How can I reflect to attribute but not trigger a rerender in lit-element? - web-component

Question

I am converting a custom element dropdown over to lit-element. The way the existing element shows the dropdown options is by setting an expanded boolean attribute on the element, and the options are shown/hidden via css:
my-element:not([expanded]) .options-container {
display: none;
}
my-element[expanded] .options-container {
display: block;
}
The component doesn't need to do any rerenders because the logic is all in the css.
How can I achieve this behavior with lit-element, and not rerender the component? Rerendering can be costly if there are a lot of dropdown options.
I have tried implementing a shouldUpdate that returns false if only expanded has changed - but this causes lit-element not to reflect expanded to the attribute when set via a property, which is necessary in order to show/hide via css.
This is what I have, which doesn't work:
class MyDropdown extends LitElement {
static get properties() {
return {
expanded: { type: Boolean, reflect: true },
...
};
}
shouldUpdate(changedProperties) {
if (changedProperties.has('expanded') && changedProperties.size === 1) {
return false;
}
return true;
}
// disable shadow-dom
createRenderRoot() {
return this;
}
}
Note that I am not using shadow dom yet, not sure if that would change the solution. I'm on lit-element 2.2.1.

Answer 1

The idea is to not use LitElement's static properties or #Property decorator. Write your own property implementation like this:
class MyDropdown extends LitElement {
_expanded = false;
get expanded() {
return this._expanded;
}
set expanded(val) {
this._expanded = val;
// Manually setting the property and reflecting attribute.
if (val) {
this.setAttribute('expanded', '');
} else {
this.removeAttribute('expanded');
}
}
// disable shadow-dom
createRenderRoot() {
return this;
}
}
Similarly, you can listen for attributeChangedCallback lifecycle event and adjust _expanded property whenever user changes the attribute and not property.