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 import android.view.ViewParent; 25 26 /** 27 * This interface should be implemented by {@link android.view.View View} subclasses that wish 28 * to support dispatching nested scrolling operations to a cooperating parent 29 * {@link android.view.ViewGroup ViewGroup}. 30 * 31 * <p>Classes implementing this interface should create a final instance of a 32 * {@link NestedScrollingChildHelper} as a field and delegate any View methods to the 33 * <code>NestedScrollingChildHelper</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 */ 40 public interface NestedScrollingChild { 41 /** 42 * Enable or disable nested scrolling for this view. 43 * 44 * <p>If this property is set to true the view will be permitted to initiate nested 45 * scrolling operations with a compatible parent view in the current hierarchy. If this 46 * view does not implement nested scrolling this will have no effect. Disabling nested scrolling 47 * while a nested scroll is in progress has the effect of {@link #stopNestedScroll() stopping} 48 * the nested scroll.</p> 49 * 50 * @param enabled true to enable nested scrolling, false to disable 51 * 52 * @see #isNestedScrollingEnabled() 53 */ setNestedScrollingEnabled(boolean enabled)54 public void setNestedScrollingEnabled(boolean enabled); 55 56 /** 57 * Returns true if nested scrolling is enabled for this view. 58 * 59 * <p>If nested scrolling is enabled and this View class implementation supports it, 60 * this view will act as a nested scrolling child view when applicable, forwarding data 61 * about the scroll operation in progress to a compatible and cooperating nested scrolling 62 * parent.</p> 63 * 64 * @return true if nested scrolling is enabled 65 * 66 * @see #setNestedScrollingEnabled(boolean) 67 */ isNestedScrollingEnabled()68 public boolean isNestedScrollingEnabled(); 69 70 /** 71 * Begin a nestable scroll operation along the given axes. 72 * 73 * <p>A view starting a nested scroll promises to abide by the following contract:</p> 74 * 75 * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case 76 * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}. 77 * In the case of touch scrolling the nested scroll will be terminated automatically in 78 * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}. 79 * In the event of programmatic scrolling the caller must explicitly call 80 * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p> 81 * 82 * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found. 83 * If it returns false the caller may ignore the rest of this contract until the next scroll. 84 * Calling startNestedScroll while a nested scroll is already in progress will return true.</p> 85 * 86 * <p>At each incremental step of the scroll the caller should invoke 87 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} 88 * once it has calculated the requested scrolling delta. If it returns true the nested scrolling 89 * parent at least partially consumed the scroll and the caller should adjust the amount it 90 * scrolls by.</p> 91 * 92 * <p>After applying the remainder of the scroll delta the caller should invoke 93 * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing 94 * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat 95 * these values differently. See 96 * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}. 97 * </p> 98 * 99 * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL} 100 * and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}. 101 * @return true if a cooperative parent was found and nested scrolling has been enabled for 102 * the current gesture. 103 * 104 * @see #stopNestedScroll() 105 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 106 * @see #dispatchNestedScroll(int, int, int, int, int[]) 107 */ startNestedScroll(int axes)108 public boolean startNestedScroll(int axes); 109 110 /** 111 * Stop a nested scroll in progress. 112 * 113 * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p> 114 * 115 * @see #startNestedScroll(int) 116 */ stopNestedScroll()117 public void stopNestedScroll(); 118 119 /** 120 * Returns true if this view has a nested scrolling parent. 121 * 122 * <p>The presence of a nested scrolling parent indicates that this view has initiated 123 * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p> 124 * 125 * @return whether this view has a nested scrolling parent 126 */ hasNestedScrollingParent()127 public boolean hasNestedScrollingParent(); 128 129 /** 130 * Dispatch one step of a nested scroll in progress. 131 * 132 * <p>Implementations of views that support nested scrolling should call this to report 133 * info about a scroll in progress to the current nested scrolling parent. If a nested scroll 134 * is not currently in progress or nested scrolling is not 135 * {@link #isNestedScrollingEnabled() enabled} for this view this method does nothing.</p> 136 * 137 * <p>Compatible View implementations should also call 138 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} before 139 * consuming a component of the scroll event themselves.