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.support.annotation.Nullable; 21 import android.support.v4.view.ViewCompat.ScrollAxis; 22 import android.view.MotionEvent; 23 import android.view.VelocityTracker; 24 import android.view.View; 25 import android.view.ViewConfiguration; 26 import android.view.ViewParent; 27 28 /** 29 * This interface should be implemented by {@link android.view.View View} subclasses that wish 30 * to support dispatching nested scrolling operations to a cooperating parent 31 * {@link android.view.ViewGroup ViewGroup}. 32 * 33 * <p>Classes implementing this interface should create a final instance of a 34 * {@link NestedScrollingChildHelper} as a field and delegate any View methods to the 35 * <code>NestedScrollingChildHelper</code> methods of the same signature.</p> 36 * 37 * <p>Views invoking nested scrolling functionality should always do so from the relevant 38 * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility 39 * shim static methods. This ensures interoperability with nested scrolling views on Android 40 * 5.0 Lollipop and newer.</p> 41 */ 42 public interface NestedScrollingChild { 43 /** 44 * Enable or disable nested scrolling for this view. 45 * 46 * <p>If this property is set to true the view will be permitted to initiate nested 47 * scrolling operations with a compatible parent view in the current hierarchy. If this 48 * view does not implement nested scrolling this will have no effect. Disabling nested scrolling 49 * while a nested scroll is in progress has the effect of {@link #stopNestedScroll() stopping} 50 * the nested scroll.</p> 51 * 52 * @param enabled true to enable nested scrolling, false to disable 53 * 54 * @see #isNestedScrollingEnabled() 55 */ setNestedScrollingEnabled(boolean enabled)56 void setNestedScrollingEnabled(boolean enabled); 57 58 /** 59 * Returns true if nested scrolling is enabled for this view. 60 * 61 * <p>If nested scrolling is enabled and this View class implementation supports it, 62 * this view will act as a nested scrolling child view when applicable, forwarding data 63 * about the scroll operation in progress to a compatible and cooperating nested scrolling 64 * parent.</p> 65 * 66 * @return true if nested scrolling is enabled 67 * 68 * @see #setNestedScrollingEnabled(boolean) 69 */ isNestedScrollingEnabled()70 boolean isNestedScrollingEnabled(); 71 72 /** 73 * Begin a nestable scroll operation along the given axes. 74 * 75 * <p>A view starting a nested scroll promises to abide by the following contract:</p> 76 * 77 * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case 78 * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}. 79 * In the case of touch scrolling the nested scroll will be terminated automatically in 80 * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}. 81 * In the event of programmatic scrolling the caller must explicitly call 82 * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p> 83 * 84 * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found. 85 * If it returns false the caller may ignore the rest of this contract until the next scroll. 86 * Calling startNestedScroll while a nested scroll is already in progress will return true.</p> 87 * 88 * <p>At each incremental step of the scroll the caller should invoke 89 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} 90 * once it has calculated the requested scrolling delta. If it returns true the nested scrolling 91 * parent at least partially consumed the scroll and the caller should adjust the amount it 92 * scrolls by.</p> 93 * 94 * <p>After applying the remainder of the scroll delta the caller should invoke 95 * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing 96 * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat 97 * these values differently. See 98 * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}. 99 * </p> 100 * 101 * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL} 102 * and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}. 103 * @return true if a cooperative parent was found and nested scrolling has been enabled for 104 * the current gesture. 105 * 106 * @see #stopNestedScroll() 107 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 108 * @see #dispatchNestedScroll(int, int, int, int, int[]) 109 */ startNestedScroll(@crollAxis int axes)110 boolean startNestedScroll(@ScrollAxis int axes); 111 112 /** 113 * Stop a nested scroll in progress. 114 * 115 * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p> 116 * 117 * @see #startNestedScroll(int) 118 */ stopNestedScroll()119 void stopNestedScroll(); 120 121 /** 122 * Returns true if this view has a nested scrolling parent. 123 * 124 * <p>The presence of a nested scrolling parent indicates that this view has initiated 125 * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p> 126 * 127 * @return whether this view has a nested scrolling parent 128 */ hasNestedScrollingParent()129 boolean hasNestedScrollingParent(); 130 131 /** 132 * Dispatch one step of a nested scroll in progress. 133 * 134 * <p>Implementations of views that support nested scrolling should call this to report 135 * info about a scroll in progress to the current nested scrolling parent. If a nested scroll 136 * is not currently in progress or nested scrolling is not 137 * {@link #isNestedScrollingEnabled() enabled} for this view this method does nothing.