了解异步code通俗地说 [英] Understanding Asynchronous Code in Layman's terms
问题描述
我了解异步岬基本的事情：事情不按顺序执行。我知道有一些事很强大......据说。但对我的生活，我不能换我的头周围的code。让我们来看看异步Node.js的code，我已经写了...但并没有真正得到的。
I understand the basic thing about asynchronous-ness: things don't execute sequentially. And I understand there is something very powerful about that... allegedly. But for the life of me I can't wrap my head around the code. Let's take a look at async Node.JS code that I HAVE WRITTEN...but don't truly get.
function newuser(response, postData) {
console.log("Request handler 'newuser' was called.");
var body = '<html>' +
'<head>' +
'<meta http-equiv="Content-Type" content="text/html; ' +
'charset=UTF-8" />' +
'</head>' +
'<body>' +
'<form action=" /thanks" method="post">' +
'<h1> First Name </h1>' +
'<textarea name="text" rows="1" cols="20"></textarea>' +
'<h1> Last Name </h1>' +
'<textarea name="text" rows="1" cols="20"></textarea>' +
'<h1> Email </h1>' +
'<textarea name="text" rows="1" cols="20"></textarea>' +
'<input type="submit" value="Submit text" />' +
'</body>' +
'</html>';
response.writeHead(200, { "Content-Type": "text/html" });
response.write(body);
response.end();
}
在哪里响应来自一遍吗？发布数据？为什么我不能在这个回调定义一个变量，然后用它的回调之外吗？有没有一种方法能有几件事是连续的，则该程序异步的休息吗？
Where did response come from again? postData? Why can't I define a variable in this "callback" and then use it outside of the callback? Is there a way to have a few things be sequential then the rest of the program async?
推荐答案
我不知道正在使用此功能在哪里，但回调的一点是，要将其传递到运行异步一些功能;它存储回调走，当该功能正在与不管它需要做的，它会的呼叫的你提供必要的参数回调完成。一个例子由前到后的可能是最好的。
I'm not sure where this function is being used, but the point of callbacks is that you pass them into some function that runs asynchronously; it stores your callback away, and when that function is done with whatever it needs to do, it will call your callback with the necessary parameters. An example from front-to-back is probably best.
假设我们有一个框架，并在它存在着很长一段时间运行时，从数据库中提取某些数据的操作。
Imagine we have a framework, and in it there is an operation that runs for a long time, fetching some data from the database.
function getStuffFromDatabase() {
// this takes a long time
};
由于我们不希望它同步运行，我们将允许用户在回调中传递。
Since we don't want it to run synchronously, we'll allow the user to pass in a callback.
function getStuffFromDatabase(callback) {
// this takes a long time
};
我们会模拟花费很长的时间跟的setTimeout 通话;我们还将pretend我们从数据库中的一些数据，但我们会只是很难code的字符串值。
We'll simulate taking a long time with a call to setTimeout; we'll also pretend we got some data from the database, but we'll just hardcode a string value.
function getStuffFromDatabase(callback) {
setTimeout(function() {
var results = "database data";
}, 5000);
};
最后，一旦我们拥有的数据，我们的呼叫的该框架的功能的用户给我们的回调。
Finally, once we have the data, we'll call the callback given to us by the user of the framework's function.
function getStuffFromDatabase(callback) {
setTimeout(function() {
var results = "database data";
callback(results);
}, 5000);
};
由于框架的用户，你会做这样的事情使用的功能：
As a user of the framework, you'd do something like this to use the function:
getStuffFromDatabase(function(data) {
console.log("The database data is " + data);
});
所以，你可以看到数据（等同于响应和 POSTDATA 在你的例子）来自您传递回调的到函数的;它给出了数据，你什么时候知道的数据应该是什么。
So, as you can see data (same as response and postData in your example) came from the function that you pass your callback into; it gives that data to you when it knows what that data should be.
您不能设置在回调的值，并用它回调以外的原因是因为回调本身不会发生，直到后来在时间。
The reason you can't set a value in your callback and use it outside the callback is because the callback itself doesn't happen until later in time.
// executed immediately executed sometime in the future
// | | by getStuffFromDatabase
// v v
getStuffFromDatabase(function(data) {
var results = data; // <- this isn't available until sometime in the future!
});
console.log(results); // <- executed immediately
在该的console.log 运行时，的 VAR结果赋值还没有发生！
When the console.log runs, the assignment of var results hasn't happened yet!
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