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 18 package android.support.v4.view; 19 20 import android.view.MotionEvent; 21 import android.view.VelocityTracker; 22 import android.view.View; 23 import android.view.ViewConfiguration; 24 25 /** 26 * This interface should be implemented by {@link android.view.ViewGroup ViewGroup} subclasses 27 * that wish to support scrolling operations delegated by a nested child view. 28 * 29 * <p>Classes implementing this interface should create a final instance of a 30 * {@link NestedScrollingParentHelper} as a field and delegate any View or ViewGroup methods 31 * to the <code>NestedScrollingParentHelper</code> methods of the same signature.</p> 32 * 33 * <p>Views invoking nested scrolling functionality should always do so from the relevant 34 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility 35 * shim static methods. This ensures interoperability with nested scrolling views on Android 36 * 5.0 Lollipop and newer.</p> 37 */ 38 public interface NestedScrollingParent { 39 /** 40 * React to a descendant view initiating a nestable scroll operation, claiming the 41 * nested scroll operation if appropriate. 42 * 43 * <p>This method will be called in response to a descendant view invoking 44 * {@link ViewCompat#startNestedScroll(View, int)}. Each parent up the view hierarchy will be 45 * given an opportunity to respond and claim the nested scrolling operation by returning 46 * <code>true</code>.</p> 47 * 48 * <p>This method may be overridden by ViewParent implementations to indicate when the view 49 * is willing to support a nested scrolling operation that is about to begin. If it returns 50 * true, this ViewParent will become the target view's nested scrolling parent for the duration 51 * of the scroll operation in progress. When the nested scroll is finished this ViewParent 52 * will receive a call to {@link #onStopNestedScroll(View)}. 53 * </p> 54 * 55 * @param child Direct child of this ViewParent containing target 56 * @param target View that initiated the nested scroll 57 * @param nestedScrollAxes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL}, 58 * {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both 59 * @return true if this ViewParent accepts the nested scroll operation 60 */ 61 public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes); 62 63 /** 64 * React to the successful claiming of a nested scroll operation. 65 * 66 * <p>This method will be called after 67 * {@link #onStartNestedScroll(View, View, int) onStartNestedScroll} returns true. It offers 68 * an opportunity for the view and its superclasses to perform initial configuration 69 * for the nested scroll. Implementations of this method should always call their superclass's 70 * implementation of this method if one is present.</p> 71 * 72 * @param child Direct child of this ViewParent containing target 73 * @param target View that initiated the nested scroll 74 * @param nestedScrollAxes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL}, 75 * {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both 76 * @see #onStartNestedScroll(View, View, int) 77 * @see #onStopNestedScroll(View) 78 */ 79 public void onNestedScrollAccepted(View child, View target, int nestedScrollAxes); 80 81 /** 82 * React to a nested scroll operation ending. 83 * 84 * <p>Perform cleanup after a nested scrolling operation. 85 * This method will be called when a nested scroll stops, for example when a nested touch 86 * scroll ends with a {@link MotionEvent#ACTION_UP} or {@link MotionEvent#ACTION_CANCEL} event. 87 * Implementations of this method should always call their superclass's implementation of this 88 * method if one is present.</p> 89 * 90 * @param target View that initiated the nested scroll 91 */ 92 public void onStopNestedScroll(View target); 93 94 /** 95 * React to a nested scroll in progress. 96 * 97 * <p>This method will be called when the ViewParent's current nested scrolling child view 98 * dispatches a nested scroll event. To receive calls to this method the ViewParent must have 99 * previously returned <code>true</code> for a call to 100 * {@link #onStartNestedScroll(View, View, int)}.</p> 101 * 102 * <p>Both the consumed and unconsumed portions of the scroll distance are reported to the 103 * ViewParent. An implementation may choose to use the consumed portion to match or chase scroll 104 * position of multiple child elements, for example. The unconsumed portion may be used to 105 * allow continuous dragging of multiple scrolling or draggable elements, such as scrolling 106 * a list within a vertical drawer where the drawer begins dragging once the edge of inner 107 * scrolling content is reached.