Add animation on the timeline content, to avoid glitch (#1323)

features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/timeline/TimelineView.kt

@@ -47,6 +47,7 @@ import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
 import androidx.compose.ui.draw.rotate
 import androidx.compose.ui.platform.LocalInspectionMode
 import androidx.compose.ui.res.pluralStringResource
@@ -63,8 +64,9 @@ import io.element.android.features.messages.impl.timeline.model.event.TimelineIt
 import io.element.android.features.messages.impl.timeline.model.event.TimelineItemEventContentProvider
 import io.element.android.features.messages.impl.timeline.model.event.TimelineItemStateContent
 import io.element.android.features.messages.impl.timeline.model.event.canBeRepliedTo
-import io.element.android.libraries.designsystem.preview.PreviewsDayNight
+import io.element.android.libraries.designsystem.animation.alphaAnimation
 import io.element.android.libraries.designsystem.preview.ElementPreview
+import io.element.android.libraries.designsystem.preview.PreviewsDayNight
 import io.element.android.libraries.designsystem.theme.components.FloatingActionButton
 import io.element.android.libraries.designsystem.theme.components.Icon
 import io.element.android.libraries.designsystem.utils.CommonDrawables
@@ -105,7 +107,10 @@ fun TimelineView(
         state.eventSink(TimelineEvents.PollAnswerSelected(pollStartId, answerId))
     }
 
-    Box(modifier = modifier) {
+    // Animate alpha when timeline is first displayed, to avoid flashes or glitching when viewing rooms
+    val alpha by alphaAnimation(label = "alpha for timeline")
+
+    Box(modifier = modifier.alpha(alpha)) {
         LazyColumn(
             modifier = Modifier.fillMaxSize(),
             state = lazyListState,
@@ -315,7 +320,9 @@ private fun JumpToBottomButton(
             contentColor = ElementTheme.colors.iconSecondary
         ) {
             Icon(
-                modifier = Modifier.size(24.dp).rotate(90f),
+                modifier = Modifier
+                    .size(24.dp)
+                    .rotate(90f),
                 resourceId = CommonDrawables.ic_compound_arrow_right,
                 contentDescription = "",
             )

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/animation/AlphaAnimation.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.element.android.libraries.designsystem.animation
+
+import androidx.compose.animation.core.animateFloatAsState
+import androidx.compose.animation.core.tween
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.State
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.platform.LocalInspectionMode
+
+@Composable
+fun alphaAnimation(
+    fromAlpha: Float = 0f,
+    toAlpha: Float = 1f,
+    delayMillis: Int = 150,
+    durationMillis: Int = 150,
+    label: String = "AlphaAnimation",
+): State<Float> {
+    val firstAlpha = if (LocalInspectionMode.current) 1f else fromAlpha
+    var alpha by remember { mutableFloatStateOf(firstAlpha) }
+    LaunchedEffect(Unit) { alpha = toAlpha }
+    return animateFloatAsState(
+        targetValue = alpha,
+        animationSpec = tween(
+            delayMillis = delayMillis,
+            durationMillis = durationMillis,
+        ),
+        label = label
+    )
+}