add(zig-interface): dependency to check interface contracts at comptime

The described interfaces for `Widget` and `Layout` are now defined and
correspondingly checked at comptime.

src/widget.zig

@@ -1,8 +1,5 @@
 //! Dynamic dispatch for widget implementations.
-//! Each widget should at least implement these functions:
-//! - handle(this: *@This(), event: Event) ?Event {}
-//! - render(this: *@This(), renderer: *Renderer) !void {}
-//! - deinit(this: *@This()) void {}
+//! Each `Widget` has to implement the `WidgetInterface`
 //!
 //! Create a `Widget` using `createFrom(object: anytype)` and use them through
 //! the defined interface. The widget will take care of calling the correct
@@ -21,9 +18,14 @@ pub fn Widget(comptime Event: type, comptime Renderer: type) type {
     if (!isTaggedUnion(Event)) {
         @compileError("Provided user event `Event` for `Layout(comptime Event: type)` is not of type `union(enum)`.");
     }
-    return struct {
+    const Type = struct {
         const WidgetType = @This();
         const Ptr = usize;
+        pub const Interface = @import("interface").Interface(.{
+            .handle = fn (anytype, Event) ?Event,
+            .render = fn (anytype, *Renderer) anyerror!void,
+            .deinit = fn (anytype) void,
+        }, .{});
 
         const VTable = struct {
             handle: *const fn (this: *WidgetType, event: Event) ?Event,
@@ -88,8 +90,13 @@ pub fn Widget(comptime Event: type, comptime Renderer: type) type {
             };
         }
 
-        // TODO: import and export of `Widget` implementations (with corresponding initialization using `Event`)
+        // import and export of `Widget` implementations
         pub const RawText = @import("widget/RawText.zig").Widget(Event, Renderer);
         pub const Spacer = @import("widget/Spacer.zig").Widget(Event, Renderer);
     };
+    // test widget implementation satisfies the interface
+    comptime Type.Interface.satisfiedBy(Type);
+    comptime Type.Interface.satisfiedBy(Type.RawText);
+    comptime Type.Interface.satisfiedBy(Type.Spacer);
+    return Type;
 }