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);

        }

    }

}