android中ListView异步加载图片时的图片错位问题解决方案

2401_84123026

于 2024-04-22 19:30:04 发布

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分类专栏：程序员文章标签： android python 开发语言

本文链接：https://blog.youkuaiyun.com/2401_84123026/article/details/138088899

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mHolder.mTextView.setText(url != null ? url.substring(url.lastIndexOf(“/”) + 1) : “”);
mHolder.mImageView.setTag(URLS[position]);
if (mDownloader == null) {
mDownloader = new ImageDownloader();
}
//这句代码的作用是为了解决convertView被重用的时候，图片预设的问题
mHolder.mImageView.setImageResource(R.drawable.ic_launcher);
if (mDownloader != null) {
//异步下载图片
mDownloader.imageDownload(url, mHolder.mImageView, “/yanbin”,MainActivity.this, new OnImageDownload() {
@Override
public void onDownloadSucc(Bitmap bitmap,
String c_url,ImageView mimageView) {
ImageView imageView = (ImageView) mListView.findViewWithTag(c_url);
if (imageView != null) {
imageView.setImageBitmap(bitmap);
imageView.setTag(“”);
}
}
});
}
return convertView;
}
/**
* 使用ViewHolder来优化listview
* @author yanbin
*
*/
private class ViewHolder {
ImageView mImageView;
TextView mTextView;
}
}
}

上面的mDownloader.imageDownload()就是异步下载图片比较核心的方法，该方法在ImageDownloader.java类下。其中的五个参数分别为：要设置在当前ImageView 上的图片的url地址，当前ImageView，文件缓存地址，当前的activity以及图片回调接口。

在ImageDownloader类中，我们首先根据url从软引用中获取图片，如果不存在，从sdcard中读取图片，如果还不存在，则启动一个AsyncTask异步下载图片。注意注意：这里我们做了一个这样的操作：用一个map将当前的url及其对应的MyAsyncTask存放起来了。由于getView会执行至少一次，这一步的操作是为了相同的url创建相同的AsyncTask。在onPostExecute()方法中，将该url对应的信息从map中删除，一定要记得执行这一步。看到很多的异步图片下载的例子中，重复创建AsyncTask都是普遍存在的，这里我们使用上面的思路解决掉了这一问题。更详细的代码自己看ImageDownloader.java类吧，首先给出OnImageDownload.java接口的代码：

[java] view plain copy

public interface OnImageDownload {
void onDownloadSucc(Bitmap bitmap,String c_url,ImageView imageView);
}

