AndroidのListViewで非同期で画像を表示する

MyAdapter.java

package com.sugaishun.sample;

import java.io.InputStream;
import java.net.URL;
import java.util.List;
import java.util.Locale;

import org.joda.time.DateTime;

import android.content.Context;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;

class MyAdapter extends BaseAdapter {
	private Context context;
	private List<Entry> entries;
	
	MyAdapter(Context context, List<Entry> entries) {
		this.context = context;
		this.entries = entries;
	}
	
	@Override
	public int getCount() {
		return entries.size();
	}

	@Override
	public Object getItem(int position) {
		return entries.get(position);
	}

	@Override
	public long getItemId(int position) {
		return position;
	}

	@Override
	public View getView(int position, View convertView, ViewGroup parent) {
		final Entry entry = (Entry) getItem(position);
		View row = convertView;
		
		if (row == null) {
			LayoutInflater inflater = 
					(LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
			row = inflater.inflate(R.layout.row, parent, false);
		}
		
		ViewHolder holder = (ViewHolder) row.getTag();
		
		if (holder == null) {
			holder = new ViewHolder(row);
			row.setTag(holder);
		}
		
		// 画像を表示するとこの処理
		holder.icon.setImageDrawable(context.getResources().getDrawable(R.drawable.invisible));
		holder.icon.setTag(entry.getLink());

                // キャッシュに画像があればそれを表示。なければAsyncTaskで取ってくる。
		Drawable image = ImageCache.get(entry.getLink());
		if (image == null) {
			FetchImageTask task = new FetchImageTask(holder.icon);
			task.execute(entry.getLink());
		} else {
			holder.icon.setImageDrawable(image);
		}
		
		holder.dateTime.setText(getFormattedDateTime(entry.getTime()));
		holder.title.setText(entry.getTitle());
		holder.blogTitle.setText(entry.getBlog_title());
		
		return row;
	}
	
	public String getFormattedDateTime(String ISO8601DateTime) {
		DateTime dt = new DateTime(ISO8601DateTime);
		return dt.toString("yyyy年MM月dd日 HH:mm", Locale.JAPAN);
	}
	
	class FetchImageTask extends AsyncTask<String, Void, Drawable> {
		private ImageView imageView;
		private String tag;
		
		public FetchImageTask(ImageView imageView) {
			this.imageView = imageView;
			this.tag = imageView.getTag().toString();
		}

		@Override
		protected Drawable doInBackground(String... urls) {
			synchronized (context) {
				try {
					Drawable image = ImageCache.get(urls[0]);
					if (image == null) {
						URL url = new URL("http://favicon.qfor.info/w/" + urls[0]);
						image = Drawable.createFromStream((InputStream) url.getContent(), "");
						ImageCache.set(urls[0], image);
					}
					return image;
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
			return null;
		}

		@Override
		protected void onPostExecute(Drawable result) {
			if (this.tag.equals(this.imageView.getTag())) {
				if (result == null) return;
				this.imageView.setImageDrawable(result);
			}
		}		
	}
}