sdk/sources/android-23/android/support/v4/widget/ListViewAutoScrollHelper.java

   1 /*
   2  * Copyright (C) 2013 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 package android.support.v4.widget;
  18
  19 import android.view.View;
  20 import android.widget.ListView;
  21
  22 /**
  23  * An implementation of {@link AutoScrollHelper} that knows how to scroll
  24  * through a {@link ListView}.
  25  */
  26 public class ListViewAutoScrollHelper extends AutoScrollHelper {
  27     private final ListView mTarget;
  28
  29     public ListViewAutoScrollHelper(ListView target) {
  30         super(target);
  31
  32         mTarget = target;
  33     }
  34
  35     @Override
  36     public void scrollTargetBy(int deltaX, int deltaY) {
  37         final ListView target = mTarget;
  38         final int firstPosition = target.getFirstVisiblePosition();
  39         if (firstPosition == ListView.INVALID_POSITION) {
  40             return;
  41         }
  42
  43         final View firstView = target.getChildAt(0);
  44         if (firstView == null) {
  45             return;
  46         }
  47
  48         final int newTop = firstView.getTop() - deltaY;
  49         target.setSelectionFromTop(firstPosition, newTop);
  50     }
  51
  52     @Override
  53     public boolean canTargetScrollHorizontally(int direction) {
  54         // List do not scroll horizontally.
  55         return false;
  56     }
  57
  58     @Override
  59     public boolean canTargetScrollVertically(int direction) {
  60         final ListView target = mTarget;
  61         final int itemCount = target.getCount();
  62         if (itemCount == 0) {
  63             return false;
  64         }
  65
  66         final int childCount = target.getChildCount();
  67         final int firstPosition = target.getFirstVisiblePosition();
  68         final int lastPosition = firstPosition + childCount;
  69
  70         if (direction > 0) {
  71             // Are we already showing the entire last item?
  72             if (lastPosition >= itemCount) {
  73                 final View lastView = target.getChildAt(childCount - 1);
  74                 if (lastView.getBottom() <= target.getHeight()) {
  75                     return false;
  76                 }
  77             }
  78         } else if (direction < 0) {
  79             // Are we already showing the entire first item?
  80             if (firstPosition <= 0) {
  81                 final View firstView = target.getChildAt(0);
  82                 if (firstView.getTop() >= 0) {
  83                     return false;
  84                 }
  85             }
  86         } else {
  87             // The behavior for direction 0 is undefined and we can return
  88             // whatever we want.
  89             return false;
  90         }
  91
  92         return true;
  93     }
  94 }