// Copyright 2021 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. #ifndef ASH_CONTROLS_SCROLL_VIEW_GRADIENT_HELPER_H_ #define ASH_CONTROLS_SCROLL_VIEW_GRADIENT_HELPER_H_ #include #include "ash/ash_export.h" #include "base/callback_list.h" #include "ui/views/controls/scroll_view.h" namespace ash { class GradientLayerDelegate; // Draws fade in / fade out gradients at the top and bottom of a ScrollView. // Uses layer masks to draw the gradient. Each gradient only shows if the view // can be scrolled in that direction. For efficiency, does not create a layer // mask if no gradient is showing (i.e. if the scroll view contents fit in the // viewport and hence the view cannot be scrolled). // // The gradient is created on the first call to UpdateGradientZone(), not in the // constructor. This allows the helper to be created any time during view tree // construction, even before the scroll view contents are known. // // Views using this helper should call UpdateGradientZone() whenever the scroll // view bounds or contents bounds change (e.g. from Layout()). // // The gradient is destroyed when this object is destroyed. This does not add // extra work in the common views::View teardown case (the layer would be // destroyed anyway). class ASH_EXPORT ScrollViewGradientHelper { public: // `scroll_view` must have a layer. ScrollViewGradientHelper(views::ScrollView* scroll_view, int gradient_height); ~ScrollViewGradientHelper(); // Updates the gradients based on `scroll_view_` bounds and scroll position. void UpdateGradientZone(); GradientLayerDelegate* gradient_layer_for_test() { return gradient_layer_.get(); } private: friend class ScopedScrollViewGradientDisabler; // Removes the scroll view mask layer. void RemoveMaskLayer(); // The scroll view being decorated. views::ScrollView* const scroll_view_; // The height of the gradient in DIPs. const int gradient_height_; // Draws the fade in/out gradients via a `scroll_view_` mask layer. std::unique_ptr gradient_layer_; // Callback subscriptions. base::CallbackListSubscription on_contents_scrolled_subscription_; base::CallbackListSubscription on_contents_scroll_ended_subscription_; }; } // namespace ash #endif // ASH_CONTROLS_SCROLL_VIEW_GRADIENT_HELPER_H_