我的应用程序使用ListView显示来自网址的一些图片和标题。但无论我做什么，图像在向上或向下滚动后都会从ListView消失，没有人能再看到它们。
我怎样才能做到这一点？我是android的新手，所以我很高兴看到任何建议。
我的代码在这里：
AndroidManifest：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.forwork">
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity
android:name=".MainActivity"
android:configChanges="keyboardHidden|orientation|screenSize">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
MainActivity.java：
import android.app.Activity;
import android.content.Context;
import android.content.res.Configuration;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.os.Bundle;
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;
import android.widget.Toast;
import org.apache.http.HttpStatus;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.ref.WeakReference;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Objects;
import java.util.concurrent.ExecutionException;
public class MainActivity extends Activity{
public final static int SIZE=30;
public String[] titles = new String[SIZE]; //array for titles
public String[] keys = new String[SIZE]; //array for id of news
public String[] image_url = new String[SIZE]; //array for image urls to download
public String data="";
//that's for parsing
JSONObject news = null;
String title = "";
JSONObject prefs = null;
JSONObject inner = null;
JSONObject elements = null;
JSONObject image =null;
String pic = "";
JSONObject json = null;
JSONArray collection = null;
JSONObject documents = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (isInternetThere(this)) //if internet is not absent
try {
ListView my_list_view = (ListView) findViewById(R.id.list_view); //custom list view
CustomList adapter = new CustomList(this,titles,image_url); //custom adapter
my_list_view.setAdapter(adapter);
Parse p = new Parse(); //instance of my async task for getting json
data = p.execute().get();
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
else {
Toast.makeText(this, "No internet connection", Toast.LENGTH_LONG).show();
finish();
}
try {
json = new JSONObject(data); //parsing json
collection = json.getJSONArray("collection"); //new's ids
documents = json.getJSONObject("documents"); //new's info
for (int i=0; i<keys.length;i++){
keys[i]=collection.getString(i);
news = documents.getJSONObject(keys[i]);
title = news.getString("title"); //title
titles[i]=title; //put title into list
prefs = news.getJSONObject("prefs");
inner = prefs.getJSONObject("inner");
elements = inner.getJSONObject("elements");
image = elements.getJSONObject("image");
if (image.has("small_url")) //whether there is a picture?
pic = image.getString("small_url");
else pic = "";
image_url[i] = pic;
}
} catch (JSONException e) {
e.printStackTrace();
}
}
@Override
public void onConfigurationChanged(Configuration newConfig) { //it is just for orientation monitoring
super.onConfigurationChanged(newConfig);
if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show();
} else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) {
Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show();
}
}
public class Parse extends AsyncTask<Void,Void,String> { //my async task #1 that just get a json from url
@Override
protected String doInBackground(Void... params) {
String str= null;
try {
str = readJSON();
} catch (Exception e) {
e.printStackTrace();
}
return str;
}
public String readJSON() { //function that get json
HttpURLConnection urlConnection = null;
BufferedReader reader = null;
String resultJson = ""; //this will be a string with json
try {
URL url = new URL("https://meduza.io/api/v3/search?chrono=news&page=0&per_page=30&locale=ru"); //url with 30 news
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
InputStream inputStream = urlConnection.getInputStream(); //stream for getting json
StringBuffer buffer = new StringBuffer(); //buffer for json saving
reader = new BufferedReader(new InputStreamReader(inputStream)); //it just reads
String line;
while ((line = reader.readLine()) != null) { //line by line..
buffer.append(line); //...and add it into buffer
}
resultJson = buffer.toString(); //result is string got from buffer
} catch (Exception e) {
e.printStackTrace();
}
return resultJson;
}
}
public class DownloadImage extends AsyncTask<String,Void,Bitmap>{ //my async task #2 for downloading image from url using Bitmap
private final WeakReference<ImageView> imgv; //it means that..
public DownloadImage(ImageView imageView){
imgv = new WeakReference<ImageView>(imageView); //the object can be deleted by garbage collector without any trubles
} //it helps to avoid memory leaks
@Override
protected Bitmap doInBackground(String... strings) {
return down_Bit(strings[0]);
}
@Override
protected void onPostExecute(Bitmap res){
if (isCancelled()){
res=null;
}
if(imgv != null){ //if the link is actual...
ImageView iv = imgv.get(); //we get it
if (iv != null){ //if the object is exists...
if (res != null) //and image exests...
iv.setImageBitmap(res); //we show it
}
}
}
private Bitmap down_Bit(String url){ //it is for downloading images
HttpURLConnection urlConnection = null;
try{
URL uri = new URL(url);
urlConnection = (HttpURLConnection) uri.openConnection();
int statusCode = urlConnection.getResponseCode();
if (statusCode != HttpStatus.SC_OK) //if our url is not right(for ex. "http" instead of "https"
return null;
InputStream is = urlConnection.getInputStream();
if (is != null){
Bitmap bitmap = BitmapFactory.decodeStream(is); //if input stream exists we get the Bitmap from it....
return bitmap; //and return it
}
} catch (Exception e){
e.printStackTrace();
} finally {
if (urlConnection != null)
urlConnection.disconnect();
}
return null;
}
}
public class CustomList extends ArrayAdapter<String>{ //that is my custom adapter named CustomList. probably for not misunderstanding
private Activity context;
private String[] item_name;
private String[] imgurl;
public CustomList(Activity context,String[] item_name, String[] imgurl){ //constructor
super(context,R.layout.my_listview_item,item_name);
this.context=context; //activity
this.item_name=item_name; //there is will be the titles
this.imgurl=imgurl; //there is will be the images
}
@Override
public int getCount(){
return item_name.length;
}
@Override
public String getItem(int position){
return item_name[position];
}
@Override
public long getItemId(int position){
return position;
}
public View getView(int position, View view, ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View rowView = inflater.inflate(R.layout.my_listview_item,null,true);
TextView txtTitle = (TextView) rowView.findViewById(R.id.txt);
if (imgurl[position] != "") { //if there is a url of an image(because there some news without it)
rowView.findViewById(R.id.img).setVisibility(View.VISIBLE); //show the picture slot
imgurl[position] = "https://meduza.io" + imgurl[position]; //and make the url absolute, not relative
new DownloadImage((ImageView) rowView.findViewById(R.id.img)).execute(imgurl[position]); //and download the picture using the absolute url
}
else
rowView.findViewById(R.id.img).setVisibility(View.GONE); //if there is no picture, just hide the slot
txtTitle.setText(item_name[position]); //it is title
return rowView;
}
}
public boolean isInternetThere(Context context) { //it is for internet connection testing
ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = manager.getActiveNetworkInfo();
if (networkInfo != null && networkInfo.isConnected()) {
return true;
} else {
return false;
}
}
}
答案 0 :(得分：0)
我认为您使用的是ListView。在ListView滚动时，Android会重新使用列表项，因此滚动时图像开始消失。
因此，您需要做的第一个更改是使用RecyclerView而不是ListView。 RecyclerView实施将强制您实施ViewHolder模式，这将使您的滚动体验更好！查看smooth scrolling.
最后，为了让您的图片按需运行，我建议您使用Picasso。这是最好的图像加载库。您不需要维护列表中元素的位置，图像将根据需要工作。
我建议使用库来实现Image加载最佳实践本身可能需要花费很多时间。 The best practices.你必须维护图像的缓存，有效地保持BitMap的大小等。
希望这些要点可以指导您尽可能以最佳方式实施图像滚动列表:)干杯！