[英]Can someone explain the following code for me?
I'm trying to understand Promise. 我试图了解Promise。 But here I'm confused.
但是在这里我很困惑。 I want to create a test function that will print 3000 after 3 second, then print 2000 after 2 second, then print 1000 after 1 second.
我想创建一个测试功能,将在3秒后打印3000,然后在2秒后打印2000,然后在1秒后打印1000。 Here is my code:
这是我的代码:
'use strict';
var Q = require('q');
function delayConsole(timeOut) {
var defer = Q.defer();
setTimeout(function(){
console.log(timeOut);
defer.resolve(2000);
},timeOut);
return defer.promise;
}
// This works
delayConsole(3000).then(function(){
return delayConsole(2000);
}).then(function(){
return delayConsole(1000);
});
// This doesn't work. Why?
delayConsole(3000).then(delayConsole(2000)).then(delayConsole(1000));
There, you call the function delayConsole
immediately : 在那里,您立即调用函数
delayConsole
:
.then(delayConsole(2000))
That is : you don't pass the function but the result of the function call, you don't wait for the promises to be chained. 那就是:您不传递函数,而是函数调用的结果,您不必等待promise被链接。
When you do 当你做
then(function(){
return delayConsole(2000);
})
then you pass a function, not the result of that function call. 那么您传递一个函数,而不是该函数调用的结果。 The function can be called when the previous element in the promise chain is solved.
当解决了promise链中的前一个元素时,可以调用该函数。
I just thought I'd share that you can make this construction work which is sometimes easier to use: 我只是想与您分享,您可以使此构造工作有时更易于使用:
promise.then(delayConsole(3000)).then(delayConsole(2000)).then(delayConsole(1000));
by changing delayConsole()
to this: 通过将
delayConsole()
更改为此:
function delayConsole(timeOut) {
return function() {
var defer = Q.defer();
setTimeout(function(){
console.log(timeOut);
defer.resolve(2000);
},timeOut);
return defer.promise;
}
}
This way, calling delayConsole()
just captures the timeout argument and returns a function that can be called later by the promise .then
handler. 这样,调用
delayConsole()
只是捕获超时参数并返回一个函数,以后可以由promise .then
处理程序调用。 So, you are still passing a function reference to the .then()
handler which lets the promise engine call the internal function sometime later rather than execute it now. 因此,您仍将函数引用传递给
.then()
处理函数,该处理函数允许promise引擎稍后再调用内部函数,而不是立即执行它。
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