Wait and don't execute a function until a light is set to green

问题

I understand that we could use .then to make sure the order of asynchronous calls:

return doTask1()
    .then(function () {
        return doTask2()
    })

But sometimes it will be convenient to have a light and to be able to say: wait and don't execute task2 until a light is set to GREEN; the light is a variable initially set to RED, and can be set to GREEN by task1 or other functions.

Does anyone know if it is possible to accomplish this?

Edit 1: I think being able to express this is particularly useful when we need several tasks to be ended to set the light green, and we don't know/mind the order of these tasks. .then cannot do this easily, because we don't know the order of these tasks.

Edit 2: Regarding my light, I had asked a more specific question. In one word, it is the message another application B sends by postMessage that we are waiting for. At the moment, I have written the following code (which is in a resolve), which works more or less (I have not tried if making only ONE function with their common part will work).

task1: ['codeService', '$window', '$q', function (codeService, $window, $q) {
    return codeService.task1().then(function () { // task1 sends a request to another application B by postMessage
        var deferred = $q.defer();
        $window.addEventListener("message", function (event) {
            if (event.data.req === "task1ReturnFromB") deferred.resolve(event.data)
        }, { once: true });
        return deferred.promise
    })
}],
task2: ['codeService', 'task1', '$window', '$q', function(codeService, task1, $window, $q) {
    return codeService.task2().then(function () { // task2 sends a request to Application B by postMessage
        var deferred = $q.defer();
        $window.addEventListener("message", function (event) {
            if (event.data.req === "task2ReturnFromB") deferred.resolve(event.data) 
        }, { once: true });
        return deferred.promise
    })
}]

So in this specific case, postMessage sent by Application B triggers the event. In a more general case, I guess we could probably trigger an event by eg, dispatchEvent, in one application?

回答1:

You haven't told us anything about the API for your light so I can only guess what it's like. Presuming it's an event driven model, you can convert it to a promise, and do this:

function waitForGreenLight() {
    return new Promise(function (resolve) {
        $window.addEventListener("message", function (event) {
           if (event.data.req === "task2ReturnFromB") {
               resolve(event.data) 
           }
        }, { once: true });
    });
}

return doTask1()
    .then(waitForGreenLight)
    .then(doTask2);

If your light doesn't provide events, you could have a waitForGreenLight that periodically polls it until it's green. The code to use waitForGreenLight would remain the same.

function waitForGreenLight() {
    return new Promise(function (resolve) {
        var handle = setInterval(function () { 
            if (myLight.color === 'green') { 
                resolve();
                clearInterval(handle);
            }
        }, 100);
    });
}

回答2:

Using Promises would be the best way to do this. Promises are objects that execute a function and when that function is done the THEN-function is executed. But only when the function give a result back.

Example

var task1 = new Promise(function(resolve, reject) {
     var value = 1;
     setTimeout(function(){
          resolve(value);
     }, 1000);

});

var task1.then(function(value){
     // After one second, do something with your value here
});

来源：https://stackoverflow.com/questions/42752239/wait-and-dont-execute-a-function-until-a-light-is-set-to-green