1 /* 2 * Copyright 2018 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 androidx.core.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 import androidx.annotation.Nullable; 27 import androidx.core.view.ViewCompat.ScrollAxis; 28 29 /** 30 * This interface should be implemented by {@link android.view.View View} subclasses that wish 31 * to support dispatching nested scrolling operations to a cooperating parent 32 * {@link android.view.ViewGroup ViewGroup}. 33 * 34 * <p>Classes implementing this interface should create a final instance of a 35 * {@link NestedScrollingChildHelper} as a field and delegate any View methods to the 36 * <code>NestedScrollingChildHelper</code> methods of the same signature.</p> 37 * 38 * <p>Views invoking nested scrolling functionality should always do so from the relevant 39 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility 40 * shim static methods. This ensures interoperability with nested scrolling views on Android 41 * 5.0 Lollipop and newer.</p> 42 */ 43 public interface NestedScrollingChild { 44 /** 45 * Enable or disable nested scrolling for this view. 46 * 47 * <p>If this property is set to true the view will be permitted to initiate nested 48 * scrolling operations with a compatible parent view in the current hierarchy. If this 49 * view does not implement nested scrolling this will have no effect. Disabling nested scrolling 50 * while a nested scroll is in progress has the effect of {@link #stopNestedScroll() stopping} 51 * the nested scroll.</p> 52 * 53 * @param enabled true to enable nested scrolling, false to disable 54 * 55 * @see #isNestedScrollingEnabled() 56 */ 57 void setNestedScrollingEnabled(boolean enabled); 58 59 /** 60 * Returns true if nested scrolling is enabled for this view. 61 * 62 * <p>If nested scrolling is enabled and this View class implementation supports it, 63 * this view will act as a nested scrolling child view when applicable, forwarding data 64 * about the scroll operation in progress to a compatible and cooperating nested scrolling 65 * parent.</p> 66 * 67 * @return true if nested scrolling is enabled 68 * 69 * @see #setNestedScrollingEnabled(boolean) 70 */ 71 boolean isNestedScrollingEnabled(); 72 73 /** 74 * Begin a nestable scroll operation along the given axes. 75 * 76 * <p>A view starting a nested scroll promises to abide by the following contract:</p> 77 * 78 * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case 79 * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}. 80 * In the case of touch scrolling the nested scroll will be terminated automatically in 81 * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}. 82 * In the event of programmatic scrolling the caller must explicitly call 83 * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p> 84 * 85 * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found. 86 * If it returns false the caller may ignore the rest of this contract until the next scroll. 87 * Calling startNestedScroll while a nested scroll is already in progress will return true.</p> 88 * 89 * <p>At each incremental step of the scroll the caller should invoke 90 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} 91 * once it has calculated the requested scrolling delta. If it returns true the nested scrolling 92 * parent at least partially consumed the scroll and the caller should adjust the amount it 93 * scrolls by.</p> 94 * 95 * <p>After applying the remainder of the scroll delta the caller should invoke 96 * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing 97 * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat 98 * these values differently. See 99 * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}. 100 * </p> 101 * 102 * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL} 103 * and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}. 104 * @return true if a cooperative parent was found and nested scrolling has been enabled for 105 * the current gesture. 106 * 107 * @see #stopNestedScroll() 108 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 109 * @see #dispatchNestedScroll(int, int, int, int, int[]) 110 */ 111 boolean startNestedScroll(@ScrollAxis int axes); 112 113 /** 114 * Stop a nested scroll in progress. 115 * 116 * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p> 117 * 118 * @see #startNestedScroll(int) 119 */ 120 void stopNestedScroll(); 121 122 /** 123 * Returns true if this view has a nested scrolling parent. 124 * 125 * <p>The presence of a nested scrolling parent indicates that this view has initiated 126 * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p> 127 * 128 * @return whether this view has a nested scrolling parent 129 */ 130 boolean hasNestedScrollingParent(); 131 132 /** 133 * Dispatch one step of a nested scroll in progress. 134 * 135 * <p>Implementations of views that support nested scrolling should call this to report 136 * info about a scroll in progress to the current nested scrolling parent. If a nested scroll 137 * is not currently in progress or nested scrolling is not 138 * {@link #isNestedScrollingEnabled() enabled} for this view this method does nothing.