Add helper text when dragging filter at a low velocity.

app/src/main/java/org/thoughtcrime/securesms/conversationlist/chatfilter/ConversationListFilterPullView.kt

@@ -1,15 +1,18 @@
 package org.thoughtcrime.securesms.conversationlist.chatfilter
 
 import android.animation.Animator
-import android.animation.FloatEvaluator
 import android.animation.ObjectAnimator
 import android.content.Context
 import android.util.AttributeSet
 import android.widget.FrameLayout
 import androidx.core.animation.doOnEnd
 import org.thoughtcrime.securesms.R
+import org.thoughtcrime.securesms.animation.AnimationCompleteListener
 import org.thoughtcrime.securesms.databinding.ConversationListFilterPullViewBinding
 import org.thoughtcrime.securesms.util.VibrateUtil
+import kotlin.math.max
+import kotlin.math.min
+import kotlin.time.Duration.Companion.milliseconds
 
 /**
  * Encapsulates the push / pull latch for enabling and disabling
@@ -24,7 +27,9 @@ class ConversationListFilterPullView @JvmOverloads constructor(
 ) : FrameLayout(context, attrs) {
 
   companion object {
-    private val EVAL = FloatEvaluator()
+    private const val ANIMATE_HELP_TEXT_VELOCITY_THRESHOLD = 1f
+    private const val ANIMATE_HELP_TEXT_THRESHOLD = 30
+    private const val ANIMATE_HELP_TEXT_START_FRACTION = 0.35f
   }
 
   private val binding: ConversationListFilterPullViewBinding
@@ -42,6 +47,9 @@ class ConversationListFilterPullView @JvmOverloads constructor(
   }
 
   private var pillAnimator: Animator? = null
+  private val velocityTracker = ProgressVelocityTracker(5)
+  private var animateHelpText = 0
+  private var helpTextStartFraction = 0.35f
 
   fun onUserDrag(progress: Float) {
     binding.filterCircle.textFieldMetrics = Pair(binding.filterText.width, binding.filterText.height)
@@ -52,11 +60,33 @@ class ConversationListFilterPullView @JvmOverloads constructor(
     } else if (state == FilterPullState.CLOSED && progress >= 1f) {
       setState(FilterPullState.OPEN_APEX)
       vibrate()
+      resetHelpText()
     } else if (state == FilterPullState.OPEN && progress >= 1f) {
       setState(FilterPullState.CLOSE_APEX)
       vibrate()
     }
 
+    if (state == FilterPullState.CLOSED && animateHelpText < ANIMATE_HELP_TEXT_THRESHOLD) {
+      velocityTracker.submitProgress(progress, System.currentTimeMillis().milliseconds)
+      val velocity = velocityTracker.calculateVelocity()
+      animateHelpText = if (velocity > 0f && velocity < ANIMATE_HELP_TEXT_VELOCITY_THRESHOLD) {
+        min(Int.MAX_VALUE, animateHelpText + 1)
+      } else {
+        max(0, animateHelpText - 1)
+      }
+
+      if (animateHelpText >= ANIMATE_HELP_TEXT_THRESHOLD) {
+        helpTextStartFraction = max(progress, ANIMATE_HELP_TEXT_START_FRACTION)
+      }
+    }
+
+    if (animateHelpText >= ANIMATE_HELP_TEXT_THRESHOLD) {
+      binding.helpText.visibility = VISIBLE
+    }
+
+    binding.helpText.alpha = max(0f, FilterLerp.getHelpTextAlphaLerp(progress, helpTextStartFraction))
+    binding.helpText.translationY = FilterLerp.getPillLerp(progress)
+
     if (state == FilterPullState.OPEN || state == FilterPullState.OPEN_APEX || state == FilterPullState.CLOSE_APEX || state == FilterPullState.CLOSING) {
       binding.filterText.translationY = FilterLerp.getPillLerp(progress)
     } else {
@@ -96,6 +126,17 @@ class ConversationListFilterPullView @JvmOverloads constructor(
     animatePillOut()
   }
 
