问题
I created a custom listview layout with images which are loaded from web like this:
http://i.stack.imgur.com/l8ZOc.png
It works fine when scrolling down. However, when you scroll down, the previous items go out of screen then destroyed. When you try to scroll up again, it gets loaded again (from cache, faster but not instant) which causes a delay and it is not fluent as it should be.
1.Is there an example of how to do this properly?
2.Is there a way to prevent listview items being destroyed when they are out of screen?
3.If so, will it cause problems to keep too many items?
Bellow is my code:
MenuAdapter:
public class MenuAdapter extends BaseAdapter{
Context context;
List<MyMenuItem> menuItems;
MenuAdapter(Context context, List<MyMenuItem> menuItems) {
this.context = context;
this.menuItems = menuItems;
}
@Override
public int getCount() {
return menuItems.size();
}
@Override
public Object getItem(int position) {
return menuItems.get(position);
}
@Override
public long getItemId(int position) {
return menuItems.indexOf(getItem(position));
}
private class ViewHolder {
ImageView ivMenu;
TextView tvMenuHeader;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
LayoutInflater mInflater = (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
if (convertView == null) {
convertView = mInflater.inflate(R.layout.menu_item, null);
holder = new ViewHolder();
holder.tvMenuHeader = (TextView) convertView.findViewById(R.id.tvMenuHeader);
holder.ivMenu = (ImageView) convertView.findViewById(R.id.ivMenuItem);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
MyMenuItem row_pos = menuItems.get(position);
Picasso.with(context)
.load(row_pos.getItem_image_url())
.into(holder.ivMenu);
holder.tvMenuHeader.setText(row_pos.getItem_header());
Log.e("Test", "headers:" + row_pos.getItem_header());
return convertView;
}
}
MyMenuItem:
public class MyMenuItem {
private String item_header;
private String item_image_url;
public MyMenuItem(String item_header, String item_image_url){
this.item_header=item_header;
this.item_image_url=item_image_url;
}
public String getItem_header(){
return item_header;
}
public void setItem_header(String item_header){
this.item_header=item_header;
}
public String getItem_image_url(){
return item_image_url;
}
public void setItem_image_url(String item_image_url){
this.item_image_url=item_image_url;
}
}
MainActivity:
public class MyActivity extends Activity implements AdapterView.OnItemClickListener {
List<MyMenuItem> menuItems;
ListView myListView;
JSONArray jsonArray;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_my);
Bundle extras = getIntent().getExtras();
if(extras!=null){
try{
jsonArray = new JSONArray(extras.getString("Data"));
}catch (Exception e){
e.printStackTrace();
}
}
menuItems = new ArrayList<MyMenuItem>();
for (int i = 0; i < jsonArray.length(); i++) {
try {
MyMenuItem item = new MyMenuItem(jsonArray.getJSONObject(i).getString("title"), jsonArray.getJSONObject(i).getString("imageURL"));
menuItems.add(item);
}catch (Exception e){
e.printStackTrace();
}
}
myListView = (ListView) findViewById(R.id.list);
MenuAdapter adapter = new MenuAdapter(this, menuItems);
myListView.setAdapter(adapter);
myListView.setOnItemClickListener(this);
}
}
MenuItem.xml:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/ivMenuItem"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:scaleType="center"
android:src="@drawable/em" />
<TextView
android:id="@+id/tvMenuHeader"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="#55000000"
android:paddingBottom="15dp"
android:paddingLeft="10dp"
android:paddingRight="10dp"
android:paddingTop="15dp"
android:textColor="@android:color/white"
android:layout_gravity="left|top"
android:layout_alignBottom="@+id/ivMenuItem"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true" />
</RelativeLayout>
回答1:
1. Is there an example of how to do this properly?
Your code looks pretty close to perfect. The Adapter's getView method is usually the critical path to optimize. Compare for example Picasso's own example SampleListDetailAdapter.java. The important points it (as well as your code) does
- check for & re-use already inflated views, inflation is expensive.
- use
ViewHolderso you don't have to callfindViewByIdevery time. Not terribly expensive on simple views. Also cached afaik. Picasso.with(context).load(url)...each time you need to display an image. This should finish instantly but still use caches and other magic.
There are some minor optimizations you can add, but I doubt that there are noticeable or even measurable changes:
pure style change: use BaseAdapter#getItem(position). This method
exists for you only. The framework doesn't use it.
@Override
public MyMenuItem getItem(int position) { // << subclasses can use subtypes in overridden methods!
return menuItems.get(position);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
...
MyMenuItem row_pos = getItem(position);
Use a sane id method
@Override
public long getItemId(int position) {
return menuItems.indexOf(getItem(position));
}
is equivalent to
@Override
public long getItemId(int position) {
return position;
}
but now infinitely faster. indexOf(Object) scales really badly with the number of objects.
Cache objects that don't change:
MenuAdapter(Context context, List<MyMenuItem> menuItems) {
this.mLayoutInflater = LayoutInflater.from(content);
this.mPicasso = Picasso.with(context);
}
..
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = mInflater.inflate(R.layout.menu_item, null);
...
mPicasso
.load(row_pos.getItem_image_url())
.into(holder.ivMenu);
2. Is there a way to prevent listview items being destroyed when they are out of screen?
No(*).
..(*) well you can essentially cache the result of getView e.g. in LruCache(position, View) or LruCache(MyMenuItem, View), then don't touch the convertView - they need to remain unconverted or you would kill those views in your cache. Also
@Override
public int getItemViewType(int position) {
return Adapter.IGNORE_ITEM_VIEW_TYPE;
}
seemed to be required because the standard adapter using code assumes that views it removes from visibility are gone. They are not and messing with them messes with your cache and caused weird display problems for me.
3. If so, will it cause problems to keep too many items?
Yes. This behavior is not intendend / expected. There is also more or less nothing you gain. You might be able to save you the call to holder.tvMenuHeader.setText(). Likewise the one to Picasso but both of them should complete instantly. Picasso should have your image cached already. By caching all Views you essentially add another cache that also contains all the images. I would rather check that the picasso cache works as intended and holds most items. The only reason you may want to do it with view caching is for cases that require complicated setup of the view, so it becomes worth caching the completely constructed view rather than just some content parts.
Profile
Profiling can actually tell you where you can / need / should improve. The first to look at IMO is traceview. You'll see if code blocks the main thread which results in choppy list scrolling. If you're doing complicated views and you see that the draw methods are executed most of the time, profile them as well.
- http://www.curious-creature.org/docs/android-performance-case-study-1.html
- http://blog.venmo.com/hf2t3h4x98p5e13z82pl8j66ngcmry/performance-tuning-on-android
- http://www.vogella.com/tutorials/AndroidTools/article.html
来源：https://stackoverflow.com/questions/26791871/how-to-correctly-implement-a-custom-listview-with-images-using-picasso-library