</p> 138 * 139 * <p>Compatible View implementations should also call 140 * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} before 141 * consuming a component of the scroll event themselves.</p> 142 * 143 * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step 144 * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step 145 * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view 146 * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view 147 * @param offsetInWindow Optional. If not null, on return this will contain the offset 148 * in local view coordinates of this view from before this operation 149 * to after it completes. View implementations may use this to adjust 150 * expected input coordinate tracking. 151 * @return true if the event was dispatched, false if it could not be dispatched. 152 * @see #dispatchNestedPreScroll(int, int, int[], int[]) 153 */ dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow)154 boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, 155 int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow); 156 157 /** 158 * Dispatch one step of a nested scroll in progress before this view consumes any portion of it. 159 * 160 * <p>Nested pre-scroll events are to nested scroll events what touch intercept is to touch. 161 * <code>dispatchNestedPreScroll</code> offers an opportunity for the parent view in a nested 162 * scrolling operation to consume some or all of the scroll operation before the child view 163 * consumes it.</p> 164 * 165 * @param dx Horizontal scroll distance in pixels 166 * @param dy Vertical scroll distance in pixels 167 * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx 168 * and consumed[1] the consumed dy. 169 * @param offsetInWindow Optional. If not null, on return this will contain the offset 170 * in local view coordinates of this view from before this operation 171 * to after it completes. View implementations may use this to adjust 172 * expected input coordinate tracking. 173 * @return true if the parent consumed some or all of the scroll delta 174 * @see #dispatchNestedScroll(int, int, int, int, int[]) 175 */ dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow)176 boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, 177 @Nullable int[] offsetInWindow); 178 179 /** 180 * Dispatch a fling to a nested scrolling parent. 181 * 182 * <p>This method should be used to indicate that a nested scrolling child has detected 183 * suitable conditions for a fling. Generally this means that a touch scroll has ended with a 184 * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds 185 * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} 186 * along a scrollable axis.</p> 187 * 188 * <p>If a nested scrolling child view would normally fling but it is at the edge of 189 * its own content, it can use this method to delegate the fling to its nested scrolling 190 * parent instead. The parent may optionally consume the fling or observe a child fling.</p> 191 * 192 * @param velocityX Horizontal fling velocity in pixels per second 193 * @param velocityY Vertical fling velocity in pixels per second 194 * @param consumed true if the child consumed the fling, false otherwise 195 * @return true if the nested scrolling parent consumed or otherwise reacted to the fling 196 */ dispatchNestedFling(float velocityX, float velocityY, boolean consumed)197 boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); 198 199 /** 200 * Dispatch a fling to a nested scrolling parent before it is processed by this view. 201 * 202 * <p>Nested pre-fling events are to nested fling events what touch intercept is to touch 203 * and what nested pre-scroll is to nested scroll. <code>dispatchNestedPreFling</code> 204 * offsets an opportunity for the parent view in a nested fling to fully consume the fling 205 * before the child view consumes it. If this method returns <code>true</code>, a nested 206 * parent view consumed the fling and this view should not scroll as a result.</p> 207 * 208 * <p>For a better user experience, only one view in a nested scrolling chain should consume 209 * the fling at a time. If a parent view consumed the fling this method will return false. 210 * Custom view implementations should account for this in two ways:</p> 211 * 212 * <ul> 213 * <li>If a custom view is paged and needs to settle to a fixed page-point, do not 214 * call <code>dispatchNestedPreFling</code>; consume the fling and settle to a valid 215 * position regardless.</li> 216 * <li>If a nested parent does consume the fling, this view should not scroll at all, 217 * even to settle back to a valid idle position.</li> 218 * </ul> 219 * 220 * <p>Views should also not offer fling velocities to nested parent views along an axis 221 * where scrolling is not currently supported; a {@link android.widget.ScrollView ScrollView} 222 * should not offer a horizontal fling velocity to its parents since scrolling along that 223 * axis is not permitted and carrying velocity along that motion does not make sense.</p> 224 * 225 * @param velocityX Horizontal fling velocity in pixels per second 226 * @param velocityY Vertical fling velocity in pixels per second 227 * @return true if a nested scrolling parent consumed the fling 228 */ dispatchNestedPreFling(float velocityX, float velocityY)229 boolean dispatchNestedPreFling(float velocityX, float velocityY); 230 } 231