javascript 调用chrome.tabs.query后，结果不可用

After calling chrome.tabs.query, the results are not available

提问by samy

I'm creating (learning) an extension for Google Chrome.

我正在为 Google Chrome 创建（学习）一个扩展程序。

To debug some code, I inserted console.log(), as follows:

为了调试一些代码，我插入了console.log()，如下所示：

var fourmTabs = new Array();
chrome.tabs.query({}, function (tabs) {
    for (var i = 0; i < tabs.length; i++) {
        fourmTabs[i] = tabs[i];
    }
});
for (var i = 0; i < fourmTabs.length; i++) {
    if (fourmTabs[i] != null)
        window.console.log(fourmTabs[i].url);
    else {
        window.console.log("??" + i);
    }
}

It's very simple code: get all tabs info into an array of my own, and print some things.

这是非常简单的代码：将所有选项卡信息放入我自己的数组中，然后打印一些内容。

To check whether the code works as it should, I run the code. Here comes the problem:

为了检查代码是否正常工作，我运行了代码。问题来了：

• When I use breakpoints (via the Developer tools), the code runs fine.
• Without breakpoints, nothing is printed.

• 当我使用断点（通过开发人员工具）时，代码运行良好。
• 如果没有断点，则不会打印任何内容。

Any idea why?

知道为什么吗？

回答by Rob W

Your problem can be simplified to:

您的问题可以简化为：

/*1.*/ var fourmTabs = [];
/*2.*/ chrome.tabs.query({}, function(tabs) {
/*3.*/     fourmTabs[0] = tabs[0];
/*4.*/ });
/*5.*/ console.log(fourmTabs[0]);

You expect that the fourmTabsarray is updated (by line 3) when line 5 is reached.
That is wrong, because the chrome.tabs.querymethod is asynchronous.

您希望在fourmTabs到达第 5 行时更新数组（按第 3 行）。
这是错误的，因为该chrome.tabs.query方法是异步的。

---

---

In an attempt to make you understand the significance of the asynchronous aspect, I show a code snippet with the same structure as your code anda story.

为了让您了解异步方面的重要性，我展示了一个与您的代码具有相同结构的代码片段和一个故事。

/*1.*/ var rope = null;
/*2.*/ requestRope(function(receivedRope) {
/*3.*/     rope = receivedRope;
/*4.*/ });
/*5.*/ grab(rope);

• At line 1, the presence of a rope is announced.
• At lines 2-4, a callback functionis created, which ought to be called by the requestRopefunction.
• At line 5, you're going to grab the rope via the grabfunction.

• 在第 1 行，宣布绳子的存在。
• 在第 2-4 行，创建了一个回调函数，它应该被requestRope函数调用。
• 在第 5 行，您将通过grab函数抓取绳索。

When requestRopeis implemented synchronously, there's no problem:
  You: "Hi, I want a rope. Please throw the rope^{"call the callback function"}when you've got one."
  She: "Sure." throws rope
  You: Jumps and grabs rope- You manage to get at the other side, alive.

当requestRope实现同步，没有任何问题：
  你：“您好，我想绳子请。抛绳^{‘调用回调函数’}，当你有一个。”
  她：“当然。” 扔绳子
  你：跳跃并抓住绳子- 你设法到达另一边，活着。

When requestRopeis implemented asynchronously, you may have a problem if you treat it as synchronous:
  You: "Please throw a rope at me."
  She: "Sure. Let's have a look..."
  You: Jumps and attempts to grab ropeBecause there's no rope, you fall and die.
  She: Throws rope_{Too late, of course.}

当requestRope是异步实现的，如果你把它当成同步的，你可能会遇到一个问题：
  你：“请向我扔一根绳子。”
  她：“当然。让我们看看……”
  你：跳跃并试图抓住绳子因为没有绳子，你跌倒并死去。
  她：扔绳子_{当然太晚了。}

---

---

Now you've seen the difference between an asynchronously and synchronously implemented function, let's solve your original question:

现在您已经看到了异步和同步实现的函数之间的区别，让我们解决您最初的问题：

var fourmTabs = new Array();
chrome.tabs.query({}, function (tabs) {
    for (var i = 0; i < tabs.length; i++) {
        fourmTabs[i] = tabs[i];
    }
    // Moved code inside the callback handler
    for (var i = 0; i < fourmTabs.length; i++) {
        if (fourmTabs[i] != null)
           window.console.log(fourmTabs[i].url);
        else {
            window.console.log("??" + i);
        }
    }
});
// <moved code inside callback function of chrome.tabs.query>

With breakpoints, your code works, because by the time that the second part of the code is reached, the callback has already been called.

使用断点，您的代码可以工作，因为在到达代码的第二部分时，回调已经被调用。