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Lifecycle

LitElement-based components update asynchronously in response to observed property changes. Property changes are batched—if more properties change after an update is requested, but before the update starts, all of the changes are captured in the same update.

At a high level, the update lifecycle is:

  1. A property is set.
  2. Check whether an update is needed. If an update is needed, request one.
  3. Perform the update:
  • Process properties and attributes.
  • Render the element.
  1. Resolve a Promise, indicating that the update is complete.

LitElement and the browser event loop

The browser executes JavaScript code by processing a queue of tasks in the event loop. In each iteration of the event loop, the browser takes a task from the queue and runs it to completion.

When the task completes, before taking the next task from the queue, the browser allocates time to perform work from other sources—including DOM updates, user interactions, and the microtask queue.

By default, LitElement updates are requested asynchronously, and queued as microtasks. This means that Step 3 above (Perform the update) is executed at the end of the next iteration of the event loop.

You can change this behavior so that Step 3 awaits a Promise before performing the update. See performUpdate for more information.

For a more detailed explanation of the browser event loop, see Jake Archibald's article.

Lifecycle callbacks

LitElement also inherits the default lifecycle callbacks from the Web Component standard:

  • connectedCallback: Invoked when a component is added to the document's DOM.
  • disconnectedCallback: Invoked when a component is removed from the document's DOM.
  • adoptedCallback: Invoked when a component is moved to a new document.
  • attributeChangedCallback: Invoked when component attribute changes.

Be aware that adoptedCallback is not polyfilled.

All lifecycle methods need to call the super method.

Example:

Promises and asynchronous functions

LitElement uses Promise objects to schedule and respond to element updates.

Using async and await makes it easy to work with Promises. For example, you can await the updateComplete Promise:

Because async functions return a Promise, you can await them, too:

See the Web Fundamentals primer on Promises for a more in-depth tutorial.

Common reasons to hook into the custom element lifecycle or the LitElement update lifecycle are initializations, managing derived data, and dealing with events that originate outside of your element's template. The following list provides some common use cases and approaches. In several cases there is more than one way to achieve a certain goal. Reading this list along with the detailed technical reference will provide you with a rather complete picture and enable you to decide what fits your component's needs best.

  • Use property.hasChanged for checking "Is this a change? Do I want to run the update lifecycle?".
  • Use the element constructor for initializing LitElement properties with default values. (Attribute values from the DOM are not available when the constructor runs.)
  • Use firstUpdated for initializing private fields from DOM attributes (as the constructor doesn't have access to them). Note that render has already run at this point and your changes might trigger another update lifecycle. If it's imperative that you get access to attribute values before the first render happens, consider using connectedCallback, but you'll need to do the extra logic for figuring out the "first" update yourself as connectedCallback can be called multiple times.
  • Use updated for keeping derived data up to date or reacting to changes. If you find, that you're causing re-renders, consider using update instead.
  • Use custom JS property getters for derived data that is "cheap" to calculate and if its not likely to change often and your element doesn't re-render often.
  • Use requestUpdate to trigger an update lifecycle when LitElement cannot pick it up. (E.g. if you have an observed property that is an array, and you add an item to that array instead of replacing the entire array, LitElement won't "see" this change, because the reference to the array didn't change.)
  • Use connectedCallback to register event handlers for outside your element's template, but don't forget to remove them in disconnectedCallback!

In call order, the methods and properties in the update lifecycle are:

  1. someProperty.hasChanged
  2. requestUpdate
  3. performUpdate
  4. shouldUpdate
  5. update
  6. render
  7. firstUpdated
  8. updated
  9. updateComplete

All declared properties have a function, hasChanged, which is called whenever the property is set; if hasChanged returns true, an update is scheduled.

See the Properties documentation for information on configuring hasChanged to customize what constitutes a property change.

| Params

  | propertyName

oldValue| Name of property to be updated.

Previous property value. | | Returns | Promise | Returns the updateComplete Promise, which resolves on completion of the update. | | Updates? | No | Property changes inside this method will not trigger an element update. |

If hasChanged returned true, requestUpdate fires, and the update proceeds.

To manually start an element update, call requestUpdate with no parameters.

To implement a custom property setter that supports property options, pass the property name and its previous value as parameters.

Example: Manually start an element update

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Example: Call requestUpdate from a custom property setter

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| Returns | void or Promise | Performs an update. | | Updates? | No | Property changes inside this method will not trigger an element update. |

By default, performUpdate is scheduled as a microtask after the end of the next execution of the browser event loop. To schedule performUpdate, implement it as an asynchronous method that awaits some state before calling super.performUpdate(). For example:

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| Params | changedProperties| Map. Keys are the names of changed properties; Values are the corresponding previous values. | | Returns | Boolean | If true, update proceeds. Default return value is true. | | Updates? | Yes | Property changes inside this method will trigger an element update. |

Controls whether an update should proceed. Implement shouldUpdate to specify which property changes should cause updates. By default, this method always returns true.

Example: Customize which property changes should cause updates

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| Params | changedProperties| Map. Keys are the names of changed properties; Values are the corresponding previous values. | | Updates? | No | Property changes inside this method do not trigger an element update. |

Reflects property values to attributes and calls render to render DOM via lit-html. Provided here for reference. You don't need to override or call this method. But if you override it, make sure to call super.update(changedProperties) or render will never be called.

| Returns | TemplateResult | Must return a lit-html TemplateResult. | | Updates? | No | Property changes inside this method will not trigger an element update. |

Uses lit-html to render the element template. You must implement render for any component that extends the LitElement base class.

See the documentation on Templates for more information.

| Params | changedProperties| Map. Keys are the names of changed properties; Values are the corresponding previous values. | | Updates? | Yes | Property changes inside this method will trigger an element update. |

Called after the element's DOM has been updated the first time, immediately before updated is called.

Implement firstUpdated to perform one-time work after the element's template has been created.

Example: Focus an input element on first update

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| Params | changedProperties| Map. Keys are the names of changed properties; Values are the corresponding previous values. | | Updates? | Yes | Property changes inside this method will trigger an element update. |

Called when the element's DOM has been updated and rendered. Implement to perform some task after an update.

Example: Focus an element after update

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| Type | Promise | Resolves with a Boolean when the element has finished updating. | | Resolves

| true if there are no more pending updates.

false if this update cycle triggered another update. |

The updateComplete Promise resolves when the element has finished updating. Use updateComplete to wait for an update:

Example

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Overriding updateComplete

To await additional state before fulfilling the updateComplete promise, override the _getUpdateComplete method. For example, it may be useful to await the update of a child element here. First await super._getUpdateComplete(), then any subsequent state.

It's recommended to override the _getUpdateComplete method instead of the updateComplete getter to ensure compatibility with users who are using TypeScript's ES5 output (see TypeScript#338).

Control when updates are processed

Implement performUpdate:

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Customize which property changes should cause an update

Implement shouldUpdate:

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Customize what constitutes a property change

Specify hasChanged for the property. See the Properties documentation.

Manage property changes and updates for object subproperties

Mutations (changes to object subproperties and array items) are not observable. Instead, either rewrite the whole object, or call requestUpdate after a mutation.

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Update in response to something that isn't a property change

Call requestUpdate:

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Request an update regardless of property changes

Call requestUpdate():

Request an update for a specific property

Call requestUpdate(propName, oldValue):

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Do something after the first update

Implement firstUpdated:

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Do something after every update

Implement updated:

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Do something when the element next updates

Await the updateComplete promise:

Wait for an element to finish updating

Await the updateComplete promise:

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