1/* 2 * Copyright (C) 2017 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 17package com.android.internal.widget; 18 19import android.view.MotionEvent; 20import android.view.VelocityTracker; 21import android.view.View; 22import android.view.ViewConfiguration; 23import android.view.ViewParent; 24 25/** 26 * This interface should be implemented by {@link android.view.View View} subclasses that wish 27 * to support dispatching nested scrolling operations to a cooperating parent 28 * {@link android.view.ViewGroup ViewGroup}. 29 * 30 * <p>Classes implementing this interface should create a final instance of a 31 * {@link NestedScrollingChildHelper} as a field and delegate any View methods to the 32 * <code>NestedScrollingChildHelper</code> methods of the same signature.</p> 33 * 34 * <p>Views invoking nested scrolling functionality should always do so from the relevant 35 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility 36 * shim static methods. This ensures interoperability with nested scrolling views on Android 37 * 5.0 Lollipop and newer.</p> 38 */ 39public interface NestedScrollingChild { 40 /** 41 * Enable or disable nested scrolling for this view. 42 * 43 * <p>If this property is set to true the view will be permitted to initiate nested 44 * scrolling operations with a compatible parent view in the current hierarchy. If this 45 * view does not implement nested scrolling this will have no effect. Disabling nested scrolling 46 * while a nested scroll is in progress has the effect of {@link #stopNestedScroll() stopping} 47 * the nested scroll.</p> 48 * 49 * @param enabled true to enable nested scrolling, false to disable 50 * 51 * @see #isNestedScrollingEnabled() 52 */ 53 void setNestedScrollingEnabled(boolean enabled); 54 55 /** 56 * Returns true if nested scrolling is enabled for this view. 57 * 58 * <p>If nested scrolling is enabled and this View class implementation supports it, 59 * this view will act as a nested scrolling child view when applicable, forwarding data 60 * about the scroll operation in progress to a compatible and cooperating nested scrolling 61 * parent.</p> 62 * 63 * @return true if nested scrolling is enabled 64 * 65 * @see #setNestedScrollingEnabled(boolean) 66 */ 67 boolean isNestedScrollingEnabled(); 68 69 /** 70 * Begin a nestable scroll operation along the given axes. 71 * 72 * <p>A view starting a nested scroll promises to abide by the following contract:</p> 73 * 74 * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case 75 * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}. 76 * In the case of touch scrolling the nested scroll will be terminated automatically in 77 * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}. 78 * In the event of programmatic scrolling the caller must explicitly call 79 * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p> 80 * 81 * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found. 82 * If it returns false the caller may ignore the rest of this contract until the next scroll. 83 * Calling startNestedScroll while a nested scroll is already in progress will return true.</p> 84 * 85 * <p>At each incremental step of the scroll the caller should invoke 86 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} 87 * once it has calculated the requested scrolling delta. If it returns true the nested scrolling 88 * parent at least partially consumed the scroll and the caller should adjust the amount it 89 * scrolls by.</p> 90 * 91 * <p>After applying the remainder of the scroll delta the caller should invoke 92 * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing 93 * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat 94 * these values differently. See 95 * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}. 96 * </p> 97 * 98 * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL} 99 * and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}. 100 * @return true if a cooperative parent was found and nested scrolling has been enabled for 101 * the current gesture. 102 * 103 * @see #stopNestedScroll() 104 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 105 * @see #dispatchNestedScroll(int, int, int, int, int[]) 106 */ 107 boolean startNestedScroll(int axes); 108 109 /** 110 * Stop a nested scroll in progress. 111 * 112 * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p> 113 * 114 * @see #startNestedScroll(int) 115 */ 116 void stopNestedScroll(); 117 118 /** 119 * Returns true if this view has a nested scrolling parent. 120 * 121 * <p>The presence of a nested scrolling parent indicates that this view has initiated 122 * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p> 123 * 124 * @return whether this view has a nested scrolling parent 125 */ 126 boolean hasNestedScrollingParent(); 127 128 /** 129 * Dispatch one step of a nested scroll in progress. 130 * 131 * <p>Implementations of views that support nested scrolling should call this to report 132 * info about a scroll in progress to the current nested scrolling parent. If a nested scroll 133 * is not currently in progress or nested scrolling is not 134 * {@link #isNestedScrollingEnabled() enabled} for this view this method does nothing.