Make StatusContentContainer play nice with hiding subviews

Tusker/Views/Status/StatusContentContainer.swift

@@ -30,48 +30,84 @@ class StatusContentContainer: UIView {
     
     let pollView = StatusPollView()
     
-    private var lastSubviewBottomConstraint: NSLayoutConstraint!
+    private var arrangedSubviews: [UIView] {
+        [contentTextView, cardView, attachmentsView, pollView]
+    }
+    
+    private var isHiddenObservations: [NSKeyValueObservation] = []
+    
+    private var verticalConstraints: [NSLayoutConstraint] = []
+    private var lastSubviewBottomConstraint: NSLayoutConstraint?
     private var zeroHeightConstraint: NSLayoutConstraint!
    
     override init(frame: CGRect) {
         super.init(frame: frame)
         
-        let subviews = [contentTextView, cardView, attachmentsView, pollView]
+        for subview in arrangedSubviews {
-        for (index, subview) in subviews.enumerated() {
             subview.translatesAutoresizingMaskIntoConstraints = false
             addSubview(subview)
             
-            let topConstraint: NSLayoutConstraint
-            if index == 0 {
-                topConstraint = subview.topAnchor.constraint(equalTo: topAnchor)
-            } else {
-                topConstraint = subview.topAnchor.constraint(equalTo: subviews[index - 1].bottomAnchor, constant: 4)
-            }
-            
             NSLayoutConstraint.activate([
-                topConstraint,
                 subview.leadingAnchor.constraint(equalTo: leadingAnchor),
                 subview.trailingAnchor.constraint(equalTo: trailingAnchor),
             ])
         }
         
-        // these constraints need to have low priority so that during the collapse/expand animation, the content container is the view that shrinks/expands
+        // this constraint needs to have low priority so that during the collapse/expand animation, the content container is the view that shrinks/expands
-        lastSubviewBottomConstraint = subviews.last!.bottomAnchor.constraint(equalTo: bottomAnchor)
-        lastSubviewBottomConstraint.isActive = true
-        lastSubviewBottomConstraint.priority = .defaultLow
         zeroHeightConstraint = heightAnchor.constraint(equalToConstant: 0)
         zeroHeightConstraint.priority = .defaultLow
         
+        setNeedsUpdateConstraints()
+        
         // mask to bounds so that the during the expand/collapse animation, subviews are clipped
         layer.masksToBounds = true
+        
+        isHiddenObservations = arrangedSubviews.map {
+            $0.observe(\.isHidden) { [unowned self] _, _ in
+                self.setNeedsUpdateConstraints()
+            }
+        }
     }
     
     required init?(coder: NSCoder) {
         fatalError("init(coder:) has not been implemented")
     }
     
+    override func updateConstraints() {
+        NSLayoutConstraint.deactivate(verticalConstraints)
+        verticalConstraints = []
+        var lastVisibleSubview: UIView?
+        
+        for subview in arrangedSubviews {
+            guard !subview.isHidden else {
+                continue
+            }
+            
+            if let lastVisibleSubview {
+                verticalConstraints.append(subview.topAnchor.constraint(equalTo: lastVisibleSubview.bottomAnchor, constant: 4))
+            } else {
+                verticalConstraints.append(subview.topAnchor.constraint(equalTo: topAnchor))
+            }
+            
+            lastVisibleSubview = subview
+        }
+        
+        NSLayoutConstraint.activate(verticalConstraints)
+        
+        lastSubviewBottomConstraint?.isActive = false
+        // this constraint needs to have low priority so that during the collapse/expand animation, the content container is the view that shrinks/expands
+        lastSubviewBottomConstraint = subviews.last(where: { !$0.isHidden })!.bottomAnchor.constraint(equalTo: bottomAnchor)
+        lastSubviewBottomConstraint!.isActive = true
+        lastSubviewBottomConstraint!.priority = .defaultLow
+        
+        super.updateConstraints()
+    }
+    
     func setCollapsed(_ collapsed: Bool) {
-        lastSubviewBottomConstraint.isActive = !collapsed
+        // ensure that we have a lastSubviewBottomConstraint
+        updateConstraintsIfNeeded()
+        // force unwrap because the content container should always have at least one view
+        lastSubviewBottomConstraint!.isActive = !collapsed
         zeroHeightConstraint.isActive = collapsed
     }