Java 使用AsyncTask在Listview中显示图像
我试图用AsyncTask在listview中显示图像,但出现错误。说 尝试调用虚拟方法“java.lang.String” 空对象引用上的java.lang.Object.toString() 这是正在发生的错误Java 使用AsyncTask在Listview中显示图像,java,android,image,listview,android-asynctask,Java,Android,Image,Listview,Android Asynctask,我试图用AsyncTask在listview中显示图像,但出现错误。说 尝试调用虚拟方法“java.lang.String” 空对象引用上的java.lang.Object.toString() 这是正在发生的错误 public LoadImage(ImageView imv) { this.imv = imv; this.path = imv.getTag().toString(); } 及 这里是LoadImage.java import android.g
public LoadImage(ImageView imv) {
this.imv = imv;
this.path = imv.getTag().toString();
}
及
这里是LoadImage.java
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.os.AsyncTask;
import android.os.Environment;
import android.view.View;
import android.widget.ImageView;
import java.io.File;
/**
* Created by Yudii on 27/05/2016.
*/
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);
}
}
}
导入android.graphics.Bitmap;
导入android.graphics.BitmapFactory;
导入android.graphics.drawable.BitmapDrawable;
导入android.os.AsyncTask;
导入android.os.Environment;
导入android.view.view;
导入android.widget.ImageView;
导入java.io.File;
/**
*由Yudii于2016年5月27日创建。
*/
类LoadImage扩展了异步任务{
私有ImageView imv;
私有字符串路径;
公共加载映像(ImageView imv){
this.imv=imv;
this.path=imv.getTag().toString();
}
@凌驾
受保护位图doInBackground(对象…参数){
位图=空;
File File=新文件(Environment.getExternalStorageDirectory()
.getAbsolutePath()+路径);
if(file.exists()){
位图=BitmapFactory.decodeFile(file.getAbsolutePath());
}
返回位图;
}
@凌驾
受保护的void onPostExecute(位图结果){
如果(!imv.getTag().toString().equals(路径)){
/*路径不同。这意味着
图像视图由其他异步任务处理。
我们什么都不做就回来*/
//返回;
}
如果(结果!=null&&imv!=null){
imv.setVisibility(View.VISIBLE);
imv.setImageBitmap(结果);
}否则{
imv.setVisibility(View.GONE);
}
}
}
这里是CustomList.java
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
import java.util.ArrayList;
public class CustomList extends ArrayAdapter<String>{
private final Activity context;
private final Integer[] imageId;
public CustomList(Activity context,
String[] web, Integer[] imageId) {
super(context, R.layout.list_single, web);
this.context = context;
this.imageId = imageId;
}
@Override
public View getView(int position, View view, ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View rowView= inflater.inflate(R.layout.list_single, null, true);
ImageView imageView = (ImageView) rowView.findViewById(R.id.img);
LoadImage loadImage = new LoadImage(imageView);
Bitmap bitmap = loadImage.doInBackground();
imageView.setImageResource(imageId[position]);
return rowView;
}
}
导入android.app.Activity;
导入android.graphics.Bitmap;
导入android.graphics.drawable.BitmapDrawable;
导入android.view.LayoutInflater;
导入android.view.view;
导入android.view.ViewGroup;
导入android.widget.ArrayAdapter;
导入android.widget.ImageView;
导入android.widget.TextView;
导入java.util.ArrayList;
公共类CustomList扩展了ArrayAdapter{
私人最终活动背景;
私有最终整数[]imageId;
公共自定义列表(活动上下文,
字符串[]web,整数[]imageId){
super(上下文、R.layout.list_single、web);
this.context=上下文;
this.imageId=imageId;
}
@凌驾
公共视图getView(内部位置、视图视图、视图组父视图){
LayoutInflater充气器=上下文。getLayoutInflater();
视图行视图=充气机。充气(R.layout.list_single,null,true);
ImageView ImageView=(ImageView)rowView.findViewById(R.id.img);
LoadImage LoadImage=新的LoadImage(imageView);
位图Bitmap=loadImage.doInBackground();
setImageResource(imageId[位置]);
返回行视图;
}
}
这里是MainActivity.java
import android.app.Activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.Toast;
public class MainActivity extends Activity {
ListView list;
String[] web = {
"Google Plus",
"Twitter",
"Tall",
};
Integer[] imageId = {
R.drawable.gggggggggg,
R.drawable.hhhhhhhhhh,
R.drawable.ttttttttttt
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
CustomList adapter = new
