README.md

Infinite Scroll for Mithril

A component to handle scrolling of an "infinite" list or grid, while only drawing what is on screen (plus a bit of pre-fetching), so safe to use on mobiles.

Compatible with Mithril 1.x.

• Examples
• Features
• Installation
• Usage
  • Handling data
    • Using pageUrl for referencing URLs
    • Data file structure
    • Using pageData for server requests
      • With m.request
      • With async
      • Returning data directly
      • Returning data as a Promise
  • Handling dynamic data
  • Advanced item function example
  • Getting the total page count
  • Using images
  • Using table data
  • Pagination
  • Custom page wrapper
• Configuration options
  • Appearance options
  • Callback functions
  • Paging options
  • Options for infinite.isElementInViewport
• Styling
  • CSS classes
  • Fixed scroll and overflow-anchor
• Size
• Dependencies
• Licence

Examples

Examples

Features

• Natural scrolling using browser defaults.
• Fast and fluent (on desktop and modern mobiles).
• Can be used for lists, grids and table-like data.
• Items that are out of sight are removed, so only a fraction of the total content is drawn on the screen. This is good for speed and memory consumption.
• Support for unequal content heights and dynamic resizing of content elements.
• As more data is loaded, the scroll view increases in size, so that the scrollbar can be used to go back to a specific point on the page.
• Items are handled per "page", which is a normal way of handling batches of search results from the server.
• Pages can contain an arbitrary and unequal amount of items.
• Pre-fetches data of the current page, the page before and after (or n pages ahead).
• When there is room on the page to show more data: automatic detection of "loadable space" (so no loading area detection div is needed).
• Optionally use previous/next paging buttons.
• Supports dynamic content (for instance when filtering results).

Not included (by design):

• Special support for older mobile browsers: no touch layer, requestAnimationFrame, absolute positioning or speed/deceleration calculations.

Installation

Use as npm module:

npm install --save mithril-infinite

or download/clone from Github.

For working with the examples, see the examples documentation.

Usage

Note: The parent of "scroll-view" must have a height. Also make sure that html has a height (typically set to 100%).

Handling data

Data can be provided:

• With pageUrl for referencing URLs
• With pageData for server requests

Using pageUrl for referencing URLs

An example using data files:

import infinite from "mithril-infinite"

m(infinite, {
  maxPages: 16,
  pageUrl: pageNum => `data/page-${pageNum}.json`,
  item
})

With these options we are:

• limiting the number of pages to 16
• passing a function to generate a JSON data URL
• passing a function that should return a Mithril element

A simple item function:

const item = (data, opts, index) => 
  m(".item", [
    m("h2", data.title),
    m("div", data.body)
  ])

The item function passes 3 parameters:

1. data contains the loaded data from pageUrl.
2. opts contains: isScrolling: Bool, pageId: String, pageNum: Number, pageSize: Number
3. index: the item index

Data file structure

Data is handled per "results" page. You are free to use any data format.

You could use a JSON data object for each page, containing a list of items. For example:

[
  {
    "src": "cat.jpg",
    "width": 500,
    "height": 375
  }
]

Or:

[
  ["red", "#ff0000"],
]

Using pageData for server requests

In most real world situations an API server will provide the data. So while passing file URLs with pageUrl is a handy shortcut, we preferably use data requests.

With m.request

import infinite from "mithril-infinite"

const pageData = pageNum => 
  m.request({
    method: "GET",
    dataType: "jsonp",
    url: dataUrl(pageNum)
  })

m(infinite, {
  pageData,
  item
})

Demo tip: in the example "Grid" we use jsonplaceholder.typicode.com to fetch our images:

const PAGE_ITEMS = 10

const dataUrl = pageNum =>
  `http://jsonplaceholder.typicode.com/photos?_start=${(pageNum - 1) * PAGE_ITEMS}&_end=${pageNum * PAGE_ITEMS}`

With async

import infinite from "mithril-infinite"

const asyncPageData = async function(pageNum) {
  try {
    const response = await fetch(dataUrl(pageNum))
    return response.json()
  } catch (ex) {
    //console.log('parsing failed', ex)
  }
}

m(infinite, {
  pageData: asyncPageData,
  item
})

Returning data directly

import infinite from "mithril-infinite"

const returnData = () =>
  [{ /* some data */ }]

m(infinite, {
  pageData: returnData,
  item
})

Returning data as a Promise

import infinite from "mithril-infinite"

const returnDelayedData = () =>
  new Promise(resolve =>
    setTimeout(() =>
      resolve(data)
    , 1000)
  )

m(infinite, {
  pageData: returnDelayedData,
  item
})

Handling dynamic data

In situations where the Infinite component needs to show different items - for instance when filtering or sorting search results - we must provide a unique key for each page. The key will enable Mithril to properly distinguish the pages.

