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/* * Copyright (C) 2015 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.support.v4.view; import android.view.MotionEvent; import android.view.VelocityTracker; import android.view.View; import android.view.ViewConfiguration; import android.view.ViewParent; /** * This interface should be implemented by {@link android.view.View View} subclasses that wish * to support dispatching nested scrolling operations to a cooperating parent * {@link android.view.ViewGroup ViewGroup}. * * <p>Classes implementing this interface should create a final instance of a * {@link NestedScrollingChildHelper} as a field and delegate any View methods to the * <code>NestedScrollingChildHelper</code> methods of the same signature.</p> * * <p>Views invoking nested scrolling functionality should always do so from the relevant * {@link ViewCompat}, {@link ViewGroupCompat} or {@link ViewParentCompat} compatibility * shim static methods. This ensures interoperability with nested scrolling views on Android * 5.0 Lollipop and newer.</p> */ public interface NestedScrollingChild { /** * Enable or disable nested scrolling for this view. * * <p>If this property is set to true the view will be permitted to initiate nested * scrolling operations with a compatible parent view in the current hierarchy. If this * view does not implement nested scrolling this will have no effect. Disabling nested scrolling * while a nested scroll is in progress has the effect of {@link #stopNestedScroll() stopping} * the nested scroll.</p> * * @param enabled true to enable nested scrolling, false to disable * * @see #isNestedScrollingEnabled() */ public void setNestedScrollingEnabled(boolean enabled); /** * Returns true if nested scrolling is enabled for this view. * * <p>If nested scrolling is enabled and this View class implementation supports it, * this view will act as a nested scrolling child view when applicable, forwarding data * about the scroll operation in progress to a compatible and cooperating nested scrolling * parent.</p> * * @return true if nested scrolling is enabled * * @see #setNestedScrollingEnabled(boolean) */ public boolean isNestedScrollingEnabled(); /** * Begin a nestable scroll operation along the given axes. * * <p>A view starting a nested scroll promises to abide by the following contract:</p> * * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}. * In the case of touch scrolling the nested scroll will be terminated automatically in * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}. * In the event of programmatic scrolling the caller must explicitly call * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p> * * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found. * If it returns false the caller may ignore the rest of this contract until the next scroll. * Calling startNestedScroll while a nested scroll is already in progress will return true.</p> * * <p>At each incremental step of the scroll the caller should invoke * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} * once it has calculated the requested scrolling delta. If it returns true the nested scrolling * parent at least partially consumed the scroll and the caller should adjust the amount it * scrolls by.</p> * * <p>After applying the remainder of the scroll delta the caller should invoke * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat * these values differently. See * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}. * </p> * * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL} * and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}. * @return true if a cooperative parent was found and nested scrolling has been enabled for * the current gesture. * * @see #stopNestedScroll() * @see #dispatchNestedPreScroll(int, int, int[], int[]) * @see #dispatchNestedScroll(int, int, int, int, int[]) */ public boolean startNestedScroll(int axes); /** * Stop a nested scroll in progress. * * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p> * * @see #startNestedScroll(int) */ public void stopNestedScroll(); /** * Returns true if this view has a nested scrolling parent. * * <p>The presence of a nested scrolling parent indicates that this view has initiated * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p> * * @return whether this view has a nested scrolling parent */ public boolean hasNestedScrollingParent(); /** * Dispatch one step of a nested scroll in progress. * * <p>Implementations of views that support nested scrolling should call this to report * info about a scroll in progress to the current nested scrolling parent. If a nested scroll * is not currently in progress or nested scrolling is not * {@link #isNestedScrollingEnabled() enabled} for this view this method does nothing.</p> * * <p>Compatible View implementations should also call * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll} before * consuming a component of the scroll event themselves.</p> * * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view * @param offsetInWindow Optional. If not null, on return this will contain the offset * in local view coordinates of this view from before this operation * to after it completes. View implementations may use this to adjust * expected input coordinate tracking. * @return true if the event was dispatched, false if it could not be dispatched. * @see #dispatchNestedPreScroll(int, int, int[], int[]) */ public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); /** * Dispatch one step of a nested scroll in progress before this view consumes any portion of it. * * <p>Nested pre-scroll events are to nested scroll events what touch intercept is to touch. * <code>dispatchNestedPreScroll</code> offers an opportunity for the parent view in a nested * scrolling operation to consume some or all of the scroll operation before the child view * consumes it.</p> * * @param dx Horizontal scroll distance in pixels * @param dy Vertical scroll distance in pixels * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx * and consumed[1] the consumed dy. * @param offsetInWindow Optional. If not null, on return this will contain the offset * in local view coordinates of this view from before this operation * to after it completes. View implementations may use this to adjust * expected input coordinate tracking. * @return true if the parent consumed some or all of the scroll delta * @see #dispatchNestedScroll(int, int, int, int, int[]) */ public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow); /** * Dispatch a fling to a nested scrolling parent. * * <p>This method should be used to indicate that a nested scrolling child has detected * suitable conditions for a fling. Generally this means that a touch scroll has ended with a * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity} * along a scrollable axis.</p> * * <p>If a nested scrolling child view would normally fling but it is at the edge of * its own content, it can use this method to delegate the fling to its nested scrolling * parent instead. The parent may optionally consume the fling or observe a child fling.</p> * * @param velocityX Horizontal fling velocity in pixels per second * @param velocityY Vertical fling velocity in pixels per second * @param consumed true if the child consumed the fling, false otherwise * @return true if the nested scrolling parent consumed or otherwise reacted to the fling */ public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); /** * Dispatch a fling to a nested scrolling parent before it is processed by this view. * * <p>Nested pre-fling events are to nested fling events what touch intercept is to touch * and what nested pre-scroll is to nested scroll. <code>dispatchNestedPreFling</code> * offsets an opportunity for the parent view in a nested fling to fully consume the fling * before the child view consumes it. If this method returns <code>true</code>, a nested * parent view consumed the fling and this view should not scroll as a result.</p> * * <p>For a better user experience, only one view in a nested scrolling chain should consume * the fling at a time. If a parent view consumed the fling this method will return false. * Custom view implementations should account for this in two ways:</p> * * <ul> * <li>If a custom view is paged and needs to settle to a fixed page-point, do not * call <code>dispatchNestedPreFling</code>; consume the fling and settle to a valid * position regardless.</li> * <li>If a nested parent does consume the fling, this view should not scroll at all, * even to settle back to a valid idle position.</li> * </ul> * * <p>Views should also not offer fling velocities to nested parent views along an axis * where scrolling is not currently supported; a {@link android.widget.ScrollView ScrollView} * should not offer a horizontal fling velocity to its parents since scrolling along that * axis is not permitted and carrying velocity along that motion does not make sense.</p> * * @param velocityX Horizontal fling velocity in pixels per second * @param velocityY Vertical fling velocity in pixels per second * @return true if a nested scrolling parent consumed the fling */ public boolean dispatchNestedPreFling(float velocityX, float velocityY); }