hubs/src/promiseTree.js

``` Caryl ------ courier3 / \ / \ courier1 courier2

module.exports =  function promiseTree(items, callback) {

    var waitingDefers = {}
    // promise tree
    var keys = Object.keys(items)
    keys.forEach(key => {
        // promise for waiting dependency
        waitingDefers[key] = defer()
        waitingDefers[key].promise = waitingDefers[key].promise
            .then((data) => {
                return callback({data, key})
            })

        if (!items[key].dep) {
            waitingDefers[key].resolve([])
        }
    });

    keys.forEach(key => {
        if (!items[key].dep) return
        // use Promise.all to wait for dependency, and then resolve the waitingDefer
        Promise.all(
            items[key].dep.map(k => {
                return waitingDefers[k].promise
            })
        ).then(waitingDefers[key].resolve)
    });

    return Promise.all(keys.map(key => waitingDefers[key].promise))
}

function defer() {
        var resolve = null;
        var reject = null;
        var promise = new Promise(function (rs, rj) {
            resolve = rs;
            reject = rj;
        });

        return {
            promise,
            resolve,
            reject
        }
    }