style: Implement Animate for track lists on grid-template-{columns|rows}.

Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1348519#c6 and
https://github.com/w3c/csswg-drafts/issues/3201:

Currently grid-template-rows/columns interpolate “per computed value”, which
means that if the number of tracks differs, or any track changes to/from a
particular keyword value to any other value, or if a line name is added/removed
at any position, the entire track listing is interpolated as “discrete”.
But we "agree" with two more granular options:

1. Check interpolation type per track, rather than for the entire list, before
   falling back to discrete. I.e. a length-percentage track can animate between
   two values while an adjacent auto track flips discretely to min-content.
2. Allow discrete interpolation of line name changes independently of track
   sizes.

Besides, for the repeat() function, it's complicated to support interpolation
between different repeat types (i.e. auto-fill, auto-fit) and different repeat
counts, so we always fall-back to discrete if the first parameter of repeat()
is different.

Differential Revision: https://phabricator.services.mozilla.com/D16129

components/style/values/animated/grid.rs

@@ -0,0 +1,153 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Animation implementation for various grid-related types.
+
+// Note: we can implement Animate on their generic types directly, but in this case we need to
+// make sure two trait bounds, L: Clone and I: PartialEq, are satisfied on almost all the
+// grid-related types and their other trait implementations because Animate needs them. So in
+// order to avoid adding these two trait bounds (or maybe more..) everywhere, we implement
+// Animate for the computed types, instead of the generic types.
+
+use super::{Animate, Procedure, ToAnimatedZero};
+use crate::values::computed::{GridTemplateComponent, TrackList, TrackSize};
+use crate::values::computed::Integer;
+use crate::values::computed::LengthPercentage;
+use crate::values::distance::{ComputeSquaredDistance, SquaredDistance};
+use crate::values::generics::grid as generics;
+
+fn discrete<T: Clone>(from: &T, to: &T, procedure: Procedure) -> Result<T, ()> {
+    if let Procedure::Interpolate { progress } = procedure {
+        Ok(if progress < 0.5 { from.clone() } else { to.clone() })
+    } else {
+        Err(())
+    }
+}
+
+fn animate_with_discrete_fallback<T: Animate + Clone>(
+    from: &T,
+    to: &T,
+    procedure: Procedure
+) -> Result<T, ()> {
+    from.animate(to, procedure).or_else(|_| discrete(from, to, procedure))
+}
+
+impl Animate for TrackSize {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self, other) {
+            (&generics::TrackSize::Breadth(ref from),
+             &generics::TrackSize::Breadth(ref to)) => {
+                animate_with_discrete_fallback(from, to, procedure)
+                    .map(generics::TrackSize::Breadth)
+            },
+            (&generics::TrackSize::Minmax(ref from_min, ref from_max),
+             &generics::TrackSize::Minmax(ref to_min, ref to_max)) => {
+                Ok(generics::TrackSize::Minmax(
+                    animate_with_discrete_fallback(from_min, to_min, procedure)?,
+                    animate_with_discrete_fallback(from_max, to_max, procedure)?,
+                ))
+            },
+            (&generics::TrackSize::FitContent(ref from),
+             &generics::TrackSize::FitContent(ref to)) => {
+                animate_with_discrete_fallback(from, to, procedure)
+                    .map(generics::TrackSize::FitContent)
+            },
+            (_, _) => discrete(self, other, procedure),
+        }
+    }
+}
+
+impl Animate for generics::TrackRepeat<LengthPercentage, Integer>
+where
+    generics::RepeatCount<Integer>: PartialEq,
+{
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        // If the keyword, auto-fit/fill, is the same it can result in different
+        // number of tracks. For both auto-fit/fill, the number of columns isn't
+        // known until you do layout since it depends on the container size, item
+        // placement and other factors, so we cannot do the correct interpolation
+        // by computed values. Therefore, return Err(()) if it's keywords. If it
+        // is Number, we support animation only if the count is the same and the
+        // length of track_sizes is the same.
+        // https://github.com/w3c/csswg-drafts/issues/3503
+        match (&self.count, &other.count) {
+            (&generics::RepeatCount::Number(from),
+             &generics::RepeatCount::Number(to)) if from == to => (),
+            (_, _) => return Err(()),
+        }
+
+        // The length of track_sizes should be matched.
+        if self.track_sizes.len() != other.track_sizes.len() {
+            return Err(());
+        }
+
+        let count = self.count;
+        let track_sizes = self.track_sizes
+            .iter()
+            .zip(other.track_sizes.iter())
+            .map(|(a, b)| a.animate(b, procedure))
+            .collect::<Result<Vec<_>, _>>()?;
+
+        // The length of |line_names| is always 0 or N+1, where N is the length
+        // of |track_sizes|. Besides, <line-names> is always discrete.
+        let line_names = discrete(&self.line_names, &other.line_names, procedure)?;
+
+        Ok(generics::TrackRepeat { count, line_names, track_sizes })
+    }
+}
+
+impl Animate for TrackList {
+    // Based on https://github.com/w3c/csswg-drafts/issues/3201:
+    // 1. Check interpolation type per track, so we need to handle discrete animations
+    //    in TrackSize, so any Err(()) returned from TrackSize doesn't make all TrackSize
+    //    fallback to discrete animation.
+    // 2. line-names is always discrete.
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        if self.values.len() != other.values.len() {
+            return Err(());
+        }
+
+        if self.list_type != other.list_type {
+            return Err(());
+        }
+
+        // For now, repeat(auto-fill/auto-fit, ...) is not animatable. TrackRepeat will
+        // return Err(()) if we use keywords. Therefore, we can early return here to avoid
+        // traversing |values| in <auto-track-list>. This may be updated in the future.
+        // https://github.com/w3c/csswg-drafts/issues/3503
+        if let generics::TrackListType::Auto(_) = self.list_type {
+            return Err(());
+        }
+
+        let list_type = self.list_type;
+        let auto_repeat = self.auto_repeat.animate(&other.auto_repeat, procedure)?;
+        let values = self.values
+            .iter()
+            .zip(other.values.iter())
+            .map(|(a, b)| a.animate(b, procedure))
+            .collect::<Result<Vec<_>, _>>()?;
+        // The length of |line_names| is always 0 or N+1, where N is the length
+        // of |track_sizes|. Besides, <line-names> is always discrete.
+        let line_names = discrete(&self.line_names, &other.line_names, procedure)?;
+
+        Ok(TrackList { list_type, values, line_names, auto_repeat })
+    }
+}
+
+impl ComputeSquaredDistance for GridTemplateComponent {
+    #[inline]
+    fn compute_squared_distance(&self, _other: &Self) -> Result<SquaredDistance, ()> {
+        // TODO: Bug 1518585, we should implement ComputeSquaredDistance.
+        Err(())
+    }
+}
+
+impl ToAnimatedZero for GridTemplateComponent {
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        // It's not clear to get a zero grid track list based on the current definition
+        // of spec, so we return Err(()) directly.
+        Err(())
+    }
+}