android實用技巧：android實現(xiàn)listview異步加載圖片

針對listview異步加載圖片這個問題，麥子學(xué)院android開發(fā)老師講了一種非常實用的方法，麥子學(xué)院android開發(fā)老師說凡是是要通過網(wǎng)絡(luò)獲取圖片資源一般使用這種方法比較好，用戶體驗好，下面就說實現(xiàn)方法，先貼上主方法的代碼：

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package cn.wangmeng.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>> p_w_picpathCache;

public AsyncImageLoader() {

p_w_picpathCache = new HashMap<String, SoftReference<Drawable>>();

}

public Drawable loadDrawable(final String p_w_picpathUrl, final ImageCallback p_w_picpathCallback) {

if (p_w_picpathCache.containsKey(p_w_picpathUrl)) {

SoftReference<Drawable> softReference = p_w_picpathCache.get(p_w_picpathUrl);

Drawable drawable = softReference.get();

if (drawable != null) {

return drawable;

}

final Handler handler = new Handler() {

public void handleMessage(Message message) {

p_w_picpathCallback.p_w_picpathLoaded((Drawable) message.obj, p_w_picpathUrl);

}

};

new Thread() {

@Override

public void run() {

Drawable drawable = loadImageFromUrl(p_w_picpathUrl);

p_w_picpathCache.put(p_w_picpathUrl, 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;

}

public interface ImageCallback {

public void p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl);

}

以上代碼是實現(xiàn)異步獲取圖片的主方法，SoftReference是軟引用，是為了更好的為了系統(tǒng)回收變量，重復(fù)的URL直接返回已有的資源，實現(xiàn)回調(diào)函數(shù)，讓數(shù)據(jù)成功后，更新到UI線程。

幾個輔助類文件：

package cn.wangmeng.test;

public class ImageAndText {

private String p_w_picpathUrl;

private String text;

public ImageAndText(String p_w_picpathUrl, String text) {

this.p_w_picpathUrl = p_w_picpathUrl;

this.text = text;

}

public String getImageUrl() {

return p_w_picpathUrl;

}

public String getText() {

return text;

}

package cn.wangmeng.test;

import android.view.View;

import android.widget.ImageView;

import android.widget.TextView;

public class ViewCache {

private View baseView;

private TextView textView;

private ImageView p_w_picpathView;

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 (p_w_picpathView == null) {

p_w_picpathView = (ImageView) baseView.findViewById(R.id.p_w_picpath);

}

return p_w_picpathView;

}

ViewCache是輔助獲取adapter的子元素布局

package cn.wangmeng.test;

import java.util.List;

import cn.wangmeng.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> p_w_picpathAndTexts, ListView listView) {

super(activity, 0, p_w_picpathAndTexts);

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.p_w_picpath_and_text_row, null);

viewCache = new ViewCache(rowView);

rowView.setTag(viewCache);

} else {

viewCache = (ViewCache) rowView.getTag();

}

ImageAndText p_w_picpathAndText = getItem(position);

// Load the p_w_picpath and set it on the ImageView

String p_w_picpathUrl = p_w_picpathAndText.getImageUrl();

ImageView p_w_picpathView = viewCache.getImageView();

p_w_picpathView.setTag(p_w_picpathUrl);

Drawable cachedImage = asyncImageLoader.loadDrawable(p_w_picpathUrl, new ImageCallback() {

public void p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl) {

ImageView p_w_picpathViewByTag = (ImageView) listView.findViewWithTag(p_w_picpathUrl);

if (p_w_picpathViewByTag != null) {

p_w_picpathViewByTag.setImageDrawable(p_w_picpathDrawable);

}

});

if (cachedImage == null) {

p_w_picpathView.setImageResource(R.drawable.default_p_w_picpath);

}else{

p_w_picpathView.setImageDrawable(cachedImage);

}

// Set the text on the TextView

TextView textView = viewCache.getTextView();

textView.setText(p_w_picpathAndText.getText());

return rowView;

}

ImageAndTextListAdapter是實現(xiàn)ListView的Adapter，里面有個技巧就是p_w_picpathView.setTag(p_w_picpathUrl)，setTag是存儲數(shù)據(jù)的，這樣是為了保證在回調(diào)函數(shù)時，listview去更新自己對應(yīng)item,大家仔細(xì)閱讀就知道了。

最后貼出布局文件：

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="horizontal"

android:layout_width="fill_parent"

android:layout_height="wrap_content">

<ImageView android:id="@+id/p_w_picpath"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

<TextView android:id="@+id/text"

android:layout_width="wrap_content"

android:layout_height="wrap_content"/>

</LinearLayout>

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