Allow empty keyframe and keyframes with non-animatable properties.

We need to create CSS animations that have empty keyframe or keyframes
which have only invalid properties or non-animatable properties to fire
animation events for such animations.

components/style/keyframes.rs

@@ -266,50 +266,51 @@ fn get_animated_properties(keyframe: &Keyframe) -> Vec<TransitionProperty> {
 impl KeyframesAnimation {
     /// Create a keyframes animation from a given list of keyframes.
     ///
-    /// This will return `None` if the list of keyframes is empty, or there are
-    /// no animated properties obtained from the keyframes.
+    /// This will return a keyframe animation with empty steps and
+    /// properties_changed if the list of keyframes is empty, or there are no
+    //  animated properties obtained from the keyframes.
     ///
     /// Otherwise, this will compute and sort the steps used for the animation,
     /// and return the animation object.
-    pub fn from_keyframes(keyframes: &[Arc<RwLock<Keyframe>>]) -> Option<Self> {
+    pub fn from_keyframes(keyframes: &[Arc<RwLock<Keyframe>>]) -> Self {
+        let mut result = KeyframesAnimation {
+            steps: vec![],
+            properties_changed: vec![],
+        };
+
         if keyframes.is_empty() {
-            return None;
+            return result;
         }
 
-        let animated_properties = get_animated_properties(&keyframes[0].read());
-        if animated_properties.is_empty() {
-            return None;
+        result.properties_changed = get_animated_properties(&keyframes[0].read());
+        if result.properties_changed.is_empty() {
+            return result;
         }
 
-        let mut steps = vec![];
-
         for keyframe in keyframes {
             let keyframe = keyframe.read();
             for percentage in keyframe.selector.0.iter() {
-                steps.push(KeyframesStep::new(*percentage, KeyframesStepValue::Declarations {
+                result.steps.push(KeyframesStep::new(*percentage, KeyframesStepValue::Declarations {
                     block: keyframe.block.clone(),
                 }));
             }
         }
 
         // Sort by the start percentage, so we can easily find a frame.
-        steps.sort_by_key(|step| step.start_percentage);
+        result.steps.sort_by_key(|step| step.start_percentage);
 
         // Prepend autogenerated keyframes if appropriate.
-        if steps[0].start_percentage.0 != 0. {
-            steps.insert(0, KeyframesStep::new(KeyframePercentage::new(0.),
-                                               KeyframesStepValue::ComputedValues));
+        if result.steps[0].start_percentage.0 != 0. {
+            result.steps.insert(0, KeyframesStep::new(KeyframePercentage::new(0.),
+                                                      KeyframesStepValue::ComputedValues));
         }
 
-        if steps.last().unwrap().start_percentage.0 != 1. {
-            steps.push(KeyframesStep::new(KeyframePercentage::new(1.),
-                                          KeyframesStepValue::ComputedValues));
+        if result.steps.last().unwrap().start_percentage.0 != 1. {
+            result.steps.push(KeyframesStep::new(KeyframePercentage::new(1.),
+                                                 KeyframesStepValue::ComputedValues));
         }
 
-        Some(KeyframesAnimation {
-            steps: steps,
-            properties_changed: animated_properties,
-        })
+        result
     }
 }