feat: Add gradient background in seismic cards

Resolves: https://gitlab.steamware.net/eqn/eqn.ios/-/issues/74

Sources/Earthquake Network/Controllers/Seismic Networks/Cells/SeismicNetworkTableViewCell.swift

@@ -75,6 +75,17 @@ class SeismicNetworkTableViewCell: UITableViewCell {
         view.layer.shadowRadius = 2
         return view
     }()
+    
+    private lazy var gradientView: UIImageView = {
+        // Per gestire il gradiente, utilizziamo una image view in cui inseriamo un'immagine
+        // creata ad-hoc con il gradiente desiderato.
+        // Le prove fatte utilizzando una view normale sono fallite perchè al momento di
+        // disegnare la view non abbiamo le misure corrette.
+        let view = UIImageView(frame: .zero)
+        view.translatesAutoresizingMaskIntoConstraints = false
+        view.contentMode = .scaleAspectFill
+        return view
+    }()
         
     private lazy var placeLabel: UILabel = {
         let label = UILabel()
@@ -195,7 +206,11 @@ class SeismicNetworkTableViewCell: UITableViewCell {
         containerView.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: -8.0).isActive = true
         containerView.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: 8.0).isActive = true
         containerView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -4.0).isActive = true
+        containerView.clipsToBounds = true
 
+        containerView.addSubview(gradientView)
+        gradientView.constraint(to: containerView)
+        
         // this variable is used to keep track of the previous view, in order to attach proper constraints
         var previousView: UIView = containerView
         
@@ -377,8 +392,12 @@ class SeismicNetworkTableViewCell: UITableViewCell {
         
         let viewModel = SeismicNetworkViewModel(seismic: seismic)
         
-        containerView.backgroundColor = colors?.startColor
-                
+        if let colors = colors {
+            gradientView.image = .gradient(from: colors.startColor, to: colors.endColor, with: .init(origin: .zero, size: .init(width: 400, height: 300)))
+        } else {
+            gradientView.image = nil
+        }
+                        
         // update seismic data
         placeLabel.text = viewModel.place
         networkLabel.text = String(format: NSLocalizedString("official_provider", comment: ""), viewModel.network)