1/*
2 * Copyright (C) 2015 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17
18package android.support.v4.view;
19
20import android.support.annotation.NonNull;
21import android.support.v4.view.ViewCompat.ScrollAxis;
22import android.view.MotionEvent;
23import android.view.VelocityTracker;
24import android.view.View;
25import android.view.ViewConfiguration;
26
27/**
28 * This interface should be implemented by {@link android.view.ViewGroup ViewGroup} subclasses
29 * that wish to support scrolling operations delegated by a nested child view.
30 *
31 * <p>Classes implementing this interface should create a final instance of a
32 * {@link NestedScrollingParentHelper} as a field and delegate any View or ViewGroup methods
33 * to the <code>NestedScrollingParentHelper</code> methods of the same signature.</p>
34 *
35 * <p>Views invoking nested scrolling functionality should always do so from the relevant
36 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility
37 * shim static methods. This ensures interoperability with nested scrolling views on Android
38 * 5.0 Lollipop and newer.</p>
39 */
40public interface NestedScrollingParent {
41    /**
42     * React to a descendant view initiating a nestable scroll operation, claiming the
43     * nested scroll operation if appropriate.
44     *
45     * <p>This method will be called in response to a descendant view invoking
46     * {@link ViewCompat#startNestedScroll(View, int)}. Each parent up the view hierarchy will be
47     * given an opportunity to respond and claim the nested scrolling operation by returning
48     * <code>true</code>.</p>
49     *
50     * <p>This method may be overridden by ViewParent implementations to indicate when the view
51     * is willing to support a nested scrolling operation that is about to begin. If it returns
52     * true, this ViewParent will become the target view's nested scrolling parent for the duration
53     * of the scroll operation in progress. When the nested scroll is finished this ViewParent
54     * will receive a call to {@link #onStopNestedScroll(View)}.
55     * </p>
56     *
57     * @param child Direct child of this ViewParent containing target
58     * @param target View that initiated the nested scroll
59     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
60     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
61     * @return true if this ViewParent accepts the nested scroll operation
62     */
63    boolean onStartNestedScroll(@NonNull View child, @NonNull View target, @ScrollAxis int axes);
64
65    /**
66     * React to the successful claiming of a nested scroll operation.
67     *
68     * <p>This method will be called after
69     * {@link #onStartNestedScroll(View, View, int) onStartNestedScroll} returns true. It offers
70     * an opportunity for the view and its superclasses to perform initial configuration
71     * for the nested scroll. Implementations of this method should always call their superclass's
72     * implementation of this method if one is present.</p>
73     *
74     * @param child Direct child of this ViewParent containing target
75     * @param target View that initiated the nested scroll
76     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
77     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
78     * @see #onStartNestedScroll(View, View, int)
79     * @see #onStopNestedScroll(View)
80     */
81    void onNestedScrollAccepted(@NonNull View child, @NonNull View target, @ScrollAxis int axes);
82
83    /**
84     * React to a nested scroll operation ending.
85     *
86     * <p>Perform cleanup after a nested scrolling operation.
87     * This method will be called when a nested scroll stops, for example when a nested touch
88     * scroll ends with a {@link MotionEvent#ACTION_UP} or {@link MotionEvent#ACTION_CANCEL} event.
89     * Implementations of this method should always call their superclass's implementation of this
90     * method if one is present.</p>
91     *
92     * @param target View that initiated the nested scroll
93     */
94    void onStopNestedScroll(@NonNull View target);
95
96    /**
97     * React to a nested scroll in progress.
98     *
99     * <p>This method will be called when the ViewParent's current nested scrolling child view
100     * dispatches a nested scroll event. To receive calls to this method the ViewParent must have
101     * previously returned <code>true</code> for a call to
102     * {@link #onStartNestedScroll(View, View, int)}.</p>
103     *
104     * <p>Both the consumed and unconsumed portions of the scroll distance are reported to the
105     * ViewParent. An implementation may choose to use the consumed portion to match or chase scroll
106     * position of multiple child elements, for example. The unconsumed portion may be used to
107     * allow continuous dragging of multiple scrolling or draggable elements, such as scrolling
108     * a list within a vertical drawer where the drawer begins dragging once the edge of inner
109     * scrolling content is reached.</p>
110     *
111     * @param target The descendent view controlling the nested scroll
112     * @param dxConsumed Horizontal scroll distance in pixels already consumed by target
113     * @param dyConsumed Vertical scroll distance in pixels already consumed by target
114     * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by target
115     * @param dyUnconsumed Vertical scroll distance in pixels not consumed by target
116     */
117    void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
118            int dxUnconsumed, int dyUnconsumed);
119
120    /**
121     * React to a nested scroll in progress before the target view consumes a portion of the scroll.