</p> 139 * 140 * <p>Compatible View implementations should also call 141 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} before 142 * consuming a component of the scroll event themselves.</p> 143 * 144 * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step 145 * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step 146 * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view 147 * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view 148 * @param offsetInWindow Optional. If not null, on return this will contain the offset 149 * in local view coordinates of this view from before this operation 150 * to after it completes. View implementations may use this to adjust 151 * expected input coordinate tracking. 152 * @return true if the event was dispatched, false if it could not be dispatched. 153 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 154 */ 155 boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, 156 int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow); 157 158 /** 159 * Dispatch one step of a nested scroll in progress before this view consumes any portion of it. 160 * 161 * <p>Nested pre-scroll events are to nested scroll events what touch intercept is to touch. 162 * <code>dispatchNestedPreScroll</code> offers an opportunity for the parent view in a nested 163 * scrolling operation to consume some or all of the scroll operation before the child view 164 * consumes it.</p> 165 * 166 * @param dx Horizontal scroll distance in pixels 167 * @param dy Vertical scroll distance in pixels 168 * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx 169 * and consumed[1] the consumed dy. 170 * @param offsetInWindow Optional. If not null, on return this will contain the offset 171 * in local view coordinates of this view from before this operation 172 * to after it completes. View implementations may use this to adjust 173 * expected input coordinate tracking. 174 * @return true if the parent consumed some or all of the scroll delta 175 * @see #dispatchNestedScroll(int, int, int, int, int[]) 176 */ 177 boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, 178 @Nullable int[] offsetInWindow); 179 180 /** 181 * Dispatch a fling to a nested scrolling parent. 182 * 183 * <p>This method should be used to indicate that a nested scrolling child has detected 184 * suitable conditions for a fling. Generally this means that a touch scroll has ended with a 185 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 186 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 187 * along a scrollable axis.</p> 188 * 189 * <p>If a nested scrolling child view would normally fling but it is at the edge of 190 * its own content, it can use this method to delegate the fling to its nested scrolling 191 * parent instead. The parent may optionally consume the fling or observe a child fling.</p> 192 * 193 * @param velocityX Horizontal fling velocity in pixels per second 194 * @param velocityY Vertical fling velocity in pixels per second 195 * @param consumed true if the child consumed the fling, false otherwise 196 * @return true if the nested scrolling parent consumed or otherwise reacted to the fling 197 */ 198 boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); 199 200 /** 201 * Dispatch a fling to a nested scrolling parent before it is processed by this view. 202 * 203 * <p>Nested pre-fling events are to nested fling events what touch intercept is to touch 204 * and what nested pre-scroll is to nested scroll. <code>dispatchNestedPreFling</code> 205 * offsets an opportunity for the parent view in a nested fling to fully consume the fling 206 * before the child view consumes it. If this method returns <code>true</code>, a nested 207 * parent view consumed the fling and this view should not scroll as a result.</p> 208 * 209 * <p>For a better user experience, only one view in a nested scrolling chain should consume 210 * the fling at a time. If a parent view consumed the fling this method will return false. 211 * Custom view implementations should account for this in two ways:</p> 212 * 213 * <ul> 214 * <li>If a custom view is paged and needs to settle to a fixed page-point, do not 215 * call <code>dispatchNestedPreFling</code>; consume the fling and settle to a valid 216 * position regardless.</li> 217 * <li>If a nested parent does consume the fling, this view should not scroll at all, 218 * even to settle back to a valid idle position.</li> 219 * </ul> 220 * 221 * <p>Views should also not offer fling velocities to nested parent views along an axis 222 * where scrolling is not currently supported; a {@link android.widget.ScrollView ScrollView} 223 * should not offer a horizontal fling velocity to its parents since scrolling along that 224 * axis is not permitted and carrying velocity along that motion does not make sense.</p> 225 * 226 * @param velocityX Horizontal fling velocity in pixels per second 227 * @param velocityY Vertical fling velocity in pixels per second 228 * @return true if a nested scrolling parent consumed the fling 229 */ 230 boolean dispatchNestedPreFling(float velocityX, float velocityY); 231 } 232