CustomList(MainActivity.this, web, imageId);
list=(ListView)findViewById(R.id.list);
list.setAdapter(adapter);
list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view,
int position, long id) {
Toast.makeText(MainActivity.this, "You Clicked at " +web[+ position], Toast.LENGTH_SHORT).show();
}
});
}
}
导入android.app.Activity;
导入android.support.v7.app.AppActivity;
导入android.os.Bundle;
导入android.view.view;
导入android.widget.AdapterView;
导入android.widget.ListView;
导入android.widget.Toast;
公共类MainActivity扩展了活动{
列表视图列表;
字符串[]web={
“谷歌+”,
“推特”,
“高”,
};
整数[]图像ID={
R.drawable.gggggggggg,
R.drawable.hhhhhhhh,
R.drawable.ttttttttt
};
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
CustomList适配器=新建
CustomList(MainActivity.this、web、imageId);
list=(ListView)findViewById(R.id.list);
list.setAdapter(适配器);
list.setOnItemClickListener(新的AdapterView.OnItemClickListener(){
@凌驾
public void onItemClick(AdapterView父级、视图、,
内部位置,长id){
Toast.makeText(MainActivity.this,“您在“+web[+position],Toast.LENGTH_SHORT.show]上单击过它”);
}
});
}
}
您需要更改loadImage.execute()
而不是位图Bitmap=loadImage.doInBackground()代码>您需要更改loadImage.execute()
而不是位图Bitmap=loadImage.doInBackground()代码>为什么需要异步任务?尝试使用库并按一个字符串打开图像:
File file = new File(Environment.getExternalStorageDirectory()
.getAbsolutePath() + path);
Picasso.with(context).load(file).into(imageView);
位图对象非常重,您需要在每个位图图像之后清理内存,以防止OutOfMemoryError。我建议您使用库。为什么需要异步任务?尝试使用库并按一个字符串打开图像:
File file = new File(Environment.getExternalStorageDirectory()
.getAbsolutePath() + path);
Picasso.with(context).load(file).into(imageView);
位图对象非常重,您需要在每个位图图像之后清理内存,以防止OutOfMemoryError。我建议您使用库。在依赖项中添加以下内容:-
compile 'com.squareup.picasso:picasso:2.5.2'
然后加载您的图像:-
Picasso.with(this)
.load(drawerImage) // ImagePath
.placeholder( R.mipmap.download)
.error( R.mipmap.add_photo)
.into(imageDrawerView); //ImageView
将此添加到您的依赖项中:-
compile 'com.squareup.picasso:picasso:2.5.2'
然后加载您的图像:-
Picasso.with(this)
.load(drawerImage) // ImagePath
.placeholder( R.mipmap.download)
.error( R.mipmap.add_photo)
.into(imageDrawerView); //ImageView
1,原因:
如我所见,在getView
函数中:
ImageView imageView = (ImageView) rowView.findViewById(R.id.img);
// You need to set tag to your image view
// Ex: imageView.setTag("your_image_file_path") below this line
您的imageView
没有标记,因此下面的代码将获得NullPointerException
public LoadImage(ImageView imv) {
this.imv = imv;
this.path = imv.getTag().toString(); // Crashed here.
}
另外,我在您的getView
函数中看到以下代码:
imageView.setImageResource(imageId[position]);
这意味着“您的ImageView
将设置与定义的imageId
数组相对应的资源”。因此,您不需要AsyncTask
类,让我们删除它并更改您的getView,如下所示:
@Override
public View getView(int position, View view, ViewGroup parent) {
LayoutInflater inflater = context.getLayoutInflater();
View rowView= inflater.inflate(R.layout.list_single, parent, false);
ImageView imageView = (ImageView) rowView.findViewById(R.id.img);
// Remove below code
// LoadImage loadImage = new LoadImage(imageView);
// Bitmap bitmap = loadImage.doInBackground();
imageView.setImageResource(imageId[position]);
return rowView;
}
无论何时使用图像文件的路径,而不是可绘制资源,都需要使用Asynctask
类
2,改进:
使用ListView
,RecyclerView
提高性能时,您可以从中了解有关ViewHolder模式的更多信息。以下是您当前代码的示例:
@Override
public View getView(int position, View view, ViewGroup parent) {
View holder = null;
if (view == null) {
holder = new ViewHolder();
LayoutInflater inflater = context.getLayoutInflater();
view = inflater.inflate(R.layout.list_single, parent, false);
holder.imageView = (ImageView) view.findViewById(R.id.img);
view.setTag(holder);
} else {
holder = (ViewHolder) view.getTag();
}
holder.imageView.setImageResource(imageId[position]);
return view;
}
static class ViewHolder { // should be static class
ImageView imageView;
}
1,原因:
正如我