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<!DOCTYPE html> | |
<html> | |
<head> | |
<meta charset="utf-8"> | |
<title>FRP Infinite Scroll using Bacon.js</title> | |
<script src="//cdnjs.cloudflare.com/ajax/libs/bacon.js/0.7.53/Bacon.min.js"></script> | |
<script src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/3.7.0/lodash.min.js"></script> | |
<style type="text/css"> | |
body { | |
margin: 0; | |
padding: 0; | |
font-family: Helvetica, arial, freesans, clean, sans-serif; | |
} | |
.phonebook { | |
position: relative; | |
/* draw fake row bg colors with css */ | |
background-image: linear-gradient(#f3f3f3 50%, transparent 50%, transparent); | |
background-size: 60px 60px; /* twice the height of a row, since the pattern spans two rows */ | |
margin: 0; | |
padding: 0; | |
list-style: none; | |
} | |
.phonebook li { | |
position: absolute; | |
height: 30px; | |
line-height: 30px; | |
padding: 0 10px; | |
color: #999; | |
font-size: 12px; | |
} | |
</style> | |
</head> | |
<body> | |
<ul id="Phonebook" class="phonebook"></ul> | |
<script> | |
(function(){ // using an IIFE lets me call a function `scrollY` in this scope, and I like that name. | |
// ---------------------------------------------- Declaration and Setup | |
var totalResults = 10000; // let's assume this is known. You'd probably have to query for the result count in a real api | |
var rowHeight = 30; | |
var rows = {}; // Cache of row dom elements by index | |
var phonebookEl = document.getElementById('Phonebook'); | |
phonebookEl.style.height = totalResults * rowHeight + "px"; | |
// ---------------------------------------------- UI Streams | |
var scrollY = function() { return window.scrollY }; | |
var windowH = function() { return window.innerHeight }; | |
var yPosition = Bacon.fromEvent(window, 'scroll').map(scrollY); | |
var firstVisibleRow = yPosition.map(function(y){ return Math.floor( y / rowHeight ) }).skipDuplicates(); | |
firstVisibleRow = firstVisibleRow.toProperty(0); // Seed the initial value. without this, the stream will be empty if you load the browser with the scroller at the top of the screen | |
var screenHeight = Bacon.fromEvent(window, 'resize').debounce(50).map(windowH); | |
screenHeight = screenHeight.toProperty(window.innerHeight); // Seed the initial value. without this, the stream is empty since no resize has happened | |
var rowCount = screenHeight.map(function(screenHeight){ return Math.ceil(screenHeight/rowHeight) }).skipDuplicates(); | |
rowCount.log('rowCount'); | |
function calcVisibleRows (firstRow, rowCount) { | |
var visibleIndices = []; | |
// Limit the number of visible rows | |
lastRow = firstRow + rowCount + 1; | |
if (lastRow > totalResults) { | |
firstRow -= lastRow - totalResults; | |
} | |
for (var i = 0; i <= rowCount; i++) { visibleIndices.push(i + firstRow) } | |
return visibleIndices; | |
} | |
var visibleRowIndices = Bacon.combineWith(calcVisibleRows, firstVisibleRow, rowCount); | |
// visibleRowIndices.log('visibleRowIndices'); | |
var rowIndicesRemoved = visibleRowIndices.diff([], _.difference); | |
var rowIndicesAdded = visibleRowIndices.diff([], function(prev, cur){ return _.difference(cur, prev) }); // longer form here so we can reverse `cur` and `prev` | |
rowIndicesAdded.onValue( function(indices){ _.map(indices, renderRow) }); | |
rowIndicesRemoved.onValue(function(indices){ _.map(indices, removeRow) }); | |
// ---------------------------------------------- Rendering | |
function renderRow(idx) { | |
var row = document.createElement('li'); | |
row.innerText = idx; | |
row.style.top = idx * rowHeight + 'px'; | |
phonebookEl.appendChild(row); | |
rows[idx] = row; | |
} | |
function removeRow(idx) { | |
if (idx == null) { return } | |
rows[idx].parentElement.removeChild(rows[idx]); | |
rows[idx] = undefined; // more performant than changing the object shape? Would need to measure. | |
} | |
})(); | |
</script> | |
</body> | |
</html> | |
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