我有一个简化的函数，看起来像这样:
function(query) {
myApi.exec('SomeCommand', function(response) {
return response;
});
}
我想让它调用myApi。Exec，返回回调lambda中给出的响应。然而，上面的代码不能工作，只是立即返回。
只是为了一个非常hack的尝试，我尝试了下面的工作，但至少你知道我想要实现什么:
function(query) {
var r;
myApi.exec('SomeCommand', function(response) {
r = response;
});
while (!r) {}
return r;
}
基本上，“node.js/事件驱动”的好方法是什么?我希望我的函数等待，直到回调被调用，然后返回传递给它的值。
当前回答
实现这一点的一种方法是将API调用封装到promise中，然后使用await来等待结果。
// Let's say this is the API function with two callbacks,
// one for success and the other for error.
function apiFunction(query, successCallback, errorCallback) {
if (query == "bad query") {
errorCallback("problem with the query");
}
successCallback("Your query was <" + query + ">");
}
// Next function wraps the above API call into a Promise
// and handles the callbacks with resolve and reject.
function apiFunctionWrapper(query) {
return new Promise((resolve, reject) => {
apiFunction(query,(successResponse) => {
resolve(successResponse);
}, (errorResponse) => {
reject(errorResponse);
});
});
}
// Now you can use await to get the result from the wrapped api function
// and you can use standard try-catch to handle the errors.
async function businessLogic() {
try {
const result = await apiFunctionWrapper("query all users");
console.log(result);
// the next line will fail
const result2 = await apiFunctionWrapper("bad query");
} catch(error) {
console.error("ERROR:" + error);
}
}
// Call the main function.
businessLogic();
输出:
Your query was <query all users>
ERROR:problem with the query
其他回答
使用async和await要容易得多。
router.post('/login',async (req, res, next) => {
i = await queries.checkUser(req.body);
console.log('i: '+JSON.stringify(i));
});
//User Available Check
async function checkUser(request) {
try {
let response = await sql.query('select * from login where email = ?',
[request.email]);
return response[0];
} catch (err) {
console.log(err);
}
}
实现这一点的一种方法是将API调用封装到promise中，然后使用await来等待结果。
// Let's say this is the API function with two callbacks,
// one for success and the other for error.
function apiFunction(query, successCallback, errorCallback) {
if (query == "bad query") {
errorCallback("problem with the query");
}
successCallback("Your query was <" + query + ">");
}
// Next function wraps the above API call into a Promise
// and handles the callbacks with resolve and reject.
function apiFunctionWrapper(query) {
return new Promise((resolve, reject) => {
apiFunction(query,(successResponse) => {
resolve(successResponse);
}, (errorResponse) => {
reject(errorResponse);
});
});
}
// Now you can use await to get the result from the wrapped api function
// and you can use standard try-catch to handle the errors.
async function businessLogic() {
try {
const result = await apiFunctionWrapper("query all users");
console.log(result);
// the next line will fail
const result2 = await apiFunctionWrapper("bad query");
} catch(error) {
console.error("ERROR:" + error);
}
}
// Call the main function.
businessLogic();
输出:
Your query was <query all users>
ERROR:problem with the query
这违背了非阻塞IO的目的——你在不需要阻塞的时候阻塞了它:)
你应该嵌套回调，而不是强迫node.js等待，或者在需要r结果的回调中调用另一个回调。
如果您需要强制阻塞，那么很可能您对体系结构的想法是错误的。
如果你想让它非常简单，没有花哨的库，在执行一些其他代码之前，等待回调函数在node中执行，是这样的:
//initialize a global var to control the callback state
var callbackCount = 0;
//call the function that has a callback
someObj.executeCallback(function () {
callbackCount++;
runOtherCode();
});
someObj2.executeCallback(function () {
callbackCount++;
runOtherCode();
});
//call function that has to wait
continueExec();
function continueExec() {
//here is the trick, wait until var callbackCount is set number of callback functions
if (callbackCount < 2) {
setTimeout(continueExec, 1000);
return;
}
//Finally, do what you need
doSomeThing();
}
“好的node.js /事件驱动”的方法是不要等待。
与node等事件驱动系统一样，函数应该接受一个回调参数，该参数将在计算完成时被调用。调用者不应该等待正常意义上的值被“返回”，而是发送将处理结果值的例程:
function(query, callback) {
myApi.exec('SomeCommand', function(response) {
// other stuff here...
// bla bla..
callback(response); // this will "return" your value to the original caller
});
}
所以你不能这样使用它:
var returnValue = myFunction(query);
但就像这样:
myFunction(query, function(returnValue) {
// use the return value here instead of like a regular (non-evented) return value
});
