So here's a weird KnockoutJS problem I've never actually come across before.
I'm working on an application that uses Knockout Components very heavily.
In one part of the app, I have an editor page that's built dynamically from a JSON driven backend, and which populates a front end page with a number of widgets, depending on what it's told from the back end data.
Example The back end might send
[{"widget": "textBox"},{"widget": "textBox"},{"widget": "comboBox"},{"widget": "checkBox"}]
Which would cause the front end to build up a page containing
<html>
....
<textbox></textbox>
<textbox></textbox>
<combobox></combobox>
<checkbox></checkbox>
....
</html>
Each of the custom tags is an individual KnockoutJS component, compiled as an AMD module and loaded using RequireJS, each component is based on the same boiler plate:
/// <amd-dependency path="text!application/components/pagecontrols/template.html" />
define(["require", "exports", "knockout", 'knockout.postbox', "text!application/components/pagecontrols/template.html"], function (require, exports, ko, postbox) {
var Template = require("text!application/components/pagecontrols/template.html");
var ViewModel = (function () {
function ViewModel(params) {
var _this = this;
this.someDataBoundVar = ko.observable("");
}
ViewModel.prototype.somePublicFunction = function () {
postbox.publish("SomeMessage", { data: "some data" });
};
return ViewModel;
})();
return { viewModel: ViewModel, template: Template };
});
The components communicate with each other and with the page using "Knockout Postbox" in a pub sub fashion.
And when I put them into the page I do so in the following manor:
<div data-bind="foreach: pageComponentsToDisplay">
<!-- ko if: widget == "textBox" -->
<textBox params="details: $data"></textBox>
<!-- /ko -->
<!-- ko if: widget == "comboBox" -->
<comboBox params="details: $data"></comboBox>
<!-- /ko -->
<!-- ko if: widget == "checkBox" -->
<checkBox params="details: $data"></checkBox>
<!-- /ko -->
</div>
and where pageComponentsToDisplay is a simple knockout observable array that I just push the objects received from the backend onto:
pageComponentsToDisplay = ko.observableArray([]);
pageComponentsToDisplay(data);
Where 'data' is as shown in JSON above
Now all of this works great, but here-in now lies the ODD part.
If I have to do a "reload" of the page, I simply
pageComponentsToDisplay = ko.observableArray([]);
to clear the array, and consequently, all my components also disappear from the page, as expected, however when I load the new data in, again using:
pageComponentsToDisplay(data);
I get my new components on screen as expected, BUT the old ones appear to be still present and active in memory, even though there not visible.
The reason I know the controls are still there, because when I issue one of my PubSub messages to ask the controls for some state info, ALL of them reply.
It seems to me that when I clear the array, and when KO clears the view model, it actually does not seem to be destroying the old copies.
Further more, if I refresh again, I then get 3 sets of components responding, refresh again and it's 4, and this keeps increasing as expected.
This is the first time I've encountered this behaviour with knockout, and I've used this kind of pattern for years without an issue.
If you want a good overview of how the entire project is set up, I have a sample skeleton layout on my github page:
https://github.com/shawty/dotnetnotts15
If anyone has any ideas on what might be happening here I'd love to hear them.
As a final note, I'm actually developing all this using Typescript, but since this is a runtime problem, I'm documenting it from a JS point of view.
Regards Shawty
So after digging further (and with a little 'new thinking' thanks to cl3m's answer) I'm a little bit further forward.
In my initial post, I did mention that I was using Ryan Niemeyer's excelent PubSub extension for Knockout 'ko postbox'.
It turn's out, that my 'Components' are being disposed of and torn down BUT the subscription handlers that are being created to respond to postbox are not.
The result is, that the VM (or more specifically the values that the subscription uses in the VM) are being kept in memory, along with the postbox subscription handler.
This means when my master broadcasts a message asking for component values, the held reference responds, followed by the visibly active component.
What I need to now do is figure out a way to dispose these subscriptions, which because I'm using postbox directly, and NOT assigning them to an observable in my model, means I don't actually have a var or object reference to target them with.
The quest continues.
See my self answer to the question below.
The problem it seems was due to Knockout hanging onto subscriptions set up by postbox when the actual components where active.
In my case, I use postbox purely as a messaging platform, so all i'm doing is
ko.postbox.subscribe("foo", function(payload) { ... });
all the time, since I was only ever using single shot subscriptions in this fashion, I was never paying ANY Attention to the values returned by the postbox subscription call.
I did things this way, simply because in many of the components I create there is a common API that they all use, but to which they all respond in different ways, so all I ever needed was a simple this is what to do when your called handler that was component specific, but not application specific.
It turns out however that when you use postbox in this manner, there is no observable for you to target, and as such there is nothing to dispose. (Your not saving the return, so you have nothing to work with)
What the Knockout and Postbox documentation does not mention, is that the return value from postbox.subscribe is a general Knockout subscription function, and by assigning the return from it to a property within your model, you then have a means to call the functionality available on it, one of those functions provides the ability to "dispose" the instance, which NOT ONLY removes the physical manifestation of the component from it's collection, BUT ALSO ensures that any subscriptions or event handlers connected to it are also correctly torn down.
Couple with that, the fact that you can pass a dispose handler to your VM when you register it, the final solution is to make sure you do the following
/// <amd-dependency path="text!application/components/pagecontrols/template.html" />
define(["require", "exports", "knockout", 'knockout.postbox', "text!application/components/pagecontrols/template.html"], function (require, exports, ko, postbox) {
var Template = require("text!application/components/pagecontrols/template.html");
var ViewModel = (function () {
function ViewModel(params) {
var _this = this;
this.someDataBoundVar = ko.observable("");
this.mySubscriptionHandler = ko.postbox.subscribe("foo", function(){
// do something here to handle subscription message
});
}
ViewModel.prototype.somePublicFunction = function () {
postbox.publish("SomeMessage", { data: "some data" });
};
return ViewModel;
ViewModel.prototype.dispose = function () {
this.mySubscriptionHandler.dispose();
};
return ViewModel;
})();
return { viewModel: ViewModel, template: Template, dispose: this.dispose };
});
You'll notice that the resulting class has a "dispose" function too, this is something that KnockoutJS provides on component classes, and if your class is managed as a component by the main KO library, KO will look for and execute if found, that function when your component class goes out of scope.
As you can see in my example, Iv'e saved the return from the subscription handler as previously mentioned, then in this hook point that we know will get called, used that to ensure that I also call dispose on each subscription.
Of course this ONLY shows one subscription, if you have multiple subscriptions, then you need multiple saves, and multiple calls at the end. An easy way of achieving this, especially if your using Typescript as I am, is to use Typescripts generics functionality and save all your subscriptions into a typed array, meaning at the end all you need to do is loop over that array and call dispose on every entry in it.