mozart: Introduce new view manager interfaces.

mojo/services/ui/views/interfaces/layouts.mojom

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+[DartPackage="mojo_services"]
+module mojo.ui;
+
+import "mojo/services/geometry/interfaces/geometry.mojom";
+import "mojo/services/surfaces/interfaces/surface_id.mojom";
+
+// Box constraints for layout.
+//
+// A box constraint allows a parent view to determine the size of its child
+// by constraining its width and height.  Each dimension is considered
+// independently.  If a constraint only admits a single value in one or both
+// dimensions (because the minimum and maximum values are equal) then it is
+// said to be "tight" in those dimension.
+//
+// A |Size| respects this constraint if, and only if, all of the following
+// relations hold:
+//
+//   size.width >= constraints.min_width
+//   size.width <= constraints.max_width
+//   size.height >= constraints.min_height
+//   size.height <= constraints.max_height
+//
+// The view manager validates all constraints before delivering them to a
+// child view; it will close its connection if the parent view supplies
+// constraints which are ill-formed.
+struct BoxConstraints {
+  // The minimum width of the view in device pixels.
+  // Must be >= 0.
+  int32 min_width;
+
+  // The maximum width of the view in device pixels.
+  // Must be >= |min_width|.
+  int32 max_width;
+
+  // The minimum height of the view in device pixels.
+  // Must be >= 0.
+  int32 min_height;
+
+  // The maximum height of the view in device pixels.
+  // Must be >= |min_height|.
+  int32 max_height;
+};
+
+// Layout parameters provided by a parent view to one of its children.
+//
+// TODO(jeffbrown): We will eventually need to pass a bunch more information
+// such as theme colors, virtual light sources for shadows, elevation, and
+// more.  It is unclear whether we'll want to put that information here
+// or somewhere else but it may be convenient to stash it here because we
+// will already have an invalidation mechanism in place.
+struct ViewLayoutParams {
+  // The size constraints for the child.
+  mojo.ui.BoxConstraints constraints;
+
+  // The ratio between the size of one display device pixel to the size
+  // of one logical pixel, assuming pixels are square.  This value changes
+  // in relation to display density and zoom level.
+  // Must be > 0.
+  float device_pixel_ratio = 1.0;
+};
+
+// Layout information for a view.
+struct ViewLayoutInfo {
+  // The view's surface id for composition by the parent.
+  mojo.SurfaceId surface_id;
+
+  // The actual size of the view in device pixels.
+  mojo.Size size;
+};