122     *
123     * <p>When working with nested scrolling often the parent view may want an opportunity
124     * to consume the scroll before the nested scrolling child does. An example of this is a
125     * drawer that contains a scrollable list. The user will want to be able to scroll the list
126     * fully into view before the list itself begins scrolling.</p>
127     *
128     * <p><code>onNestedPreScroll</code> is called when a nested scrolling child invokes
129     * {@link View#dispatchNestedPreScroll(int, int, int[], int[])}. The implementation should
130     * report how any pixels of the scroll reported by dx, dy were consumed in the
131     * <code>consumed</code> array. Index 0 corresponds to dx and index 1 corresponds to dy.
132     * This parameter will never be null. Initial values for consumed[0] and consumed[1]
133     * will always be 0.</p>
134     *
135     * @param target View that initiated the nested scroll
136     * @param dx Horizontal scroll distance in pixels
137     * @param dy Vertical scroll distance in pixels
138     * @param consumed Output. The horizontal and vertical scroll distance consumed by this parent
139     */
140    void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed);
141
142    /**
143     * Request a fling from a nested scroll.
144     *
145     * <p>This method signifies that a nested scrolling child has detected suitable conditions
146     * for a fling. Generally this means that a touch scroll has ended with a
147     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
148     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
149     * along a scrollable axis.</p>
150     *
151     * <p>If a nested scrolling child view would normally fling but it is at the edge of
152     * its own content, it can use this method to delegate the fling to its nested scrolling
153     * parent instead. The parent may optionally consume the fling or observe a child fling.</p>
154     *
155     * @param target View that initiated the nested scroll
156     * @param velocityX Horizontal velocity in pixels per second
157     * @param velocityY Vertical velocity in pixels per second
158     * @param consumed true if the child consumed the fling, false otherwise
159     * @return true if this parent consumed or otherwise reacted to the fling
160     */
161    boolean onNestedFling(@NonNull View target, float velocityX, float velocityY, boolean consumed);
162
163    /**
164     * React to a nested fling before the target view consumes it.
165     *
166     * <p>This method siginfies that a nested scrolling child has detected a fling with the given
167     * velocity along each axis. Generally this means that a touch scroll has ended with a
168     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
169     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
170     * along a scrollable axis.</p>
171     *
172     * <p>If a nested scrolling parent is consuming motion as part of a
173     * {@link #onNestedPreScroll(View, int, int, int[]) pre-scroll}, it may be appropriate for
174     * it to also consume the pre-fling to complete that same motion. By returning
175     * <code>true</code> from this method, the parent indicates that the child should not
176     * fling its own internal content as well.</p>
177     *
178     * @param target View that initiated the nested scroll
179     * @param velocityX Horizontal velocity in pixels per second
180     * @param velocityY Vertical velocity in pixels per second
181     * @return true if this parent consumed the fling ahead of the target view
182     */
183    boolean onNestedPreFling(@NonNull View target, float velocityX, float velocityY);
184
185    /**
186     * Return the current axes of nested scrolling for this NestedScrollingParent.
187     *
188     * <p>A NestedScrollingParent returning something other than {@link ViewCompat#SCROLL_AXIS_NONE}
189     * is currently acting as a nested scrolling parent for one or more descendant views in
190     * the hierarchy.</p>
191     *
192     * @return Flags indicating the current axes of nested scrolling
193     * @see ViewCompat#SCROLL_AXIS_HORIZONTAL
194     * @see ViewCompat#SCROLL_AXIS_VERTICAL
195     * @see ViewCompat#SCROLL_AXIS_NONE
196     */
197    @ScrollAxis
198    int getNestedScrollAxes();
199}
200