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*滑的滚动listview到一个指定位

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/*
* Copyright (C) 2010 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 com.android.contacts.widget;

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ListView;

/**
* A ListView that can be asked to scroll (smoothly or otherwise) to a specific
* position. This class takes advantage of similar functionality that exists
* in {@link ListView} and enhances it.
*/
public class AutoScrollListView extends ListView {

/**
* Position the element at about 1/3 of the list height
*/
private static final float PREFERRED_SELECTION_OFFSET_FROM_TOP = 0.33f;

private int mRequestedScrollPosition = -1;
private boolean mSmoothScrollRequested;

public AutoScrollListView(Context context) {
super(context);
}

public AutoScrollListView(Context context, AttributeSet attrs) {
super(context, attrs);
}

public AutoScrollListView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
}

/**
* Brings the specified position to view by optionally performing a jump-scroll maneuver:
* first it jumps to some position near the one requested and then does a smooth
* scroll to the requested position. This creates an impression of full smooth
* scrolling without actually traversing the entire list. If smooth scrolling is
* not requested, instantly positions the requested item at a preferred offset.
*/
public void requestPositionToScreen(int position, boolean smoothScroll) {
mRequestedScrollPosition = position;
mSmoothScrollRequested = smoothScroll;
requestLayout();
}

@Override
protected void layoutChildren() {
super.layoutChildren();
if (mRequestedScrollPosition == -1) {
return;
}

final int position = mRequestedScrollPosition;
mRequestedScrollPosition = -1;

int firstPosition = getFirstVisiblePosition() + 1;
int lastPosition = getLastVisiblePosition();
if (position >= firstPosition && position <= lastPosition) {
return; // Already on screen
}

final int offset = (int) (getHeight() * PREFERRED_SELECTION_OFFSET_FROM_TOP);
if (!mSmoothScrollRequested) {
setSelectionFromTop(position, offset);

// Since we have changed the scrolling position, we need to redo child layout
// Calling "requestLayout" in the middle of a layout pass has no effect,
// so we call layoutChildren explicitly
super.layoutChildren();

} else {
// We will first position the list a couple of screens before or after
// the new selection and then scroll smoothly to it.
int twoScreens = (lastPosition - firstPosition) * 2;
int preliminaryPosition;
if (position < firstPosition) { preliminaryPosition = position + twoScreens; if (preliminaryPosition >= getCount()) {
preliminaryPosition = getCount() - 1;
}
if (preliminaryPosition < firstPosition) {
setSelection(preliminaryPosition);
super.layoutChildren();
}
} else {
preliminaryPosition = position - twoScreens;
if (preliminaryPosition < 0) { preliminaryPosition = 0; } if (preliminaryPosition > lastPosition) {
setSelection(preliminaryPosition);
super.layoutChildren();
}
}

smoothScrollToPositionFromTop(position, offset);
}
}
}




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