0; i < length; i++) {
iteratee(obj[i], i, obj);
}
} else {
var keys = _.keys(obj);
for (i = 0, length = keys.length; i < length; i++) {
iteratee(obj[keys[i]], keys[i], obj);
}
}
return obj; };

在underscore1.8.3中,做了如下改进,并附有相应注释:

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// Helper for collection methods to determine whether a collection
// should be iterated as an array or as an object
// Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
// Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
var getLength = property('length');
var isArrayLike = function(collection) {
var length = getLength(collection);
return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
};
var property = function(key){
return function(obj){
return obj == null ? void 0 : obj[key];
}
}
// Collection Functions
// --------------------
// The cornerstone, an `each` implementation, aka `forEach`.
// Handles raw objects in addition to array-likes. Treats all
// sparse array-likes as if they were dense.
_.each = _.forEach = function(obj, iteratee, context) {
iteratee = optimizeCb(iteratee, context);
var i, length;
if (isArrayLike(obj)) {
for (i = 0, length = obj.length; i < length; i++) {
iteratee(obj[i], i, obj);
}
} else {
var keys = _.keys(obj);
for (i = 0, length = keys.length; i < length; i++) {
iteratee(obj[keys[i]], keys[i], obj);
}
}
return obj;
};

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