Memory management in knockout
Memory management in knockout
Explicit use of computed?
What does a computed expression like this:
this.isBusy = ko.computed(function() {
return self._isBusy() || self._queryIsBusy();
});
have to do with
<div data-bind="visible: _isBusy() || _queryIsBusy()" ></div>
Internally knockout creates a computed out of an expression in a binding handler. Ko does not parse the expression in the html above to register the variables _isBusy or _queryIsBusy.
Consequences
What does this mean for your code?
If you have something below but want to move the logic to your class from the view:
<div data-bind="visible: _isBusy() || _queryIsBusy()" ></div>
You have two main options:
Option 1
Object.defineProperty(self, 'isBusy', {
get: function() {
return _isBusy() || _queryIsBusy();
}
});
<div data-bind="visible:isBusy" ></div>
Option 2
this.isBusy = function() {
return _isBusy() || _queryIsBusy();
};
<div data-bind="visible:isBusy()" ></div>
Notice ‘()’
Life cycle
How long does the computed that knockout creates live?
A bit simplified if you read the code for KO you find that knockout makes new computed (internally dependent observable) with option:
disposeWhenNodeIsRemoved
This means that knockout will remove the list of references to observables that the computed depends on when the DOM-node that the computed is associated with have been removed.
When is it safe to use a computed?
When you know the life span of the computed you have created.
Why is it generally bad to use computed willy nilly
It is generally hard to reason about when they will be disposed. It is easy to access exposed ko.computed from different objects in the system with different life span.
Tags
Do you want to send a comment or give me a hint about any issues with a blog post: Open up an issue on GitHub.
Do you want to fix an error or add a comment published on the blog? You can do a fork of this post and do a pull request on github.
Comments