</p> 135 * 136 * <p>Compatible View implementations should also call 137 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} before 138 * consuming a component of the scroll event themselves.</p> 139 * 140 * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step 141 * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step 142 * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view 143 * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view 144 * @param offsetInWindow Optional. If not null, on return this will contain the offset 145 * in local view coordinates of this view from before this operation 146 * to after it completes. View implementations may use this to adjust 147 * expected input coordinate tracking. 148 * @return true if the event was dispatched, false if it could not be dispatched. 149 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 150 */ 151 boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, 152 int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); 153 154 /** 155 * Dispatch one step of a nested scroll in progress before this view consumes any portion of it. 156 * 157 * <p>Nested pre-scroll events are to nested scroll events what touch intercept is to touch. 158 * <code>dispatchNestedPreScroll</code> offers an opportunity for the parent view in a nested 159 * scrolling operation to consume some or all of the scroll operation before the child view 160 * consumes it.</p> 161 * 162 * @param dx Horizontal scroll distance in pixels 163 * @param dy Vertical scroll distance in pixels 164 * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx 165 * and consumed[1] the consumed dy. 166 * @param offsetInWindow Optional. If not null, on return this will contain the offset 167 * in local view coordinates of this view from before this operation 168 * to after it completes. View implementations may use this to adjust 169 * expected input coordinate tracking. 170 * @return true if the parent consumed some or all of the scroll delta 171 * @see #dispatchNestedScroll(int, int, int, int, int[]) 172 */ 173 boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow); 174 175 /** 176 * Dispatch a fling to a nested scrolling parent. 177 * 178 * <p>This method should be used to indicate that a nested scrolling child has detected 179 * suitable conditions for a fling. Generally this means that a touch scroll has ended with a 180 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 181 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 182 * along a scrollable axis.</p> 183 * 184 * <p>If a nested scrolling child view would normally fling but it is at the edge of 185 * its own content, it can use this method to delegate the fling to its nested scrolling 186 * parent instead. The parent may optionally consume the fling or observe a child fling.</p> 187 * 188 * @param velocityX Horizontal fling velocity in pixels per second 189 * @param velocityY Vertical fling velocity in pixels per second 190 * @param consumed true if the child consumed the fling, false otherwise 191 * @return true if the nested scrolling parent consumed or otherwise reacted to the fling 192 */ 193 boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); 194 195 /** 196 * Dispatch a fling to a nested scrolling parent before it is processed by this view. 197 * 198 * <p>Nested pre-fling events are to nested fling events what touch intercept is to touch 199 * and what nested pre-scroll is to nested scroll. <code>dispatchNestedPreFling</code> 200 * offsets an opportunity for the parent view in a nested fling to fully consume the fling 201 * before the child view consumes it. If this method returns <code>true</code>, a nested 202 * parent view consumed the fling and this view should not scroll as a result.</p> 203 * 204 * <p>For a better user experience, only one view in a nested scrolling chain should consume 205 * the fling at a time. If a parent view consumed the fling this method will return false. 206 * Custom view implementations should account for this in two ways:</p> 207 * 208 * <ul> 209 * <li>If a custom view is paged and needs to settle to a fixed page-point, do not 210 * call <code>dispatchNestedPreFling</code>; consume the fling and settle to a valid 211 * position regardless.</li> 212 * <li>If a nested parent does consume the fling, this view should not scroll at all, 213 * even to settle back to a valid idle position.</li> 214 * </ul> 215 * 216 * <p>Views should also not offer fling velocities to nested parent views along an axis 217 * where scrolling is not currently supported; a {@link android.widget.ScrollView ScrollView} 218 * should not offer a horizontal fling velocity to its parents since scrolling along that 219 * axis is not permitted and carrying velocity along that motion does not make sense.</p> 220 * 221 * @param velocityX Horizontal fling velocity in pixels per second 222 * @param velocityY Vertical fling velocity in pixels per second 223 * @return true if a nested scrolling parent consumed the fling 224 */ 225 boolean dispatchNestedPreFling(float velocityX, float velocityY); 226} 227