我正在创建一个图像下载应用程序，该应用程序将获取图像URL，下载图像并将其显示在列表视图中。为此，我正在通过程序处理xml。问题是我如何通过程序来操纵不是我的主要xml文件的xml文件。
ImageDownloader类：
public class ImageDownloader extends AsyncTask<String, Void, Bitmap> {
private ImageView imageView;
private ProgressBar progressBar;
public ImageDownloader(ImageView imageView, ProgressBar progressBar) {
this.imageView = imageView;
this.progressBar = progressBar;
}
@Override
protected void onPreExecute() {
super.onPreExecute();
imageView.setVisibility(View.GONE);
progressBar.setVisibility(View.VISIBLE);
}
@Override
protected Bitmap doInBackground(String... params) {
try{
URL url = new URL(params[0]);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.connect();
if(connection.getContentType().contains("image")){
InputStream inputStream = connection.getInputStream();
return BitmapFactory.decodeStream(inputStream);
}
}
catch(Exception e){
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
try {
imageView.setImageBitmap(bitmap);
progressBar.setVisibility(View.GONE);
imageView.setVisibility(View.VISIBLE);
} catch (Exception e) {
e.printStackTrace();
}
}
}
ImageUI类：
public class ImageUI {
private Context context;
private ArrayList<String> urls;
private ImageView imageView;
private ProgressBar progressBar;
private RelativeLayout relativeLayout;
public ImageUI(Context context, ArrayList<String> urls, RelativeLayout relativeLayout) {
this.context = context;
this.urls = urls;
this.relativeLayout = relativeLayout;
}
private void buildUI(){
imageView = new ImageView(context);
imageView.setVisibility(View.GONE);
progressBar = new ProgressBar(context);
progressBar.setIndeterminate(true);
progressBar.setVisibility(View.GONE);
relativeLayout = new RelativeLayout(context);
relativeLayout.setGravity(1);
relativeLayout.addView(imageView);
relativeLayout.addView(progressBar);
}
public RelativeLayout downloadImages(){
int i = 0;
while(i < urls.size()){
buildUI();
ImageDownloader imageDownloader = new ImageDownloader(imageView, progressBar);
imageDownloader.execute(urls.get(i));
imageDownloader = null;
i++;
return relativeLayout;
}
return null;
}
}
RowItem类：
public class RowItem {
private RelativeLayout relativeLayout;
public RowItem(RelativeLayout relativeLayout) {
this.relativeLayout = relativeLayout;
}
public RelativeLayout getRelativeLayout() {
return relativeLayout;
}
public void setRelativeLayout(RelativeLayout relativeLayout) {
this.relativeLayout = relativeLayout;
}
}
CustomListViewAdapter类：
public class CustomListViewAdapter extends ArrayAdapter<RowItem>{
Context context;
public CustomListViewAdapter(Context context, int resourseID, List<RowItem> items){
super(context, resourseID, items);
this.context = context;
}
private class ViewHolder{
RelativeLayout relativeLayout;
}
public View getView(int position, View convertView, ViewGroup parent){
ViewHolder holder = null;
RowItem rowItem = getItem(position);
LayoutInflater inflater = (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
if(convertView == null){
convertView = inflater.inflate(R.layout.list_item, null);
holder = new ViewHolder();
holder.relativeLayout = (RelativeLayout) convertView.findViewById(R.id.relativeLayout);
convertView.setTag(holder);
}
else{
holder = (ViewHolder) convertView.getTag();
}
return convertView;
}
}
MainACtivity类别：
public class MainActivity extends AppCompatActivity {
ArrayList<RelativeLayout> images;
ArrayList<String> urls = new ArrayList<>();
RelativeLayout relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);
List<RowItem> rowItems;
ListView listView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
addUrls();
images = new ArrayList<>();
for (int i = 0; i < urls.size(); i++) {
ImageUI imageUI = new ImageUI(this, urls, relativeLayout);
images.add(imageUI.downloadImages());
}
rowItems = new ArrayList<>();
for (int i = 0; i < images.size(); i++) {
RowItem item = new RowItem(images.get(i));
rowItems.add(item);
}
listView = (ListView) findViewById(R.id.listView);
CustomListViewAdapter adapter = new CustomListViewAdapter(this,
R.layout.list_item, rowItems);
listView.setAdapter(adapter);
}
public void addUrls() {
urls.add("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTXEZRoYOhIJxL5foNz_NlatDlgYStzZgVIiKuo6vtRtz2wY-8b4Q");
urls.add("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSFL3WYbqNOX-dwjtT1LroBlY5W-3YuwSIuCMRaLpnjMXbVPEJy");
urls.add("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQiwgrJeAJN-7lcy92N51uP7XzccK_p-fTSJNCXPLPSVih8wqPf");
urls.add("https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcT19dYLCEZlMRqojedJB-05jTrflD74nasvkXs-SdVeyM2BEpCSFA");
urls.add("http://wallpaperswide.com/download/high_tech_earth-wallpaper-2880x1800.jpg");
urls.add("https://www.gettyimages.ca/gi-resources/images/Homepage/Hero/UK/CMS_Creative_164657191_Kingfisher.jpg");
urls.add("https://images.unsplash.com/photo-1418489098061-ce87b5dc3aee?ixlib=rb-0.3.5&ixid=eyJhcHBfaWQiOjEyMDd9&s=2f033882f3c25404e3f904fbfe2351be&w=1000&q=80");
urls.add("https://techcrunch.com/wp-content/uploads/2018/03/gettyimages-705351545.jpg?w=730&crop=1");
}
}
此代码给出错误：
08-07 13:29:31.865 17741-17741/com.example.danishrizvi.test3 E/MemoryLeakMonitorManager: MemoryLeakMonitor.jar is not exist!
08-07 13:29:31.916 17741-17741/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.danishrizvi.test3, PID: 17741
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.danishrizvi.test3/com.example.danishrizvi.test3.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3092)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7425)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:155)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at com.example.danishrizvi.test3.MainActivity.<init>(MainActivity.java:18)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1178)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3082)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7425)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
在此行：
RelativeLayout relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);
答案 0 :(得分：1)
移动
RelativeLayout relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout);
内部
onCreate：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
RelativeLayout relativeLayout = (RelativeLayout) findViewById(R.id.relativeLayout)
addUrls();
// the rest of your code
}
您需要先调用setContentView，然后才能使用findViewById，换句话说：您需要对View进行充气，然后才能在其中搜索子视图。
这是因为在内部，findViewById确实在做getWindow().findViewById(id)