Refactor wallet invoice URL handling

This is a minor refactor on the way wallet invoice URLs are handled, in
order to better fit the interface, enforce the design pattern, and avoid
side-effects in a particular function that handles opening URLs.

This design pattern was introduced to prevent issues on the previous
pattern, where URL handling was done with side-effects inside multiple
levels of nested logic and separate function calls, which would make
debugging very difficult, and cause the app to fail silently.

Closes: https://github.com/damus-io/damus/issues/3023
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Components/InvoiceView.swift

@@ -94,26 +94,30 @@ enum OpenWalletError: Error {
 }
 
 func open_with_wallet(wallet: Wallet.Model, invoice: String) throws {
+    let url = try getUrlToOpen(invoice: invoice, with: wallet)
+    this_app.open(url)
+}
+
+func getUrlToOpen(invoice: String, with wallet: Wallet.Model) throws(OpenWalletError) -> URL {
     if let url = URL(string: "\(wallet.link)\(invoice)"), this_app.canOpenURL(url) {
-        this_app.open(url)
+        return url
     } else {
         guard let store_link = wallet.appStoreLink else {
-            throw OpenWalletError.no_wallet_to_open
+            throw .no_wallet_to_open
         }
         
         guard let url = URL(string: store_link) else {
-            throw OpenWalletError.store_link_invalid
+            throw .store_link_invalid
         }
         
         guard this_app.canOpenURL(url) else {
-            throw OpenWalletError.system_cannot_open_store_link
+            throw .system_cannot_open_store_link
         }
         
-        this_app.open(url)
+        return url
     }
 }
 
-
 let test_invoice = Invoice(description: .description("this is a description"), amount: .specific(10000), string: "lnbc100n1p357sl0sp5t9n56wdztun39lgdqlr30xqwksg3k69q4q2rkr52aplujw0esn0qpp5mrqgljk62z20q4nvgr6lzcyn6fhylzccwdvu4k77apg3zmrkujjqdpzw35xjueqd9ejqcfqv3jhxcmjd9c8g6t0dcxqyjw5qcqpjrzjqt56h4gvp5yx36u2uzqa6qwcsk3e2duunfxppzj9vhypc3wfe2wswz607uqq3xqqqsqqqqqqqqqqqlqqyg9qyysgqagx5h20aeulj3gdwx3kxs8u9f4mcakdkwuakasamm9562ffyr9en8yg20lg0ygnr9zpwp68524kmda0t5xp2wytex35pu8hapyjajxqpsql29r", expiry: 604800, payment_hash: Data(), created_at: 1666139119)
 
 struct InvoiceView_Previews: PreviewProvider {

damus/ContentView.swift

@@ -742,6 +742,8 @@ struct ContentView: View {
         case route(Route)
         /// Open a sheet
         case sheet(Sheets)
+        /// Open an external URL
+        case external_url(URL)
         /// Do nothing.
         ///
         /// ## Implementation notes
@@ -758,6 +760,8 @@ struct ContentView: View {
             navigationCoordinator.push(route: route)
         case .sheet(let sheet):
             self.active_sheet = sheet
+        case .external_url(let url):
+            this_app.open(url)
         case .no_action:
             return
         }

damus/Models/URLHandler.swift

@@ -47,13 +47,10 @@ struct DamusURLHandler {
             if damus_state.settings.show_wallet_selector {
                 return .sheet(.select_wallet(invoice: invoice.string))
             } else {
-                do {
-                    try open_with_wallet(wallet: damus_state.settings.default_wallet.model, invoice: invoice.string)
-                    return .no_action
-                }
-                catch {
+                guard let url = try? getUrlToOpen(invoice: invoice.string, with: damus_state.settings.default_wallet.model) else {
                     return .sheet(.select_wallet(invoice: invoice.string))
                 }
+                return .external_url(url)
             }
         case nil:
             break