[Slider] Fix slider label not moving while scrolling

Resolves #3848
Resolves #3847

GIT_ORIGIN_REV_ID=630698384082464cfb40d75156ec09abfc829bed
PiperOrigin-RevId: 584067588

Author: manabu-nakamura

lib/java/com/google/android/material/slider/BaseSlider.java

@@ -65,6 +65,7 @@
 import android.view.ViewConfiguration;
 import android.view.ViewGroup;
 import android.view.ViewParent;
+import android.view.ViewTreeObserver;
 import android.view.accessibility.AccessibilityEvent;
 import android.view.accessibility.AccessibilityManager;
 import android.widget.SeekBar;
@@ -338,6 +339,26 @@ abstract class BaseSlider<
   private float touchPosition;
   @SeparationUnit private int separationUnit = UNIT_PX;
 
+  @NonNull private final ViewTreeObserver.OnScrollChangedListener onScrollChangedListener = () -> {
+    if (shouldAlwaysShowLabel() && isEnabled()) {
+      Rect contentViewBounds = new Rect();
+      ViewUtils.getContentView(this).getHitRect(contentViewBounds);
+      boolean isSliderVisibleOnScreen = getLocalVisibleRect(contentViewBounds);
+      for (int i = 0; i < labels.size(); i++) {
+        TooltipDrawable label = labels.get(i);
+        // Get associated value for label
+        if (i < values.size()) {
+          positionLabel(label, values.get(i));
+        }
+        if (isSliderVisibleOnScreen) {
+          ViewUtils.getContentViewOverlay(this).add(label);
+        } else {
+          ViewUtils.getContentViewOverlay(this).remove(label);
+        }
+      }
+    }
+  };
+
   /**
    * Determines the behavior of the label which can be any of the following.
    *
@@ -1865,6 +1886,7 @@ public void setEnabled(boolean enabled) {
   @Override
   protected void onAttachedToWindow() {
     super.onAttachedToWindow();
+    getViewTreeObserver().addOnScrollChangedListener(onScrollChangedListener);
     // The label is attached on the Overlay relative to the content.
     for (TooltipDrawable label : labels) {
       attachLabelToContentView(label);
@@ -1885,7 +1907,7 @@ protected void onDetachedFromWindow() {
     for (TooltipDrawable label : labels) {
       detachLabelFromContentView(label);
     }
-
+    getViewTreeObserver().removeOnScrollChangedListener(onScrollChangedListener);
     super.onDetachedFromWindow();
   }
 
@@ -2646,7 +2668,11 @@ private String formatValue(float value) {
 
   private void setValueForLabel(TooltipDrawable label, float value) {
     label.setText(formatValue(value));
+    positionLabel(label, value);
+    ViewUtils.getContentViewOverlay(this).add(label);
+  }
 
+  private void positionLabel(TooltipDrawable label, float value) {
     int left =
         trackSidePadding
             + (int) (normalizeValue(value) * trackWidth)
@@ -2659,8 +2685,6 @@ private void setValueForLabel(TooltipDrawable label, float value) {
     Rect rect = new Rect(label.getBounds());
     DescendantOffsetUtils.offsetDescendantRect(ViewUtils.getContentView(this), this, rect);
     label.setBounds(rect);
-
-    ViewUtils.getContentViewOverlay(this).add(label);
   }
 
   private void invalidateTrack() {