Use option pageKey to provide a function that returns a unique identifying string. For example:

import infinite from "mithril-infinite"
import stream from "mithril/stream"

const query = stream("")

const Search = {
  view: () =>
    m("div", 
      m("input", {
        oninput: e => query(e.target.value),
        value: query()
      })
    )
}

const MyComponent = {
  view: () => {
    const queryStr = query()
    return m(infinite, {
      before: m(Search),
      pageKey: pageNum => `${pageNum}-${queryString}`,
      // other options
    })
  }
}

Advanced item function example

To enhance the current loading behavior, we:

• Load images when they are visible in the viewport
• Stop loading images when the page is scrolling. This makes a big difference in performance, but it will not always result in a good user experience - the page will seem "dead" when during the scrolling. So use with consideration.

The item function can now look like this:

const item = (data, opts) =>
  m("a.grid-item",
    m(".image-holder",
      m(".image", {
        oncreate: vnode => maybeShowImage(vnode, data, opts.isScrolling),
        onupdate: vnode => maybeShowImage(vnode, data, opts.isScrolling)
      })
    )
  )

// Don't load the image if the page is scrolling
const maybeShowImage = (vnode, data, isScrolling) => {
  if (isScrolling || vnode.state.inited) {
    return
  }
  // Only load the image when visible in the viewport
  if (infinite.isElementInViewport({ el: vnode.dom })) {
    showImage(vnode.dom, data.thumbnailUrl)
    vnode.state.inited = true
  }
el.style.backgroundImage = `url(${url})`

Getting the total page count

How the total page count is delivered will differ per server. jsonplaceholder.typicode.com passes the info in the request header.

Example "Fixed" shows how to get the total page count from the request, and use that to calculate the total content height.

We place the pageData function in the oninit function so that we have easy access to the state.pageCount variable:

const state = vnode.state
state.pageCount = 1

state.pageData = pageNum => 
  m.request({
    method: "GET",
    dataType: "jsonp",
    url: dataUrl(pageNum),
    extract: xhr => (
      // Read the total count from the header
      state.pageCount = Math.ceil(parseInt(xhr.getResponseHeader("X-Total-Count"), 10) / PAGE_ITEMS),
      JSON.parse(xhr.responseText)
    )
  })

Then pass state.pageData to infinite:

m(infinite, {
  pageData: state.pageData,
  maxPages: state.pageCount,
  ...
})

Using images

For a better loading experience (and data usage), images should be loaded only when they appear on the screen. To check if the image is in the viewport, you can use the function infinite.isElementInViewport({ el }). For example:

if (infinite.isElementInViewport({ el: vnode.dom })) {
  loadImage(vnode.dom, data.thumbnailUrl)
}

Images should not be shown with the <img/> tag: while this works fine on desktop browsers, this causes redrawing glitches on iOS Safari. The solution is to use background-image. For example:

el.style.backgroundImage = `url(${url})`

Using table data

Using <table> tags causes reflow problems. Use divs instead, with CSS styling for table features. For example:

.page {
  display: table;
  width: 100%;
}
.list-item {
  width: 100%;
  display: table-row;
}
.list-item > div {
  display: table-cell;
}

Pagination

See the "Paging" example.

Custom page wrapper

Use processPageData to either:

• Process content data before passing to item
• Wrap in a custom element wrapper
• Use a custom item function

Simple example with a wrapper:

m(infinite, {
  processPageData: (content, options)  => {
    return m(".my-page", content.map((data, index) => item(data, options, index)));
  },
  ...
});

