src/com/android/contacts/PinnedHeaderListView.java

/*
 * 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;

import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.View;
import android.widget.ListAdapter;
import android.widget.ListView;

/**
 * A ListView that maintains a header pinned at the top of the list. The
 * pinned header can be pushed up and dissolved as needed.
 */
public class PinnedHeaderListView extends ListView {

    /**
     * Adapter interface.  The list adapter must implement this interface.
     */
    public interface PinnedHeaderAdapter {

        /**
         * Pinned header state: don't show the header.
         */
        public static final int PINNED_HEADER_GONE = 0;

        /**
         * Pinned header state: show the header at the top of the list.
         */
        public static final int PINNED_HEADER_VISIBLE = 1;

        /**
         * Pinned header state: show the header. If the header extends beyond
         * the bottom of the first shown element, push it up and clip.
         */
        public static final int PINNED_HEADER_PUSHED_UP = 2;

        /**
         * Computes the desired state of the pinned header for the given
         * position of the first visible list item. Allowed return values are
         * {@link #PINNED_HEADER_GONE}, {@link #PINNED_HEADER_VISIBLE} or
         * {@link #PINNED_HEADER_PUSHED_UP}.
         */
        int getPinnedHeaderState(int position);

        /**
         * Configures the pinned header view to match the first visible list item.
         *
         * @param header pinned header view.
         * @param position position of the first visible list item.
         * @param alpha fading of the header view, between 0 and 255.
         */
        void configurePinnedHeader(View header, int position, int alpha);
    }

    private static final int MAX_ALPHA = 255;

    private PinnedHeaderAdapter mAdapter;
    private View mHeaderView;
    private boolean mHeaderViewVisible;

    private int mHeaderViewWidth;

    private int mHeaderViewHeight;

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

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

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

    public void setPinnedHeaderView(View view) {
        mHeaderView = view;

        // Disable vertical fading when the pinned header is present
        // TODO change ListView to allow separate measures for top and bottom fading edge;
        // in this particular case we would like to disable the top, but not the bottom edge.
        if (mHeaderView != null) {
            setFadingEdgeLength(0);
        }
        requestLayout();
    }

    @Override
    public void setAdapter(ListAdapter adapter) {
        super.setAdapter(adapter);
        mAdapter = (PinnedHeaderAdapter)adapter;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if (mHeaderView != null) {
            measureChild(mHeaderView, widthMeasureSpec, heightMeasureSpec);
            mHeaderViewWidth = mHeaderView.getMeasuredWidth();
            mHeaderViewHeight = mHeaderView.getMeasuredHeight();
        }
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        if (mHeaderView != null) {
            mHeaderView.layout(0, 0, mHeaderViewWidth, mHeaderViewHeight);
            configureHeaderView(getFirstVisiblePosition());
        }
    }

    public void configureHeaderView(int position) {
        if (mHeaderView == null) {
            return;
        }

        int state = mAdapter.getPinnedHeaderState(position);
        switch (state) {
            case PinnedHeaderAdapter.PINNED_HEADER_GONE: {
                mHeaderViewVisible = false;
                break;
            }

            case PinnedHeaderAdapter.PINNED_HEADER_VISIBLE: {
                mAdapter.configurePinnedHeader(mHeaderView, position, MAX_ALPHA);
                if (mHeaderView.getTop() != 0) {
                    mHeaderView.layout(0, 0, mHeaderViewWidth, mHeaderViewHeight);
                }
                mHeaderViewVisible = true;
                break;
            }

            case PinnedHeaderAdapter.PINNED_HEADER_PUSHED_UP: {
                View firstView = getChildAt(0);
                int bottom = firstView.getBottom();
                int itemHeight = firstView.getHeight();
                int headerHeight = mHeaderView.getHeight();
                int y;
                int alpha;
                if (bottom < headerHeight) {
                    y = (bottom - headerHeight);
                    alpha = MAX_ALPHA * (headerHeight + y) / headerHeight;
                } else {
                    y = 0;
                    alpha = MAX_ALPHA;
                }
                mAdapter.configurePinnedHeader(mHeaderView, position, alpha);
                if (mHeaderView.getTop() != y) {
                    mHeaderView.layout(0, y, mHeaderViewWidth, mHeaderViewHeight + y);
                }
                mHeaderViewVisible = true;
                break;
            }
        }
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);
        if (mHeaderViewVisible) {
            drawChild(canvas, mHeaderView, getDrawingTime());
        }
    }
}