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import { TimerHandle } from 'calypso/types';
export interface Cancelable {
cancel: () => void;
}
/**
* Creates a delayed function that invokes `func` as soon as possible after the next layout
* flush. At that time, it is very unlikely that the DOM has been written to since the last
* layout and it should be safe to read from DOM without causing a synchronous reflow, i.e.,
* cheaply and without any performance hit.
*
* The returned wrapped function comes with a `cancel` method that cancels the delayed invocations.
*
* Inspired by the Firefox performance best practices MDN article at:
* https://developer.mozilla.org/en-US/Firefox/Performance_best_practices_for_Firefox_fe_engineers
* @param func - The function to be invoked after the layout flush
* @returns The new delayed function
*/
export default function afterLayoutFlush< T extends ( ...args: any[] ) => any >( func: T ) {
let timeoutHandle: TimerHandle | undefined = undefined;
let rafHandle: number | undefined = undefined;
const hasRAF = typeof requestAnimationFrame === 'function';
let lastThis: any;
let lastArgs: any[] | undefined;
const scheduleRAF = function ( rafFunc: T ) {
return function ( this: any, ...args: any[] ) {
// eslint-disable-next-line @typescript-eslint/no-this-alias
lastThis = this;
lastArgs = args;
// if a RAF is already scheduled and not yet executed, don't schedule another one
if ( rafHandle !== undefined ) {
return;
}
rafHandle = requestAnimationFrame( () => {
// clear the handle to signal that the scheduled RAF has been executed
rafHandle = undefined;
rafFunc();
} );
} as T;
};
const scheduleTimeout = function ( timeoutFunc: T ) {
return function ( this: any, ...args: any[] ) {
if ( ! hasRAF ) {
// eslint-disable-next-line @typescript-eslint/no-this-alias
lastThis = this;
lastArgs = args;
}
// If a timeout is already scheduled and not yet executed, don't schedule another one.
// Multiple `requestAnimationFrame` handlers can be scheduled and executed before the
// browser decides to finally execute the timeout handler.
if ( timeoutHandle !== undefined ) {
return;
}
timeoutHandle = setTimeout( () => {
const callArgs = lastArgs !== undefined ? lastArgs : [];
const callThis = lastThis;
lastArgs = undefined;
lastThis = undefined;
// clear the handle to signal that the timeout handler has been executed
timeoutHandle = undefined;
timeoutFunc.apply( callThis, callArgs );
}, 0 );
} as T;
};
// if RAF is not supported (in Node.js test environment), the wrapped function
// will only set a timeout.
let wrappedFunc: T = scheduleTimeout( func );
if ( hasRAF ) {
wrappedFunc = scheduleRAF( wrappedFunc );
}
const wrappedWithCancel = wrappedFunc as T & Cancelable;
wrappedWithCancel.cancel = () => {
lastThis = undefined;
lastArgs = undefined;
if ( rafHandle !== undefined ) {
cancelAnimationFrame( rafHandle );
rafHandle = undefined;
}
if ( timeoutHandle !== undefined ) {
clearTimeout( timeoutHandle );
timeoutHandle = undefined;
}
};
return wrappedWithCancel;
}
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