在Listview中更改特定的TextView颜色

Question

i Have Two Database first one Contain All The Items, and the ListView Display it and the second db contain the the Favorite item , [selected from the first database]

what i want is that when the listview display all the items check if the item is already exist in Favoritelist then make that textview background RED for this item

i have this code that work fine

    public static void ChangeMyFavoritesBG(){

for (int i = 0; i < Items.size(); i++)  {

if(db.verification(Items.get(i).toString())){

try {

TextView favtextview = (TextView) listview.getChildAt(i-listview.getFirstVisiblePosition()).findViewById(R.id.item_name);
 favtextview.setBackgroundResource(R.drawable.redcolor);

}catch (NullPointerException e) {

}}}}

db.verification check if item exist in favorites database if true . then it should change the background of this item to red

this code work fine but only if i put it in button click i need to make the code work automatically but if i made it start automatically when the activity is loaded i get NullPointer Error

i guess because the function ChangeMyFavoritesBG(); work before the listview display items

any idea guys? and sorry for my bad english

Answer 1

在listView所使用的Adapter的getView(int position, View convertView, ViewGroup parent)方法内执行此控件。

Answer 2

If your favorite is not currently visible in the ListView then getChildAt() will return null.

You are looping over all items in the list view and my guess is that it holds more items than can fit on the screen. When your favorite item is one of them then this fragment of your code

listview.getChildAt(i-listview.getFirstVisiblePosition())

will return null. And that will cause the NullPointerException when you call findViewById(R.id.item_name) on it.

Just add a check for null on the result of getChildAt(). If it is null then do nothing, if it is non-null then call the second part. This will protect against the exception when your favorite item is not on the screen, and will allow it to be colored red when your favorite is visible on the screen.

update

My apologies, I read to quickly and misunderstood your problem to be about the NullPointerException but you say that your code works fine when you call it from a button click handler but not when you call it automatically at start-up.

You are right, the ListView does not yet have any items loaded when you are still in onCreate(). You can add a delay before running you code. The following works for me:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my);

    // initialize the ListView with data for the list items. (I'm using a string array in this 
    // example. You are loading it from a database table, but that is the same in principle.)
    ListAdapter adapter = new ArrayAdapter<String>(this, R.layout.item_list, R.id.item_name, Items);
    ListView listview = (ListView) findViewById(R.id.listview);
    listview.setAdapter(adapter);

    // ask the system to wait before setting the background color of the favorite item so that
    // the ListView has time to load the items.
    final int DELAY_IN_MILLISECONDS = 100;
    listview.postDelayed(new Runnable() {
        @Override
        public void run() {
            ChangeMyFavoritesBG();
        }
    }, DELAY_IN_MILLISECONDS);
}

As you can see in the above example, after initializing the ListView, you ask the system to wait 100 milliseconds before calling ChangeMyFavoritesBG() . Hopefully that is enough time to load the items from the database into the ListView. If it is not enough time then you can, of course, use a longer delay.

The alternative

The above should work, but to be honest I would not write it this way. The above code is very brittle because it depends on the timing of how long it takes to load the items. I recommend that you put your background coloring into a customized adapter.

Because you want the items displayed in a customized way -- you want them to have a red background when it is the favorite one -- you should use a customized adapter. Override the bindView() function to make the background red when it is the favorite one or give it a normal background when it is not the favorite.

I don't know how you currently get the items from the database into your ListView, but inheriting from SimpleCursorAdaptor would work pretty well.

public class FavoritesItemAdapter extends SimpleCursorAdapter {

    public FavoritesItemAdapter(Context context, int layout, Cursor c, String[] from, int[] to, int flags) {
        super(context, layout, c, from, to, flags);
    }

    @Override
    public void bindView(View view, Context context, Cursor cursor) {
        super.bindView(view, context, cursor);

        // read the name from the database
        int nameColumnIndex = cursor.getColumnIndexOrThrow("name");
        String name = cursor.getString(nameColumnIndex);

        // write the name to the TextView
        TextView nameText = (TextView) view.findViewById(R.id.item_name);
        nameText.setText(name);

        // set the background to normal or to red, depending on if it is the favorite one or not
        boolean isFavorite = db_verification(name);
        if (isFavorite) {
            nameText.setBackgroundResource(R.drawable.redcolor);
        } else {
            nameText.setBackgroundResource(android.R.color.transparent);
        }
    }

    public boolean db_verification(String name) {
        // this is a stub. You must use your own code here
        return name.equals("the favorite one");
    }
}

You can then throw away ChangeMyFavoritesBG() and initialize your ListView with the adapter in onCreate() like this.

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my);

    Cursor cursor = readItemsFromDatabase();        
    String[] from = new String[] { "name_column" }; // mapping from database column name ...
    int[] to = new int[] { R.id.item_name };        // ... to View ID in the item's layout.
    FavoritesItemAdapter adapter = new FavoritesItemAdapter(this, R.layout.item_list, cursor, from, to, 0);

    ListView listview = (ListView) findViewById(R.id.listview);
    listview.setAdapter(adapter);
}

Good luck!