Allow animated FloatingPanelController relayout

Framework/Sources/FloatingPanelController.swift

@@ -572,6 +572,26 @@ open class FloatingPanelController: UIViewController {
         activateLayout()
     }
 
+    /// Prepares an update to the layout object from the delegate.
+    ///
+    /// This method must eventually be balanced with a call to endUpdateLayout().
+    /// It can be called in an animation block.
+    public func beginUpdateLayout() {
+        floatingPanel.layoutAdapter.beginUpdateHeight()
+    }
+
+    /// Balances a previous call to beginUpdateLayout(), and updates the layout
+    /// object from the delegate and lays out the views managed by the
+    /// controller immediately.
+    ///
+    /// This method updates the `FloatingPanelLayout` object from the delegate and
+    /// then it calls `layoutIfNeeded()` of the root view to force the view
+    /// to update the floating panel's layout immediately. It can be called in an
+    /// animation block.
+    public func endUpdateLayout() {
+        updateLayout()
+    }
+
     /// Returns the y-coordinate of the point at the origin of the surface view.
     public func originYOfSurface(for pos: FloatingPanelPosition) -> CGFloat {
         return floatingPanel.layoutAdapter.positionY(for: pos)

Framework/Sources/FloatingPanelLayout.swift

@@ -440,6 +440,26 @@ class FloatingPanelLayoutAdapter {
         surfaceView.bottomOverflow = vc.view.bounds.height + layout.topInteractionBuffer
     }
 
+    // Support for FloatingPanelController.beginUpdateLayout/endUpdateLayout.
+    //
+    /// This method must eventually be balanced with a call to updateHeight().
+    func beginUpdateHeight() {
+        guard vc != nil else { return }
+
+        // Deactivate the hard height constraint until updateHeight()
+        if let const = self.heightConstraint {
+            NSLayoutConstraint.deactivate([const])
+        }
+
+        // Activate a very weak temporary height constraint which makes sure the
+        // surface view is not totally free-form.
+        let heightConstraint = surfaceView.heightAnchor.constraint(equalToConstant: 0)
+        heightConstraint.priority = UILayoutPriority(1)
+
+        NSLayoutConstraint.activate([heightConstraint])
+        self.heightConstraint = heightConstraint
+    }
+
     func updateInteractiveTopConstraint(diff: CGFloat, allowsTopBuffer: Bool, with behavior: FloatingPanelBehavior) {
         defer {
             layoutSurfaceIfNeeded() // MUST be called to update `surfaceView.frame`