Configuration options

Appearance options

Parameter	Mandatory	Type	Default	Description
scrollView	optional	Selector String		Pass an element's selector to assign another element as scrollView
class	optional	String		Extra CSS class appended to mithril-infinite__scroll-view
contentTag	optional	String	"div"	HTML tag for the content element
pageTag	optional	String	"div"	HTML tag for the page element; note that pages have class mithril-infinite__page plus either mithril-infinite__page--odd or mithril-infinite__page--even
maxPages	optional	Number	Number.MAX_VALUE	Maximum number of pages to draw
preloadPages	optional	Number	1	Number of pages to preload when the app starts; if room is available, this number will increase automatically
axis	optional	String	"y"	The scroll axis, either "y" or "x"
autoSize	optional	Boolean	true	Set to false to not set the width or height in CSS
before	optional	Mithril template or component		Content shown before the pages; has class mithril-infinite__before
after	optional	Mithril template or component		Content shown after the pages; has class mithril-infinite__after; will be shown only when content exists and the last page is in view (when maxPages is defined)
contentSize	optional	Number (pixels)		Use when you know the number of items to display and the height of the content, and when predictable scrollbar behaviour is desired (without jumps when content is loaded); pass a pixel value to set the size (height or width) of the scroll content, thereby overriding the dynamically calculated height; use together with pageSize
setDimensions	optional	Function ({scrolled: Number, size: Number})		Sets the initial size and scroll position of scrollView; this function is called once

Callback functions

Parameter	Mandatory	Type	Default	Description
pageUrl	either pageData or pageUrl	Function (page: Number) => String		Function that accepts a page number and returns a URL String
pageData	either pageData or pageUrl	Function (page: Number) => Promise		Function that fetches data; accepts a page number and returns a promise
item	required: either item or processPageData	Function (data: Array, options: Object, index: Number) => Mithril Template		Function that creates a Mithril element from received data
pageSize	optional	Function (content: Array) => Number	Pass a pixel value to set the size (height or width) of each page; the function accepts the page content and returns the size
pageChange	optional	Function (page: Number)		Get notified when a new page is shown
processPageData	required: either item or processPageData	Function (data: Array, options: Object) => Array		Function that maps over the page data and returns an item for each
getDimensions	optional	Function () => {scrolled: Number, size: Number}		Returns an object with state dimensions of scrollView: scrolled (either scrollTop or scrollLeft) and size (either height or width); this function is called on each view update
pageKey	optional	Function (page: Number) => String	key is based on page number	Function to provide a unique key for each Page component; use this when showing dynamic page data, for instance based on sorting or filtering

Paging options

Parameter	Mandatory	Type	Default	Description
currentPage	optional	Number		Sets the current page
from	optional	Number		Not needed when only one page is shown (use currentPage); use page data from this number and higher
to	optional	Number		Not needed when only one page is shown (use currentPage); Use page data to this number and lower

Options for infinite.isElementInViewport

All options are passed in an options object: infinite.isElementInViewport({ el, leeway })

Parameter	Mandatory	Type	Default	Description
el	required	HTML Element		The element to check
axis	optional	String	"y"	The scroll axis, either "y" or "x"
leeway	optional	Number	300	The extended area; by default the image is already fetched when it is 100px outside of the viewport; both bottom and top leeway are calculated

Styling

Note: The parent of "scroll-view" must have a height. Also make sure that html has a height (typically set to 100%).

Styles are added using j2c. This library is also used in the examples.

CSS classes

Element	Key	Class
Scroll view	scrollView	mithril-infinite__scroll-view
Scroll content	scrollContent	mithril-infinite__scroll-content
Content container	content	mithril-infinite__content
Pages container	pages	mithril-infinite__pages
Page	page	mithril-infinite__page
Content before	before	mithril-infinite__before
Content after	after	mithril-infinite__after

State	Key	Class
Scroll view, x axis	scrollViewX	mithril-infinite__scroll-view--x
Scroll view, y axis	scrollViewY	mithril-infinite__scroll-view--y
Even numbered page	pageEven	mithril-infinite__page--even
Odd numbered page	pageOdd	mithril-infinite__page--odd
Page, now placeholder	placeholder	mithril-infinite__page--placeholder

Fixed scroll and overflow-anchor

Some browsers use overflow-anchor to prevent content from jumping as the page loads more data above the viewport. This may conflict how Infinite inserts content in "placeholder slots".

To prevent miscalculations of content size, the "scroll content" element has style overflow-anchor: none.

Size

Minified and gzipped: ~ 3.9 Kb

Dependencies

• j2c - for creating js stylesheets
• Mithril 1.0
• resize-observer-polyfill
• Verge - for measuring the viewport

Licence

MIT