ImageDownloader.java的代码(有两百多行，拷贝到eclipse中看会舒服一点)：

[java] view plain copy

public class ImageDownloader {
private static final String TAG = “ImageDownloader”;
private HashMap<String, MyAsyncTask> map = new HashMap<String, MyAsyncTask>();
private Map<String, SoftReference> imageCaches = new HashMap<String, SoftReference>();
/**
*
* @param url 该mImageView对应的url
* @param mImageView
* @param path 文件存储路径
* @param mActivity
* @param download OnImageDownload回调接口，在onPostExecute()中被调用
*/
public void imageDownload(String url,ImageView mImageView,String path,Activity mActivity,OnImageDownload download){
SoftReference currBitmap = imageCaches.get(url);
Bitmap softRefBitmap = null;
if(currBitmap != null){
softRefBitmap = currBitmap.get();
}
String imageName = “”;
if(url != null){
imageName = Util.getInstance().getImageName(url);
}
Bitmap bitmap = getBitmapFromFile(mActivity,imageName,path);
//先从软引用中拿数据
if(currBitmap != null && mImageView != null && softRefBitmap != null && url.equals(mImageView.getTag())){
mImageView.setImageBitmap(softRefBitmap);
}
//软引用中没有，从文件中拿数据
else if(bitmap != null && mImageView != null && url.equals(mImageView.getTag())){
mImageView.setImageBitmap(bitmap);
}
//文件中也没有，此时根据mImageView的tag，即url去判断该url对应的task是否已经在执行，如果在执行，本次操作不创建新的线程，否则创建新的线程。
else if(url != null && needCreateNewTask(mImageView)){
MyAsyncTask task = new MyAsyncTask(url, mImageView, path,mActivity,download);
if(mImageView != null){
Log.i(TAG, "执行MyAsyncTask --> " + Util.flag);
Util.flag ++;
task.execute();
//将对应的url对应的任务存起来
map.put(url, task);
}
}
}
/**
* 判断是否需要重新创建线程下载图片，如果需要，返回值为true。
* @param url
* @param mImageView
* @return
*/
private boolean needCreateNewTask(ImageView mImageView){
boolean b = true;
if(mImageView != null){
String curr_task_url = (String)mImageView.getTag();
if(isTasksContains(curr_task_url)){
b = false;
}
}
return b;
}
/**
* 检查该url（最终反映的是当前的ImageView的tag，tag会根据position的不同而不同）对应的task是否存在
* @param url
* @return
*/
private boolean isTasksContains(String url){
boolean b = false;
if(map != null && map.get(url) != null){
b = true;
}
return b;
}
/**
* 删除map中该url的信息，这一步很重要，不然MyAsyncTask的引用会“一直”存在于map中
* @param url
*/
private void removeTaskFormMap(String url){
if(url != null && map != null && map.get(url) != null){
map.remove(url);
System.out.println(“当前map的大小==”+map.size());
}
}
/**
* 从文件中拿图片
* @param mActivity
* @param imageName 图片名字
* @param path 图片路径
* @return
*/
private Bitmap getBitmapFromFile(Activity mActivity,String imageName,String path){
Bitmap bitmap = null;
if(imageName != null){
File file = null;
String real_path = “”;
try {
if(Util.getInstance().hasSDCard()){
real_path = Util.getInstance().getExtPath() + (path != null && path.startsWith(“/”) ? path : “/” + path);
}else{
real_path = Util.getInstance().getPackagePath(mActivity) + (path != null && path.startsWith(“/”) ? path : “/” + path);
}
file = new File(real_path, imageName);
if(file.exists())
bitmap = BitmapFactory.decodeStream(new FileInputStream(file));
} catch (Exception e) {
e.printStackTrace();
bitmap = null;
}
}
return bitmap;
}
/**
* 将下载好的图片存放到文件中
* @param path 图片路径
* @param mActivity
* @param imageName 图片名字
* @param bitmap 图片
* @return
*/
private boolean setBitmapToFile(String path,Activity mActivity,String imageName,Bitmap bitmap){
File file = null;
String real_path = “”;
try {
if(Util.getInstance().hasSDCard()){
real_path = Util.getInstance().getExtPath() + (path != null && path.startsWith(“/”) ? path : “/” + path);
}else{
real_path = Util.getInstance().getPackagePath(mActivity) + (path != null && path.startsWith(“/”) ? path : “/” + path);
}
file = new File(real_path, imageName);
if(!file.exists()){
File file2 = new File(real_path + “/”);
file2.mkdirs();
}
file.createNewFile();
FileOutputStream fos = null;
if(Util.getInstance().hasSDCard()){
fos = new FileOutputStream(file);
}else{
fos = mActivity.openFileOutput(imageName, Context.MODE_PRIVATE);
}
if (imageName != null && (imageName.contains(“.png”) || imageName.contains(“.PNG”))){
bitmap.compress(Bitmap.CompressFormat.PNG, 90, fos);
}
else{
bitmap.compress(Bitmap.CompressFormat.JPEG, 90, fos);
}
fos.flush();
if(fos != null){
fos.close();
}
return true;
} catch (Exception e) {
e.printStackTrace();
return false;
}
}
/**
* 辅助方法，一般不调用
* @param path
* @param mActivity
* @param imageName
*/
private void removeBitmapFromFile(String path,Activity mActivity,String imageName){
File file = null;
String real_path = “”;
try {
if(Util.getInstance().hasSDCard()){
real_path = Util.getInstance().getExtPath() + (path != null && path.startsWith(“/”) ? path : “/” + path);
}else{
real_path = Util.getInstance().getPackagePath(mActivity) + (path != null && path.startsWith(“/”) ? path : “/” + path);
}
file = new File(real_path, imageName);
if(file != null)
file.delete();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 异步下载图片的方法
* @author yanbin
*
*/
private class MyAsyncTask extends AsyncTask<String, Void, Bitmap>{
private ImageView mImageView;
private String url;
private OnImageDownload download;
private String path;
private Activity mActivity;
public MyAsyncTask(String url,ImageView mImageView,String path,Activity mActivity,OnImageDownload download){
this.mImageView = mImageView;
this.url = url;
this.path = path;
this.mActivity = mActivity;
this.download = download;
}
@Override
protected Bitmap doInBackground(String… params) {
Bitmap data = null;
if(url != null){
try {
URL c_url = new URL(url);
InputStream bitmap_data = c_url.openStream();
data = BitmapFactory.decodeStream(bitmap_data);
String imageName = Util.getInstance().getImageName(url);
if(!setBitmapToFile(path,mActivity,imageName, data)){
removeBitmapFromFile(path,mActivity,imageName);
}
imageCaches.put(url, new SoftReference(data.createScaledBitmap(data, 100, 100, true)));
} catch (Exception e) {
e.printStackTrace();
}
}
return data;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
}
@Override
protected void onPostExecute(Bitmap result) {
//回调设置图片
if(download != null){
download.onDownloadSucc(result,url,mImageView);
//该url对应的task已经下载完成，从map中将其删除
removeTaskFormMap(url);
}
super.onPostExecute(result);
}
}
}