使用 AsyncTask 在 ListView 中加载图像 [英] Using AsyncTask to load Images in ListView
问题描述
我有一个可以容纳图像的 ListView.这取决于图像是否存在于 SDCARD 中.
I have one ListView which can hold an image. It depends if image exists or not in SDCARD.
这是我的示例代码:
public class MainActivity extends Activity {
ListView mListView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mListView = new ListView(this);
setContentView(mListView);
String[] arr = new String[] {
"/example/images/1.jpg", "/example/images/2.jpg",
"/example/images/3.jpg", "/example/images/4.jpg",
"/example/images/5.jpg", "/example/images/6.jpg",
"/example/images/7.jpg", "/example/images/8.jpg",
"/example/images/9.jpg", "/example/images/1.jpg",
"/example/images/2.jpg", "/example/images/3.jpg",
"/example/images/4.jpg", "/example/images/5.jpg",
"/example/images/6.jpg", "/example/images/7.jpg",
"/example/images/8.jpg", "/example/images/9.jpg",
"/example/images/1.jpg", "/example/images/2.jpg",
"/example/images/3.jpg", "/example/images/4.jpg",
"/example/images/5.jpg", "/example/images/6.jpg",
"/example/images/7.jpg", "/example/images/8.jpg",
"/example/images/9.jpg", "/example/images/1.jpg",
"/example/images/2.jpg", "/example/images/3.jpg",
"/example/images/4.jpg", "/example/images/5.jpg",
"/example/images/6.jpg", "/example/images/7.jpg",
"/example/images/8.jpg", "/example/images/9.jpg"};
List<String> list = Arrays.asList(arr);
MyAdapter adapter = new MyAdapter(this, R.layout.listitem_imv, list);
mListView.setAdapter(adapter);
}
class MyAdapter extends ArrayAdapter<String>{
List<String> mList;
LayoutInflater mInflater;
int mResource;
public MyAdapter(Context context, int resource,
List<String> objects) {
super(context, resource, objects);
mResource = resource;
mInflater = getLayoutInflater();
mList = objects;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view;
if(convertView == null){
view = mInflater.inflate(mResource, null);
}else{
view = convertView;
}
ImageView imageView = (ImageView) view.findViewById(R.id.imv);
TextView textView = (TextView) view.findViewById(R.id.txv);
imageView.setTag(mList.get(position));//tag of imageView == path to image
new LoadImage().execute(imageView);
textView.setText(mList.get(position).toString());
return view;
}
}
class LoadImage extends AsyncTask<Object, Void, Bitmap>{
private ImageView imv;
private String path;
@Override
protected Bitmap doInBackground(Object... params) {
imv = (ImageView) params[0];
path = imv.getTag().toString();
Bitmap bitmap = null;
File file = new File(
Environment.getExternalStorageDirectory().getAbsolutePath() + path);
if(file.exists()){
bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap result) {
if(result != null && imv != null){
imv.setVisibility(View.VISIBLE);
imv.setImageBitmap(result);
}else{
imv.setVisibility(View.GONE);
}
}
}
}
'sdcard/example/images' 目录有图片:1.jpg、2.jpg、3.jpg、4.jpg、6.jpg、7.jpg 和 9.jpg.预期结果是:
The 'sdcard/example/images' directory has the images: 1.jpg, 2.jpg, 3.jpg, 4.jpg, 6.jpg, 7.jpg and 9.jpg. the expected result is:
但是，如果我快速滚动列表，某些图像会插入错误的项目中.这是由于在 getView() 方法中使用了 convertView.
But, if I scroll the list quickly, some images are inserted in the wrong items. It happens due to use of convertView in getView() method.
如果我使用下面的代码，代码工作正常:
If I use the following code, the code works fine:
//if(convertView == null){
// view = mInflater.inflate(mResource, null);
//}else{
// view = convertView;
//}
view = mInflater.inflate(mResource, null);
当列表快速滚动时，由于使用了convertView，两个asyncTask可以引用同一个View.当 View 不再可见时，我如何取消 AsyncTask?(并且被 ListView 的另一个项目使用)
When list scrolled quickly, two asyncTasks can reference one same View, due to use of convertView. How Can I cancel AsyncTask when the View is no longer visible?(and is useb by another item of ListView)
编辑
@Override
protected void onPostExecute(Bitmap result) {
if(result != null && imv != null){
if(imv.getTag().equals(path)){
imv.setVisibility(View.VISIBLE);
imv.setImageBitmap(result);
}else{
imv.setVisibility(View.GONE);
}
}else{
imv.setVisibility(View.GONE);
}
}
推荐答案
您可以将 ImageView 发送到任务构造函数并在那里保留对图像路径的引用.现在在 onPostExecute，检查 ImageView 的当前标签是否与您开始使用的标签相同.如果是，则设置图像.如果没有，请不要做任何事情.
You can send in the ImageView to the task constructor and keep a reference to the image path there. Now at onPostExecute, check if the current tag of the ImageView is the same as the one that you started with. If yes, then set the image. If no, don't do anything.
然而，这意味着无论如何都会下载图像.您不会在视图上设置错误的图像.
However, this means that the image will be downloaded in any case. You'll just not set the wrong image on the view.
首先将 ImageView 传递给任务构造函数:
First pass the ImageView to the task constructor:
new LoadImage(imageView).execute()
然后在 LoadImage 构造函数中保存对 ImageView 和图像路径的引用.将路径保存在构造函数中而不是 doInBackground 中很重要，以确保我们不会遇到多线程问题.然后在 onPostExecute 我们检查当前路径.
Then save a reference to the ImageView and image path in LoadImage constructor. It is important to save the path in the constructor and not in doInBackground to ensure that we don't run into multi threading problems. Then at onPostExecute we check the current path.
class LoadImage extends AsyncTask<Object, Void, Bitmap>{
private ImageView imv;
private String path;
public LoadImage(ImageView imv) {
this.imv = imv;
this.path = imv.getTag().toString();
}
@Override
protected Bitmap doInBackground(Object... params) {
Bitmap bitmap = null;
File file = new File(
Environment.getExternalStorageDirectory().getAbsolutePath() + path);
if(file.exists()){
bitmap = BitmapFactory.decodeFile(file.getAbsolutePath());
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap result) {
if (!imv.getTag().toString().equals(path)) {
/* The path is not same. This means that this
image view is handled by some other async task.
We don't do anything and return. */
return;
}
if(result != null && imv != null){
imv.setVisibility(View.VISIBLE);
imv.setImageBitmap(result);
}else{
imv.setVisibility(View.GONE);
}
}
}
这篇关于使用 AsyncTask 在 ListView 中加载图像的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