</p> 140 * 141 * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step 142 * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step 143 * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view 144 * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view 145 * @param offsetInWindow Optional. If not null, on return this will contain the offset 146 * in local view coordinates of this view from before this operation 147 * to after it completes. View implementations may use this to adjust 148 * expected input coordinate tracking. 149 * @return true if the event was dispatched, false if it could not be dispatched. 150 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 151 */ dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow)152 public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, 153 int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); 154 155 /** 156 * Dispatch one step of a nested scroll in progress before this view consumes any portion of it. 157 * 158 * <p>Nested pre-scroll events are to nested scroll events what touch intercept is to touch. 159 * <code>dispatchNestedPreScroll</code> offers an opportunity for the parent view in a nested 160 * scrolling operation to consume some or all of the scroll operation before the child view 161 * consumes it.</p> 162 * 163 * @param dx Horizontal scroll distance in pixels 164 * @param dy Vertical scroll distance in pixels 165 * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx 166 * and consumed[1] the consumed dy. 167 * @param offsetInWindow Optional. If not null, on return this will contain the offset 168 * in local view coordinates of this view from before this operation 169 * to after it completes. View implementations may use this to adjust 170 * expected input coordinate tracking. 171 * @return true if the parent consumed some or all of the scroll delta 172 * @see #dispatchNestedScroll(int, int, int, int, int[]) 173 */ dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow)174 public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow); 175 176 /** 177 * Dispatch a fling to a nested scrolling parent. 178 * 179 * <p>This method should be used to indicate that a nested scrolling child has detected 180 * suitable conditions for a fling. Generally this means that a touch scroll has ended with a 181 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 182 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 183 * along a scrollable axis.</p> 184 * 185 * <p>If a nested scrolling child view would normally fling but it is at the edge of 186 * its own content, it can use this method to delegate the fling to its nested scrolling 187 * parent instead. The parent may optionally consume the fling or observe a child fling.</p> 188 * 189 * @param velocityX Horizontal fling velocity in pixels per second 190 * @param velocityY Vertical fling velocity in pixels per second 191 * @param consumed true if the child consumed the fling, false otherwise 192 * @return true if the nested scrolling parent consumed or otherwise reacted to the fling 193 */ dispatchNestedFling(float velocityX, float velocityY, boolean consumed)194 public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); 195 196 /** 197 * Dispatch a fling to a nested scrolling parent before it is processed by this view. 198 * 199 * <p>Nested pre-fling events are to nested fling events what touch intercept is to touch 200 * and what nested pre-scroll is to nested scroll. <code>dispatchNestedPreFling</code> 201 * offsets an opportunity for the parent view in a nested fling to fully consume the fling 202 * before the child view consumes it. If this method returns <code>true</code>, a nested 203 * parent view consumed the fling and this view should not scroll as a result.</p> 204 * 205 * <p>For a better user experience, only one view in a nested scrolling chain should consume 206 * the fling at a time. If a parent view consumed the fling this method will return false. 207 * Custom view implementations should account for this in two ways:</p> 208 * 209 * <ul> 210 * <li>If a custom view is paged and needs to settle to a fixed page-point, do not 211 * call <code>dispatchNestedPreFling</code>; consume the fling and settle to a valid 212 * position regardless.</li> 213 * <li>If a nested parent does consume the fling, this view should not scroll at all, 214 * even to settle back to a valid idle position.</li> 215 * </ul> 216 * 217 * <p>Views should also not offer fling velocities to nested parent views along an axis 218 * where scrolling is not currently supported; a {@link android.widget.ScrollView ScrollView} 219 * should not offer a horizontal fling velocity to its parents since scrolling along that 220 * axis is not permitted and carrying velocity along that motion does not make sense.</p> 221 * 222 * @param velocityX Horizontal fling velocity in pixels per second 223 * @param velocityY Vertical fling velocity in pixels per second 224 * @return true if a nested scrolling parent consumed the fling 225 */ dispatchNestedPreFling(float velocityX, float velocityY)226 public boolean dispatchNestedPreFling(float velocityX, float velocityY); 227 } 228