javascript - 如何判断Javascript Q deferred()是否已经执行完链中的所有函数

Question

是否可以知道，当使用 Javascript Q promise 库执行链中注册的所有函数时？

我将从 here 发布示例代码(事实上​​我的问题是跟进):

testJSCallbacks();

function testJSCallbacks(){
    var i = 0,
        promise;

    for (i = 0; i < 5; i++) {
        //Make initial promise if one doesn't exist
        if (!promise) {
            promise = Q.fcall(getStep(i));
        }
        //Append to existing promise chain
        else {
            promise = promise.then(getStep(i));
        }
        //then function returns another promise that can be used for chaining.
        //We are essentially chaining each function together here in the loop.
        promise = promise.then(function (key) {
            //Log the output of step here
            console.log("Step 1 " + key);
            return key;
        })
        //then function takes a callback function with one parammeter (the data).
        //foo signature meets this criteria and will use the resolution of the last promise (key).
        .then(foo)
        //myCB will execute after foo resolves its promise, which it does in the onsuccess callback
        .then(myCB);
    }
}

function getStep(step) {
    return function () {
        return step;
    }
}

function foo(key) {
    //retrieve png image blob from indexedDB for the key 'key'. Assume that the database is
    //created and started properly
    var getRequest = transaction.objectStore("store").get(key),
        //Need to return a promise
        deferred = Q.defer();
    getRequest.onsuccess = function (event) {
        var result = event.target.result;
        if(result){
            console.log("Step 2 " + key + " Found");
        }else{
            console.log("Step 2 " + key + " not Found");  
        }
        deferred.resolve(result);
    }

    return deferred.promise;
}

function myCB (result){
    console.log("Step 3: " + result);
}

=============

如果您在代码中注意到 foo() 和 myCB() 都将执行 5 次。

我想知道的是，当函数 myCB() 上次执行时，是否有可能从 Q 库中获得回调或某种通知，本质上意味着队列是“空的”并且所有已注册/执行延迟函数？

提前致谢。

Answer 1

是的，有!哎呀 - 我什至可以说这就是 promise 的全部意义所在。

那个方法叫做drumroll .then!

使用它时，我们还将使用 Q.all 来等待 promise 集合。这是因为你有一个循环。如果你没有 - 我们只需要 .then

function testJSCallbacks(){
       var promises = [];
       for(var i = 0; i < 5; i++){
       ...
       promise = promise.then(function (key) {
            //Log the output of step here
            console.log("Step 1 " + key);
            return key;
        })
        //then function takes a callback function with one parammeter (the data).
        //foo signature meets this criteria and will use the resolution of the last promise (key).
        .then(foo)
        //myCB will execute after foo resolves its promise, which it does in the onsuccess callback
        .then(myCB);
        promises[i] = promise;
    }
    return Q.all(promises); // note the return statement
}

然后，你可以 Hook 它的完成:

 testJSCallbacks().then(function(results){
        // everything is complete and none of it rejected
 });

如果你从这篇文章中得到一件事——它应该是从方法中返回 promise :)。此外，不是运行 myCb 而是简单地返回值并从外部运行 myCb - 它更强大:)