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Android进阶：ListView性能优化异步加载图片 使滑动效果流畅

ListView 是一种可以显示一系列项目并能进行滚动显示的 View，每一行的Item可能包含复杂的结构，可能会从网络上获取icon等的一些图标信息，就现在的网络速度要想保持ListView运行的很好滚动流畅是做不到的

 

所以这里就需要把这些信息利用多线程实现异步加载

 

实现这样功能的类

 

 

  1. public class AsyncImageLoader {  
  2.     private HashMap<String, SoftReference<Drawable>> imageCache;  
  3.    
  4.     public AsyncImageLoader() {  
  5.         imageCache = new HashMap<String, SoftReference<Drawable>>();  
  6.     }  
  7.    
  8.     public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {  
  9.         if (imageCache.containsKey(imageUrl)) {  
  10.             SoftReference<Drawable> softReference = imageCache.get(imageUrl);  
  11.             Drawable drawable = softReference.get();  
  12.             if (drawable != null) {  
  13.                 return drawable;  
  14.             }  
  15.         }  
  16.         final Handler handler = new Handler() {  
  17.             @Override  
  18.             public void handleMessage(Message message) {  
  19.                 imageCallback.imageLoaded((Drawable) message.obj, imageUrl);  
  20.             }  
  21.         };  
  22.         new Thread() {  
  23.             @Override  
  24.             public void run() {  
  25.                 Drawable drawable = loadImageFromUrl(imageUrl);  
  26.                 imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));  
  27.                 Message message = handler.obtainMessage(0, drawable);  
  28.                 handler.sendMessage(message);  
  29.             }  
  30.         }.start();  
  31.         return null;  
  32.     }  
  33.    
  34.     public static Drawable loadImageFromUrl(String url) {  
  35.         // ...   
  36.     }  
  37.    
  38.     public interface ImageCallback {  
  39.         public void imageLoaded(Drawable imageDrawable, String imageUrl);  
  40.     }  
  41. }  

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<Drawable> softReference = imageCache.get(imageUrl); Drawable drawable = softReference.get(); if (drawable != null) { return drawable; } } final Handler handler = new Handler() { @Override 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) { // ... } public interface ImageCallback { public void imageLoaded(Drawable imageDrawable, String imageUrl); }}

 

注意这里使用了 SoftReference来缓存图片，允许 GC在需要的时候可以对缓存中的图片进行清理。它这样工作：

·         调用 loadDrawable(ImageUrl, imageCallback)，传入一个匿名实现的 ImageCallback接口

·         如果图片在缓存中不存在的话，图片将从单一的线程中下载并在下载结束时通过 ImageCallback回调

·         如果图片确实存在于缓存中，就会马上返回，不会回调 ImageCallback

 

        然后我们还可以根据09google I/0开发者大会提到的方式来继续优化Adapter 使用ViewHolder来减少一些比较费时的操作，譬如inflate XML 和 findViewById()等操作

      

  1. public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {  
  2.    
  3.     private ListView listView;  
  4.     private AsyncImageLoader asyncImageLoader;  
  5.    
  6.     public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {  
  7.         super(activity, 0, imageAndTexts);  
  8.         this.listView = listView;  
  9.         asyncImageLoader = new AsyncImageLoader();  
  10.     }  
  11.    
  12.     @Override  
  13.     public View getView(int position, View convertView, ViewGroup parent) {  
  14.         Activity activity = (Activity) getContext();  
  15.    
  16.         // Inflate the views from XML   
  17.         View rowView = convertView;  
  18.         ViewCache viewCache;  
  19.         if (rowView == null) {  
  20.             LayoutInflater inflater = activity.getLayoutInflater();  
  21.             rowView = inflater.inflate(R.layout.image_and_text_row, null);  
  22.             viewCache = new ViewCache(rowView);  
  23.             rowView.setTag(viewCache);  
  24.         } else {  
  25.             viewCache = (ViewCache) rowView.getTag();  
  26.         }  
  27.         ImageAndText imageAndText = getItem(position);  
  28.    
  29.         // Load the image and set it on the ImageView   
  30.         String imageUrl = imageAndText.getImageUrl();  
  31.         ImageView imageView = viewCache.getImageView();  
  32.         imageView.setTag(imageUrl);  
  33.         Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {  
  34.             public void imageLoaded(Drawable imageDrawable, String imageUrl) {  
  35.                 ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);  
  36.                 if (imageViewByTag != null) {  
  37.                     imageViewByTag.setImageDrawable(imageDrawable);  
  38.                 }  
  39.             }  
  40.         });  
  41.         imageView.setImageDrawable(cachedImage);  
  42.    
  43.         // Set the text on the TextView   
  44.         TextView textView = viewCache.getTextView();  
  45.         textView.setText(imageAndText.getText());  
  46.    
  47.         return rowView;  
  48.     }  
  49. }   

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(); } @Override 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); 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); } } }); imageView.setImageDrawable(cachedImage); // Set the text on the TextView TextView textView = viewCache.getTextView(); textView.setText(imageAndText.getText()); return rowView; }} 

 

 

      这里我们没有加载完iamge之后直接设定到相应的ImageView上 ，而是通过Tag查找，这里我们重用的View 这里有个listView的引用来通过Tag查找 可见 CallBack的实现

 

     

  1. ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);  
  2.                if (imageViewByTag != null) {  
  3.                    imageViewByTag.setImageDrawable(imageDrawable);  
  4.                }  

ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl); if (imageViewByTag != null) { imageViewByTag.setImageDrawable(imageDrawable); }

 

      这里通过ViewCatch来减少了 findViewById的使用

 

    

  1. public class ViewCache {  
  2.    
  3.     private View baseView;  
  4.     private TextView textView;  
  5.     private ImageView imageView;  
  6.    
  7.     public ViewCache(View baseView) {  
  8.         this.baseView = baseView;  
  9.     }  
  10.    
  11.     public TextView getTextView() {  
  12.         if (textView == null) {  
  13.             textView = (TextView) baseView.findViewById(R.id.text);  
  14.         }  
  15.         return titleView;  
  16.     }  
  17.    
  18.     public ImageView getImageView() {  
  19.         if (imageView == null) {  
  20.             imageView = (ImageView) baseView.findViewById(R.id.image);  
  21.         }  
  22.         return imageView;  
  23.     }  
  24. }   

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 titleView; } public ImageView getImageView() { if (imageView == null) { imageView = (ImageView) baseView.findViewById(R.id.image); } return imageView; }} 

 

     总结 ：这里主要做了三点优化

 

 

  • 在单一线程里加载图片
  •   重用列表中行
  • 缓存行中的 View

 

最后更新：2017-04-02 06:51:45

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