</p> 108 * 109 * @param target The descendent view controlling the nested scroll 110 * @param dxConsumed Horizontal scroll distance in pixels already consumed by target 111 * @param dyConsumed Vertical scroll distance in pixels already consumed by target 112 * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by target 113 * @param dyUnconsumed Vertical scroll distance in pixels not consumed by target 114 */ 115 public void onNestedScroll(View target, int dxConsumed, int dyConsumed, 116 int dxUnconsumed, int dyUnconsumed); 117 118 /** 119 * React to a nested scroll in progress before the target view consumes a portion of the scroll. 120 * 121 * <p>When working with nested scrolling often the parent view may want an opportunity 122 * to consume the scroll before the nested scrolling child does. An example of this is a 123 * drawer that contains a scrollable list. The user will want to be able to scroll the list 124 * fully into view before the list itself begins scrolling.</p> 125 * 126 * <p><code>onNestedPreScroll</code> is called when a nested scrolling child invokes 127 * {@link View#dispatchNestedPreScroll(int, int, int[], int[])}. The implementation should 128 * report how any pixels of the scroll reported by dx, dy were consumed in the 129 * <code>consumed</code> array. Index 0 corresponds to dx and index 1 corresponds to dy. 130 * This parameter will never be null. Initial values for consumed[0] and consumed[1] 131 * will always be 0.</p> 132 * 133 * @param target View that initiated the nested scroll 134 * @param dx Horizontal scroll distance in pixels 135 * @param dy Vertical scroll distance in pixels 136 * @param consumed Output. The horizontal and vertical scroll distance consumed by this parent 137 */ 138 public void onNestedPreScroll(View target, int dx, int dy, int[] consumed); 139 140 /** 141 * Request a fling from a nested scroll. 142 * 143 * <p>This method signifies that a nested scrolling child has detected suitable conditions 144 * for a fling. Generally this means that a touch scroll has ended with a 145 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 146 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 147 * along a scrollable axis.</p> 148 * 149 * <p>If a nested scrolling child view would normally fling but it is at the edge of 150 * its own content, it can use this method to delegate the fling to its nested scrolling 151 * parent instead. The parent may optionally consume the fling or observe a child fling.</p> 152 * 153 * @param target View that initiated the nested scroll 154 * @param velocityX Horizontal velocity in pixels per second 155 * @param velocityY Vertical velocity in pixels per second 156 * @param consumed true if the child consumed the fling, false otherwise 157 * @return true if this parent consumed or otherwise reacted to the fling 158 */ 159 public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed); 160 161 /** 162 * React to a nested fling before the target view consumes it. 163 * 164 * <p>This method siginfies that a nested scrolling child has detected a fling with the given 165 * velocity along each axis. Generally this means that a touch scroll has ended with a 166 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 167 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 168 * along a scrollable axis.</p> 169 * 170 * <p>If a nested scrolling parent is consuming motion as part of a 171 * {@link #onNestedPreScroll(View, int, int, int[]) pre-scroll}, it may be appropriate for 172 * it to also consume the pre-fling to complete that same motion. By returning 173 * <code>true</code> from this method, the parent indicates that the child should not 174 * fling its own internal content as well.</p> 175 * 176 * @param target View that initiated the nested scroll 177 * @param velocityX Horizontal velocity in pixels per second 178 * @param velocityY Vertical velocity in pixels per second 179 * @return true if this parent consumed the fling ahead of the target view 180 */ 181 public boolean onNestedPreFling(View target, float velocityX, float velocityY); 182 183 /** 184 * Return the current axes of nested scrolling for this NestedScrollingParent. 185 * 186 * <p>A NestedScrollingParent returning something other than {@link ViewCompat#SCROLL_AXIS_NONE} 187 * is currently acting as a nested scrolling parent for one or more descendant views in 188 * the hierarchy.</p> 189 * 190 * @return Flags indicating the current axes of nested scrolling 191 * @see ViewCompat#SCROLL_AXIS_HORIZONTAL 192 * @see ViewCompat#SCROLL_AXIS_VERTICAL 193 * @see ViewCompat#SCROLL_AXIS_NONE 194 */ 195 public int getNestedScrollAxes(); 196 } 197
