vxfw: add Center widget

Center widget centers a child widget within itself

src/vxfw/Center.zig

@@ -0,0 +1,118 @@
+const std = @import("std");
+const vaxis = @import("../main.zig");
+
+const Allocator = std.mem.Allocator;
+
+const vxfw = @import("vxfw.zig");
+
+const Center = @This();
+
+child: vxfw.Widget,
+
+pub fn widget(self: *const Center) vxfw.Widget {
+    return .{
+        .userdata = @constCast(self),
+        .eventHandler = typeErasedEventHandler,
+        .drawFn = typeErasedDrawFn,
+    };
+}
+
+fn typeErasedEventHandler(ptr: *anyopaque, ctx: *vxfw.EventContext, event: vxfw.Event) anyerror!void {
+    const self: *const Center = @ptrCast(@alignCast(ptr));
+    return self.child.handleEvent(ctx, event);
+}
+
+fn typeErasedDrawFn(ptr: *anyopaque, ctx: vxfw.DrawContext) Allocator.Error!vxfw.Surface {
+    const self: *const Center = @ptrCast(@alignCast(ptr));
+    return self.draw(ctx);
+}
+
+/// Cannot have unbounded constraints
+pub fn draw(self: *const Center, ctx: vxfw.DrawContext) Allocator.Error!vxfw.Surface {
+    const child_ctx = ctx.withConstraints(.{ .width = 0, .height = 0 }, ctx.max);
+    const max_size = ctx.max.size();
+    const child = try self.child.draw(child_ctx);
+
+    const x = (max_size.width - child.size.width) / 2;
+    const y = (max_size.height - child.size.height) / 2;
+
+    const children = try ctx.arena.alloc(vxfw.SubSurface, 1);
+    children[0] = .{
+        .origin = .{ .col = x, .row = y },
+        .z_index = 0,
+        .surface = child,
+    };
+
+    return .{
+        .size = max_size,
+        .widget = self.widget(),
+        .buffer = &.{},
+        .children = children,
+    };
+}
+
+test Center {
+    const Text = @import("Text.zig");
+    // Will be height=1, width=3
+    const text: Text = .{ .text = "abc" };
+
+    const center: Center = .{ .child = text.widget() };
+
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+    const ucd = try vaxis.Unicode.init(arena.allocator());
+    vxfw.DrawContext.init(&ucd, .unicode);
+
+    {
+        // Center expands to the max size. It must therefore have non-null max width and max height.
+        // These values are asserted in draw
+        const ctx: vxfw.DrawContext = .{
+            .arena = arena.allocator(),
+            .min = .{},
+            .max = .{ .width = 10, .height = 10 },
+        };
+
+        const surface = try center.draw(ctx);
+        // Center does not produce any drawable cells
+        try std.testing.expectEqual(0, surface.buffer.len);
+        // Center has 1 child
+        try std.testing.expectEqual(1, surface.children.len);
+        // Center is the max size
+        try std.testing.expectEqual(surface.size, ctx.max.size());
+        const child = surface.children[0];
+        // The child is 1x3
+        try std.testing.expectEqual(3, child.surface.size.width);
+        try std.testing.expectEqual(1, child.surface.size.height);
+        // A centered 1x3 in 10x10 should be at origin 3, 4. The bias is toward the top left corner
+        try std.testing.expectEqual(4, child.origin.row);
+        try std.testing.expectEqual(3, child.origin.col);
+    }
+    {
+        // Center expands to the max size. It must therefore have non-null max width and max height.
+        // These values are asserted in draw
+        const ctx: vxfw.DrawContext = .{
+            .arena = arena.allocator(),
+            .min = .{},
+            .max = .{ .width = 5, .height = 3 },
+        };
+
+        const surface = try center.draw(ctx);
+        // Center does not produce any drawable cells
+        try std.testing.expectEqual(0, surface.buffer.len);
+        // Center has 1 child
+        try std.testing.expectEqual(1, surface.children.len);
+        // Center is the max size
+        try std.testing.expectEqual(surface.size, ctx.max.size());
+        const child = surface.children[0];
+        // The child is 1x3
+        try std.testing.expectEqual(3, child.surface.size.width);
+        try std.testing.expectEqual(1, child.surface.size.height);
+        // A centered 1x3 in 3x5 should be at origin 1, 1. This is a perfectly centered child
+        try std.testing.expectEqual(1, child.origin.row);
+        try std.testing.expectEqual(1, child.origin.col);
+    }
+}
+
+test "refAllDecls" {
+    std.testing.refAllDecls(@This());
+}

src/vxfw/vxfw.zig

@@ -11,6 +11,7 @@ const Allocator = std.mem.Allocator;
 pub const App = @import("App.zig");
 
 // Widgets
+pub const Center = @import("Center.zig");
 pub const ListView = @import("ListView.zig");
 pub const RichText = @import("RichText.zig");
 pub const Text = @import("Text.zig");