[Shape] Restore alpha06 corner animation behavior

[jayyaj12]

Summary

Fix corner morph animation regression in MaterialButtonGroup introduced after commit ea9d250.

Problem

After ea9d250, corner morph animations in MaterialButtonGroup are interrupted during state changes (pressed, focused, etc.).

Root cause:
In MaterialShapeDrawable.onBoundsChange(), animations are now always skipped:

// After ea9d25050
updateShape(getState(), /* skipAnimation= */ true);  // Always true!

This causes springAnimatedCornerSizes to be immediately set to target values, interrupting any ongoing animations.

Reproduction:

1. Use MaterialButtonGroup with stateful corner shapes
2. Press a button
3. Observe: corner animation is interrupted (not smooth)

Why MaterialButtonGroup Cannot Fix This

I initially attempted to fix this in MaterialButtonGroup by:

1. Caching shapes to avoid redundant setShapeAppearance() calls
2. Comparing corner sizes before applying shapes
3. One-time initialization to prevent re-setting shapes

However, all approaches failed because:

// MaterialButton state change flow:
Button.setPressed(true)
  → Drawable.setState(new int[]{android.R.attr.state_pressed})
    → MaterialShapeDrawable.onStateChange(state)
      → updateShape(state, skipAnimation=false)  // Animation requested
        → [Animation starts...]
    → MaterialShapeDrawable.onBoundsChange(bounds)  // Called by framework
      → updateShape(state, skipAnimation=true)     // Animation force-skipped!
        → springAnimatedCornerSizes = targetValues  // Animation interrupted

The critical issue:

• onBoundsChange() is called by the Android framework after state changes
• With skipAnimation=true hardcoded, there's no way for MaterialButtonGroup to preserve animations
• Any attempt at the MaterialButtonGroup level is overridden by onBoundsChange()

Why bounds change during state changes:

• Ripple effects may adjust drawable bounds
• Elevation changes affect shadow bounds
• Padding adjustments for pressed states
• Even if bounds values are identical, onBoundsChange() is still called

Solution

The fix must be in MaterialShapeDrawable to distinguish between:

• Initial layout (first non-empty bounds) → Skip for performance
• Subsequent calls (state changes, etc.) → Allow animation for smooth UX

Restore the boundsIsEmpty flag approach from alpha06:

private boolean boundsIsEmpty = true;

protected void onBoundsChange(Rect bounds) {
    // ...
    if (drawableState.shapeAppearance.isStateful() && !bounds.isEmpty()) {
        // Skip only on FIRST non-empty bounds
        updateShape(getState(), /* skipAnimation= */ boundsIsEmpty);
    }
    boundsIsEmpty = bounds.isEmpty();
}

Why this works:

• First call (initial layout): boundsIsEmpty=true → Skip (fast initialization)
• Subsequent calls: boundsIsEmpty=false → Animate (smooth UX)
• No way to break this from external code

Alternative Approaches Considered

❌ Option 1: Fix in MaterialButtonGroup

Tried: Cache shapes, prevent redundant updates
Failed: onBoundsChange() always overrides with skipAnimation=true

❌ Option 2: Track actual bounds changes

Tried: Compare previousBounds to detect real size changes
Failed: Bounds may change slightly during state transitions (ripple, elevation)

✅ Option 3: Restore boundsIsEmpty logic (this PR)

Works: Distinguishes initial layout from subsequent calls at the source

Implementation

• Added boundsIsEmpty flag to track initial layout
• Modified onBoundsChange() to skip animation only on first non-empty bounds
• Added inline comments explaining the logic and referencing issue [Button] Corner morph animation broken in ButtonGroup #4990

Testing

• ✅ Verified smooth corner animations in MaterialButtonGroup during state changes
• ✅ Confirmed no performance regression on initial layout
• ✅ Tested across multiple state transitions (pressed, focused, disabled)
• ✅ Verified no impact on other components using MaterialShapeDrawable

Changes

Modified:

• lib/java/com/google/android/material/shape/MaterialShapeDrawable.java
  • Restored boundsIsEmpty flag
  • Updated onBoundsChange() animation skip logic

Fixes #4990

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Discussion

@pekingme I understand the intent of ea9d250 was to improve performance by skipping animations during bounds changes. However, this also affects state-only changes where animation should be preserved.

Would you be open to this refinement that maintains the performance benefit for initial layout while allowing animations for state transitions?