import inline from '../../assets/middleware.inline.jpg'; # Middleware Middleware are plain objects that modify the positioning coordinates in some fashion, or provide useful data for rendering, as calculated by the positioning cycle. This allows you to customize the behavior of the positioning and be as granular as you want, adding your own custom logic. `computePosition(){:js}` starts with initial positioning via `placement{:.objectKey}` — then middleware are executed as an in-between "middle" step of the initial placement computation and eventual return of data for rendering once the promise has resolved. Each middleware is executed in order, like a stack: ```js computePosition(referenceEl, floatingEl, { placement: 'right', middleware: [], }); ``` ## Example ```js const shiftByOnePixel = { name: 'shiftByOnePixel', fn({x, y}) { return { x: x + 1, y: y + 1, }; }, }; ``` This (not particularly useful) middleware adds `1{:js}` pixel to the coordinates. To use this middleware, add it to your `middleware{:.objectKey}` array: ```js computePosition(referenceEl, floatingEl, { placement: 'right', middleware: [shiftByOnePixel], }); ``` Here, `computePosition(){:js}` will compute coordinates that will place the floating element to the right center of the reference element, lying flush with it. Middleware are then executed, resulting in these coordinates getting shifted by one pixel. Then that data is returned to the caller. ### Shape A middleware is an object which has a `name{:.objectKey}` property and a `fn{:.function}` property. The `fn{:.function}` property provides the logic of the middleware, which returns new positioning coordinates or useful data. ### Data Any data can be passed via an optional `data{:.objectKey}` property of the object that is returned from `fn{:.function}`. This will be accessible to the consumer via the `middlewareData{:.param}` property: ```js {7-9} const shiftByOnePixel = { name: 'shiftByOnePixel', fn({x, y}) { return { x: x + 1, y: y + 1, data: { amount: 1, }, }; }, }; ``` ```js computePosition(referenceEl, floatingEl, { middleware: [shiftByOnePixel], }).then(({middlewareData}) => { console.log(middlewareData.shiftByOnePixel); }); ``` ### Function You may notice that Floating UI's core middleware are actually functions. This is so you can pass options in: ```js const shiftByAmount = (amount = 0) => ({ name: 'shiftByAmount', options: amount, fn: ({x, y}) => ({ x: x + amount, y: y + amount, }), }); ``` It returns an object and uses a closure to pass the configured behavior: ```js const middleware = [shiftByAmount(10)]; ``` The `options{:.objectKey}` key on a middleware object allows libraries like React to compare and update middleware on component re-renders. ### Always return an object Inside `fn{:.function}` make sure to return an object. It doesn't need to contain properties, but to remind you that it should be pure, you must return an object. Never mutate any values that get passed in from `fn{:.function}`. ## MiddlewareState An object is passed to `fn{:.function}` containing useful data about the middleware lifecycle being executed. In the previous examples, we destructured `x{:.param}` and `y{:.param}` out of the `fn{:.function}` parameter object. These are only two properties that get passed into middleware, but there are many more. The properties passed are below: ```ts type MiddlewareState = { x: number; y: number; initialPlacement: Placement; placement: Placement; strategy: Strategy; middlewareData: MiddlewareData; elements: Elements; rects: ElementRects; platform: Platform; }; ``` ### x This is the x-axis coordinate to position the floating element to. ### y This is the y-axis coordinate to position the floating element to. ### elements This is an object containing the reference and floating elements. ### rects This is an object containing the `Rect{:.class}`s of the reference and floating elements, an object of shape `{width, height, x, y}`. ### middlewareData This is an object containing all the data of any middleware at the current step in the lifecycle. The lifecycle loops over the `middleware{:.objectKey}` array, so later middleware have access to data from any middleware run prior. ### strategy The positioning strategy. ### initialPlacement The initial (or preferred) placement passed in to `computePosition(){:js}`. ### placement The stateful resultant placement. Middleware like `flip{:.function}` change `initialPlacement{:.objectKey}` to a new one. ### platform An object containing methods to make Floating UI work on the current platform, e.g. DOM or React Native. ## Ordering The order in which middleware are placed in the array matters, as middleware **use** the coordinates that were returned from previous ones. This means they perform their work based on the current positioning state. Three `shiftByOnePixel{:.const}` in the middleware array means the coordinates get shifted by 3 pixels in total: ```js const shiftByOnePixel = { name: 'shiftByOnePixel', fn: ({x, y}) => ({x: x + 1, y: y + 1}), }; const middleware = [ shiftByOnePixel, shiftByOnePixel, shiftByOnePixel, ]; ``` If the later `shiftByOnePixel` implementations had a condition based on the current value of `x{:.param}` and `y{:.param}`, the condition can change based on their placement in the array. Understanding this can help in knowing which order to place middleware in, as placing a middleware before or after another can produce a different result. In general, `offset(){:js}` should always go at the beginning of the middleware array, while `arrow(){:js}` and `hide(){:js}` at the end. The other core middleware can be shifted around depending on the desired behavior. ```js const middleware = [ offset(), // ... arrow({element: arrowElement}), hide(), ]; ``` ## Resetting the lifecycle There are use cases for needing to reset the middleware lifecycle so that other middleware perform fresh logic. - When `flip(){:js}` and `autoPlacement(){:js}` change the placement, they reset the lifecycle so that other middleware that modify the coordinates based on the current `placement{:.objectKey}` do not perform stale logic. - `size(){:js}` resets the lifecycle with the newly applied dimensions, as many middleware read the dimensions to perform their logic. - `inline(){:js}` resets the lifecycle when it changes the reference rect to a custom implementation, similar to a [Virtual Element](/docs/virtual-elements). In order to do this, add a `reset{:.objectKey}` property to the returned object from `fn{:.function}`. ```ts type Reset = | true | { placement?: Placement; // `true` will compute the new `rects` if the // dimensions were mutated. Otherwise, you can // return your own new rects. rects?: true | ElementRects; }; ``` ```js const middleware = { name: 'middleware', fn() { if (someCondition) { return { reset: { placement: nextPlacement, }, }; } return {}; }, }; ``` Data supplied to `middlewareData{:.param}` is preserved by doing this, so you can read it at any point after you've reset the lifecycle.