Android优化系列——控件优化(ListView 异步加载图片优化,SoftReference)

整理来自Android优化技术详解一书

ListView控件优化

网络上有很多关于ListView优化的博文，这里就不详细讲述了

• 重写getView方法中，if语句判断convertView是否为空，如果不为空，调用getTag()方法

ViewHolder holder;if(convertView == null){//这里初始化holder类中变量...convertView.setTag(holder);}else{    holder = (ViewHolder)convertView.getTag();}

• 创建内部类ViewHolder

static class ViewHolder{    TextView text1;    ...}

以上代码同样适用于重写Adapter。

ListView 异步加载图片优化

• 主要方法

package cn.xxx.test;import java.io.IOException;import java.io.InputStream;import java.lang.ref.SoftReference;import java.net.MalformedURLException;import java.net.URL;import java.util.HashMap;import android.graphics.drawable.Drawable;import android.os.Handler;import android.os.Message;public class AsyncImageLoader {     private HashMap<String, SoftReference<Drawable>> imageCache;         public AsyncImageLoader() {             imageCache = new HashMap<String, SoftReference<Drawable>>();         }         public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {             if (imageCache.containsKey(imageUrl)) {                 //SoftReference 是软引用,具体介绍在后面                 SoftReference<Drawable> softReference = imageCache.get(imageUrl);                 Drawable drawable = softReference.get();                 if (drawable != null) {                     return drawable;                 }             }             final Handler handler = new Handler() {                 public void handleMessage(Message message) {                     imageCallback.imageLoaded((Drawable) message.obj, imageUrl);                 }             };             new Thread() {                 @Override                 public void run() {                     Drawable drawable = loadImageFromUrl(imageUrl);                     imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));                     Message message = handler.obtainMessage(0, drawable);                     handler.sendMessage(message);                 }             }.start();             return null;         }        public static Drawable loadImageFromUrl(String url) {            URL m;            InputStream i = null;            try {                m = new URL(url);                i = (InputStream) m.getContent();            } catch (MalformedURLException e1) {                e1.printStackTrace();            } catch (IOException e) {                e.printStackTrace();            }            Drawable d = Drawable.createFromStream(i, "src");            return d;        }          //这里是一个回调接口，imageLoaded方法在后面的代码中实现         public interface ImageCallback {             public void imageLoaded(Drawable imageDrawable, String imageUrl);         }}

上述代码是实现异步获取图片的主方法，SoftReference 是软引用，目的是更好的实现系统回收变量，重复的URL直接返回已有的资源，实现回调函数，让数据成功后，更新到UI线程。
SoftReference的特点是它的一个实例保存对一个Java对象的软引用，该软引用的存在不妨碍垃圾收集线程对该Java对象的回收。也就是说，一旦SoftReference保存了对一个Java对象的软引用后，在垃圾线程对这个Java对象回收前，SoftReference类所提供的get()方法返回Java对象的强引用。另外，一旦垃圾线程回收该Java对象之后，get()方法将返回null。
SoftReferences 只有在 jvm Out Of Memory 之前才会被回收, 所以它非常适合缓存应用。

• 辅助类文件ImageAndText，类似持久化类

package cn.xxx.test;public class ImageAndText {        private String imageUrl;        private String text;        public ImageAndText(String imageUrl, String text) {            this.imageUrl = imageUrl;            this.text = text;        }        public String getImageUrl() {            return imageUrl;        }        public String getText() {            return text;        }}

• 辅助类文件ViewCache，用途类似ViewHolder。

package cn.xxx.test;import cn.xxx.test.R;import android.view.View;import android.widget.ImageView;import android.widget.TextView;public class ViewCache {        private View baseView;        private TextView textView;        private ImageView imageView;        public ViewCache(View baseView) {            this.baseView = baseView;        }        public TextView getTextView() {            if (textView == null) {                textView = (TextView) baseView.findViewById(R.id.text);            }            return textView;        }        public ImageView getImageView() {            if (imageView == null) {                imageView = (ImageView) baseView.findViewById(R.id.image);            }            return imageView;        }}

• 实现ListView的Adapter类

package cn.xxx.test;import java.util.List;import cn.xxx.test.R;import cn.xxx.test.AsyncImageLoader.ImageCallback;import android.app.Activity;import android.graphics.drawable.Drawable;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.ArrayAdapter;import android.widget.ImageView;import android.widget.ListView;import android.widget.TextView;public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {        private ListView listView;        private AsyncImageLoader asyncImageLoader;        public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {            super(activity, 0, imageAndTexts);            this.listView = listView;            asyncImageLoader = new AsyncImageLoader();        }        public View getView(int position, View convertView, ViewGroup parent) {            Activity activity = (Activity) getContext();            // Inflate the views from XML            View rowView = convertView;            ViewCache viewCache;            if (rowView == null) {                LayoutInflater inflater = activity.getLayoutInflater();                rowView = inflater.inflate(R.layout.image_and_text_row, null);                viewCache = new ViewCache(rowView);                rowView.setTag(viewCache);            } else {                viewCache = (ViewCache) rowView.getTag();            }            ImageAndText imageAndText = getItem(position);            // Load the image and set it on the ImageView            String imageUrl = imageAndText.getImageUrl();            ImageView imageView = viewCache.getImageView();            imageView.setTag(imageUrl);            //这里是最关键的回调函数的使用，当线程加载完图片时，通知handler调用回调函数，回调函数再调用以下代码            Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {                public void imageLoaded(Drawable imageDrawable, String imageUrl) {                    ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);                    if (imageViewByTag != null) {                        imageViewByTag.setImageDrawable(imageDrawable);                    }                }            });            if (cachedImage == null) {                imageView.setImageResource(R.drawable.default_image);            }else{                imageView.setImageDrawable(cachedImage);            }            // Set the text on the TextView            TextView textView = viewCache.getTextView();            textView.setText(imageAndText.getText());            return rowView;        }}