android联系人源码解析

源代码请从官方网站下载，本文针对源代码增加上自己的理解。
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. * 联系人应用中自定义组件，增强了ListView的功能。 * 用于滑动到某一个位置 以及 在到达某个位置时是否平滑 */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.     * 提供的一个方法，是否平滑滑动到position处     */    public void requestPositionToScreen(int position, boolean smoothScroll) {        mRequestedScrollPosition = position;        mSmoothScrollRequested = smoothScroll;        requestLayout();    }    /** * 这个方法在ListView（ListView包含了一系列的Item【项】）中用于布局ListViewItem。 */    @Override    protected void layoutChildren() {//重写的ListView中的方法，这个方法，主要就是封装了ListView中的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            //如果不需要平滑的滑动到position处，直接调用父类的layoutChildren方法。            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.        //现在是处理平滑滑动到某个位置了，如何处理那？ //我们首先需要将当前的list条目焦点设定在离要滑动的位置2屏内。为什么这么处理那？ //设想一下，list有1000个条目，一屏可以存放10条条目，我们当前处在最后一屏（也就是当前显示990~1000），那么我们要平滑滑动的位置为1 //我们的listView要从990一直平滑滑动到1，这要多费功夫， //所以，直接把焦点设定在list的20处（2屏数目），然后从20位置平滑过度到1。            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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