Android ViewHolder模式

這個ViewHolder到底是什麼呢？我們可以在官方sample看到這段程式碼
http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/view/List14.html

static class ViewHolder {
            TextView text;
            ImageView icon;
        }

可以看到它只是一個靜態類，它的作用就在於減少不必要的呼叫findViewById
完整的官方例子，官方例子中convertView 也是避免inflating View。
然後把對底下的控制元件引用存在ViewHolder裡面，再在View.setTag(holder)把它放在view裡，下次就可以直接取了。

/*
 * Copyright (C) 2008 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.example.android.apis.view;
 
import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;
import android.widget.ImageView;
import android.graphics.BitmapFactory;
import android.graphics.Bitmap;
import com.example.android.apis.R;
 
/**
 * Demonstrates how to write an efficient list adapter. The adapter used in this example binds
 * to an ImageView and to a TextView for each row in the list.
 *
 * To work efficiently the adapter implemented here uses two techniques:
 * - It reuses the convertView passed to getView() to avoid inflating View when it is not necessary
 * - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
 *
 * The ViewHolder pattern consists in storing a data structure in the tag of the view returned by
 * getView(). This data structures contains references to the views we want to bind data to, thus
 * avoiding calls to findViewById() every time getView() is invoked.
 */
public class List14 extends ListActivity {
 
    private static class EfficientAdapter extends BaseAdapter {
        private LayoutInflater mInflater;
        private Bitmap mIcon1;
        private Bitmap mIcon2;
 
        public EfficientAdapter(Context context) {
            // Cache the LayoutInflate to avoid asking for a new one each time.
            mInflater = LayoutInflater.from(context);
 
            // Icons bound to the rows.
            mIcon1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_1);
            mIcon2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_2);
        }
 
        /**
         * The number of items in the list is determined by the number of speeches
         * in our array.
         *
         * @see android.widget.ListAdapter#getCount()
         */
        public int getCount() {
            return DATA.length;
        }
 
        /**
         * Since the data comes from an array, just returning the index is
         * sufficent to get at the data. If we were using a more complex data
         * structure, we would return whatever object represents one row in the
         * list.
         *
         * @see android.widget.ListAdapter#getItem(int)
         */
        public Object getItem(int position) {
            return position;
        }
 
        /**
         * Use the array index as a unique id.
         *
         * @see android.widget.ListAdapter#getItemId(int)
         */
        public long getItemId(int position) {
            return position;
        }
 
        /**
         * Make a view to hold each row.
         *
         * @see android.widget.ListAdapter#getView(int, android.view.View,
         *      android.view.ViewGroup)
         */
        public View getView(int position, View convertView, ViewGroup parent) {
            // A ViewHolder keeps references to children views to avoid unneccessary calls
            // to findViewById() on each row.
            ViewHolder holder;
 
            // When convertView is not null, we can reuse it directly, there is no need
            // to reinflate it. We only inflate a new View when the convertView supplied
            // by ListView is null.
            if (convertView == null) {
                convertView = mInflater.inflate(R.layout.list_item_icon_text, null);
 
                // Creates a ViewHolder and store references to the two children views
                // we want to bind data to.
                holder = new ViewHolder();
                holder.text = (TextView) convertView.findViewById(R.id.text);
                holder.icon = (ImageView) convertView.findViewById(R.id.icon);
 
                convertView.setTag(holder);
            } else {
                // Get the ViewHolder back to get fast access to the TextView
                // and the ImageView.
                holder = (ViewHolder) convertView.getTag();
            }
 
            // Bind the data efficiently with the holder.
            holder.text.setText(DATA[position]);
            holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);
 
            return convertView;
        }
 
        static class ViewHolder {
            TextView text;
            ImageView icon;
        }
    }
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setListAdapter(new EfficientAdapter(this));
    }
 
    private static final String[] DATA = Cheeses.sCheeseStrings;