扩展WinJS Promise对象

Question

I wanted an easy way to add pauses into some stuff that I chain together with promises.

I thought it would be nice to include a "wait" method so I could write stuff like:

var promise = new WinJS.Promise(function(complete){
    whatever()
}).wait(1000).then(function(){
    whatever2();
}).wait(500).done(function(){
    alldone();
}

So to do this I added a wait method to the Promise class like so:

if (WinJS.Promise.prototype.wait == null) {
    WinJS.Promise.prototype.wait = function (milliseconds) {
        var promise = new WinJS.Promise(function (complete) {
            setTimeout(complete, milliseconds);
        });
        return promise;
    }
}

It seemed to be working, but I noticed that if I use a "then", the object I get back from it, while the documentation says is a WinJS.Promise, won't have a wait function. The promises I create all DO have the wait function, but calling .then() on a promise will cause the subsequent .wait to fail, so...

promise.wait(300).then().done();

is no problem but:

promise.then().wait(300).done();

will error out saying that there is no wait method on the Promise returned from then().

Can anyone explain what I'm doing wrong?

Answer 1

There are two reasons why your code doesn't work.

The first is that Microsoft has used their own approach to creating object instances, which they do through the WinJS.Class namespace - this means that the prototype you are altering with the addition of your wait function doesn't ever get applied to the WinJS.Promise objects you are consuming in your code.

The second problem is that you are targeting the wrong object - the WinJS.Promise.then method calls WinJS.Promise.as which returns a CompletePromise object - so even if you could make your wait function stick, it would be in the wrong place. The CompletePromise definition is not in the public scope, so you'd have to do a lot of hacking to be able make the change you want.

There is a solution, but you have to use the WinJS.Promise.timeout method. You can't use this inline, which means that to get the effect you want, you will need some slightly awkward code, as follows;

var promise = new WinJS.Promise(function (complete) {
    whatever();
    complete();
}).then(function () {
    return WinJS.Promise.timeout(1000);
}).then(whatever2).then(function() {
    return WinJS.Promise.timeout(500);
}).then(alldone);

Answer 2

This not a direct answer to your question. Adding a wait() method to Promise 's prototype should indeed work, unless then() returns an object that looks like a Promise , quacks like a Promise , but is not actually a Promise .

That said, you do not have to implement a wait() method in the first place, because Promise already exposes a timeout() method that does the same thing (and more, actually). You're looking for its single-argument form:

var promise = new WinJS.Promise(function(complete) {
    whatever();
}).timeout(1000).then(function() {
    whatever2();
}).timeout(500).done(function() {
    alldone();
});