如何在循环中的回调中引用`i`的正确值？ [英] How to reference right value of `i` in a callback in a loop?

问题描述

我有这段代码：

for (var i = 0; i < result.length; i++) {
    // call a function that open a new "thread"
    myObject.geocode({ param1: "param" }, function(results, status) {
        alert(result.title[i]);
    });                                             
}

.geocode 功能（不是我的，所以我无法编辑）打开一个新的执行线程。

The .geocode function (that is not mine, so I can't edit) open a new "thread" of execution.

当我尝试在每一步上打印标题时，我总是得到 i 的最后一个可能值。

When I try to print title on each step, I get always the last possible value of i.

如何为每次迭代保留对 i 的正确值的引用？

How can I keep a reference to the right value of i for each iteration?

推荐答案

你可以在循环中创建一个闭包;

You can create a closure within the loop;

for (var i = 0; i < result.length; i++) {
    // call a function that open a new "thread"
    (function (i) {
        myObject.geocode({ param1: "param" }, function(results, status) {
            alert(result.title[i]);
        });
    }(i));                                             
}

所以我们在这里创建一个函数;

So here we're creating a function;

(function (i) {
    myObject.geocode({ param1: "param" }, function(results, status) {
        alert(result.title[i]);
    });
});

...接受一个名为的参数i ，并启动地理编码请求。通过在函数表达式声明的末尾添加（i），我们立即运行该函数并将其传递给 i的当前值。

... which accepts one parameter named i, and launches the geocode request. By adding the (i) to the end of the declaration of a function expression, we run the function straight away and pass it the current value of i.

(function (i) {
    myObject.geocode({ param1: "param" }, function(results, status) {
        alert(result.title[i]);
    });
}(i));

变量 i 已经存在于比闭包更高的范围，因为 i 的本地声明会覆盖它。我们将传递给闭包的变量，或者闭包调用变量的名称可能不同;

It doesn't matter that a variable i already exists at a higher scope than the closure, because the local declaration of i overrides it. Either the variable we pass to the closure, or the name the closure calls the variable could be different;

(function (anotherVariable) {
    myObject.geocode({ param1: "param" }, function(results, status) {
        alert(result.title[anotherVariable]);
    });
}(aVariable));

另外你也可以将逻辑提取到另一个函数（我更喜欢它） ，但它不那么很酷）：

Alternately you could also extract the logic to another function (I prefer it, but it's less cool):

function geocode(i) {
   myObject.geocode({ param1: "param" }, function(results, status) {
       alert(result.title[i]);
   });
}

for (var i = 0; i < result.length; i++) {
    geocode(i);                                            
}

问题在于相同 i 回调函数使用的变量;正如您所发现的那样，在回调执行时已经移动了。上述两种解决方案都为每次迭代创建了另一个变量，这就是回调操作。

The problem is down to the same i variable being used by the callback functions; which, as you've discovered, has moved on by the time the callback executes. Both of the above solutions creates another variable for each iteration, and it is this that the callback operates on.

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