The question occasionally comes up about the exact specifications for the built-in Windows animations that express the Windows look and feel. Often this is because someone wants to replicate the effect outside of Windows, such as on a website, or within a webview element where WinJS isn’t accessible.

Fortunately, it’s quite simple to see the implementation of the animations directly in the WinJS source code. All the animations stuff is right near the top of the ui.js file, with the WinJS.UI.Animation namespace starting around like 530/540. You can also just search for the specific animation’s name, and you’ll find code like this (omitting comments):

fadeIn: function (shown) {
return thisWinUI.executeTransition(
property: “opacity”,
delay: 0,
duration: 250,
timing: “linear”,
from: 0,
to: 1

WinUI.executeTransition is a helper function that takes the property bag indicated here and creates the transition/animation. In this case it animates the opacity property from 0 to 1 with no delay over 250ms using linear timing. And this is exactly the “spec” for the system-wide Windows fade-in effect, which is also expressed in the XAML animations and so forth.

It’s worth mentioning too that an app can also retrieve these characteristics–called animation metrics–through the Windows.UI.Core.AnimationMetrics API in WinRT (refer also to the Animation metrics sample.) This API exists primarily for DirectX apps written in C++ that don’t have a built-in animation library to draw from and therefore must replicate the behaviors at runtime. But you can use the metrics from any language to do the same if needed.

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