Add mobile device 'emulation'

By passing the ZGT_MOBILE_EMULATED environment variable, zgt
will force the window size and source DPI to be equal to the
given phone's one. In the future, this will allow testing
responsive design easily and with slow compile times.
In essence, this is similar to Chrome's and Firefox's
mobile emulation mode.

src/backends/gtk/backend.zig

@@ -57,6 +57,8 @@ pub const Window = struct {
     /// A VBox is required to contain the menu and the window's child (wrapped in wbin)
     vbox: *c.GtkWidget,
     menuBar: ?*c.GtkWidget = null,
+    source_dpi: u32 = 96,
+    scale: f32 = 1.0,
 
     pub usingnamespace Events(Window);
 
@@ -79,7 +81,9 @@ pub const Window = struct {
     }
 
     pub fn resize(self: *Window, width: c_int, height: c_int) void {
-        c.gtk_window_resize(@ptrCast(*c.GtkWindow, self.peer), width, height);
+        c.gtk_window_resize(@ptrCast(*c.GtkWindow, self.peer),
+            @floatToInt(c_int, @intToFloat(f32, width) * self.scale),
+            @floatToInt(c_int, @intToFloat(f32, height) * self.scale));
     }
 
     pub fn setTitle(self: *Window, title: [*:0]const u8) void {
@@ -107,6 +111,14 @@ pub const Window = struct {
         self.menuBar = menuBar;
     }
 
+    pub fn setSourceDpi(self: *Window, dpi: u32) void {
+        self.source_dpi = 96;
+        // TODO: Handle GtkWindow moving between screens with different DPIs
+        const resolution = @as(f32, 96.0);
+        self.scale = resolution / @intToFloat(f32, dpi);
+        std.log.debug("Scale: {d}", .{ self.scale });
+    }
+
     fn initMenu(menu: *c.GtkMenuShell, items: []const lib.MenuItem_Impl) void {
         for (items) |item| {
             const menuItem = c.gtk_menu_item_new_with_label(item.config.label);

src/window.zig

@@ -1,18 +1,33 @@
 const std = @import("std");
 const backend = @import("backend.zig");
+const internal = @import("internal.zig");
 const Widget = @import("widget.zig").Widget;
 const ImageData = @import("image.zig").ImageData;
 const MenuBar_Impl = @import("menu.zig").MenuBar_Impl;
+const Size = @import("data.zig").Size;
+
+const Display = struct {
+    resolution: Size,
+    dpi: u32
+};
+
+const devices = std.ComptimeStringMap(Display, .{
+    .{ "iphone-13-mini", .{ .resolution = Size.init(1080, 2340), .dpi = 476 }},
+    .{ "iphone-13", .{ .resolution = Size.init(1170, 2532), .dpi = 460 }},
+    .{ "pixel-6", .{ .resolution = Size.init(1080, 2400), .dpi = 411 }},
+    .{ "pixel-6-pro", .{ .resolution = Size.init(1440, 3120), .dpi = 512 }},
+});
 
 pub const Window = struct {
-    /// The DPI the GUI has been developed against
-    source_dpi: u32 = 96,
     peer: backend.Window,
     _child: ?Widget = null,
 
     pub fn init() !Window {
         const peer = try backend.Window.create();
-        return Window{ .peer = peer };
+        var window = Window{ .peer = peer };
+        window.setSourceDpi(96);
+        window.resize(640, 480);
+        return window;
     }
 
     pub fn show(self: *Window) void {
@@ -53,7 +68,28 @@ pub const Window = struct {
         return self._child;
     }
 
+    var did_invalid_warning = false;
+
     pub fn resize(self: *Window, width: u32, height: u32) void {
+        const EMULATOR_KEY = "ZGT_MOBILE_EMULATION";
+        if (std.process.hasEnvVarConstant(EMULATOR_KEY)) {
+            const id = std.process.getEnvVarOwned(
+                internal.scratch_allocator, EMULATOR_KEY) catch unreachable;
+            defer internal.scratch_allocator.free(id);
+            if (devices.get(id)) |device| {
+                self.peer.resize(@intCast(c_int, device.resolution.width),
+                    @intCast(c_int, device.resolution.height));
+                self.setSourceDpi(device.dpi);
+                return;
+            } else if (!did_invalid_warning) {
+                std.log.warn("Invalid property \"" ++ EMULATOR_KEY ++ "={s}\"", .{ id });
+                std.debug.print("Expected one of:\r\n", .{});
+                for (devices.kvs) |entry| {
+                    std.debug.print("    - {s}\r\n", .{ entry.key });
+                }
+                did_invalid_warning = true;
+            }
+        }
         self.peer.resize(@intCast(c_int, width), @intCast(c_int, height));
     }
 
@@ -73,6 +109,11 @@ pub const Window = struct {
         self.peer.setMenuBar(bar);
     }
 
+    /// Specify for which DPI the GUI was developed against.
+    pub fn setSourceDpi(self: *Window, dpi: u32) void {
+        self.peer.setSourceDpi(dpi);
+    }
+
     pub fn deinit(self: *Window) void {
         if (self._child) |*child| {
             child.deinit();