+  private fun resetHelpText() {
+    velocityTracker.clear()
+    animateHelpText = 0
+    helpTextStartFraction = ANIMATE_HELP_TEXT_START_FRACTION
+    binding.helpText.animate().alpha(0f).setListener(object : AnimationCompleteListener() {
+      override fun onAnimationEnd(animation: Animator?) {
+        binding.helpText.visibility = INVISIBLE
+      }
+    })
+  }
+
   private fun animatePillIn() {
     binding.filterText.visibility = VISIBLE
     binding.filterText.alpha = 0f

app/src/main/java/org/thoughtcrime/securesms/conversationlist/chatfilter/FilterLerp.kt

@@ -1,6 +1,7 @@
 package org.thoughtcrime.securesms.conversationlist.chatfilter
 
 import android.animation.FloatEvaluator
+import androidx.annotation.FloatRange
 import org.signal.core.util.dp
 
 /**
@@ -36,6 +37,15 @@ object FilterLerp {
     Point(1f, FILTER_APEX * 0.55f)
   )
 
+  private fun helpTextAlphaLerp(@FloatRange(from = 0.0, to = 1.0) startFraction: Float) = getFn(
+    Point(startFraction, 0f),
+    Point(1f, 1f)
+  )
+
+  fun getHelpTextAlphaLerp(fraction: Float, startFraction: Float): Float {
+    return getLerp(fraction, helpTextAlphaLerp(startFraction))
+  }
+
   /**
    * Get the LERP for the "Filter enabled" pill.
    */

app/src/main/java/org/thoughtcrime/securesms/conversationlist/chatfilter/ProgressVelocityTracker.kt

@@ -0,0 +1,49 @@
+package org.thoughtcrime.securesms.conversationlist.chatfilter
+
+import androidx.annotation.FloatRange
+import org.whispersystems.signalservice.api.util.Preconditions
+import kotlin.time.Duration
+
+/**
+ * Velocity tracker based off % progress.
+ * Units are thus in %/s
+ *
+ * This class only supports a single axial value.
+ */
+class ProgressVelocityTracker(@androidx.annotation.IntRange(from = 0) capacity: Int) {
+
+  private val progressBuffer = RingBuffer<Float>(capacity)
+  private val durationBuffer = RingBuffer<Duration>(capacity)
+
+  fun submitProgress(@FloatRange(from = 0.0, to = 1.0) progress: Float, duration: Duration) {
+    progressBuffer.add(progress)
+    durationBuffer.add(duration)
+  }
+
+  fun clear() {
+    progressBuffer.clear()
+    durationBuffer.clear()
+  }
+
+  /**
+   * Calculates the average velocity. The units are %/s
+   */
+  fun calculateVelocity(): Float {
+    Preconditions.checkState(progressBuffer.size() == durationBuffer.size())
+
+    if (progressBuffer.size() < 2) {
+      return 0f
+    }
+
+    var progressDelta: Float
+    var timeDelta: Duration
+
+    val percentPerMillisecond = (0 until progressBuffer.size()).windowed(2).map { (indexA, indexB) ->
+      progressDelta = progressBuffer[indexB] - progressBuffer[indexA]
+      timeDelta = durationBuffer[indexB] - durationBuffer[indexA]
+      progressDelta / timeDelta.inWholeMilliseconds
+    }.sum() / (progressBuffer.size() - 1)
+
+    return percentPerMillisecond * 1000
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/conversationlist/chatfilter/RingBuffer.kt

@@ -0,0 +1,21 @@
+package org.thoughtcrime.securesms.conversationlist.chatfilter
+
+import androidx.collection.CircularArray
+
+class RingBuffer<T>(@androidx.annotation.IntRange(from = 0) private val capacity: Int) {
+  private val buffer = CircularArray<T>(capacity)
+
+  fun add(t: T) {
+    if (size() == capacity) {
+      buffer.popFirst()
+    }
+
+    buffer.addLast(t)
+  }
+
+  fun size() = buffer.size()
+
+  fun clear() = buffer.clear()
+
+  operator fun get(index: Int): T = buffer.get(index)
+}