ListView的Layout_width

Question

好的,我遇到了listView的attribut layout_width的问题.

在我的项目中,我的listview的背景图像不符合屏幕的宽度.所以我在wrap_content上设置了layout_width,并且我将listView水平居中.

看起来这是一个坏主意,因为在适配器中,GetView方法被调用的次数是显示的单元格数的3倍以上.如果我在fill_parent上设置layout_width,问题就解决了.我创建了一个显示问题的简单项目.

这是活动的xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<ListView
    android:id="@android:id/list"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
/>
</RelativeLayout>

item_list xml:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="10dp"
    android:textSize="16sp" >
</TextView>

最后活动:

package fr.example.android.listviewbug;

import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;

public class ExampleListViewBugActivity extends ListActivity{
    public static int cpt;

    public static final String[] COUNTRIES = new String[] {
        "Afghanistan", "Albania", "Algeria", "American Samoa", "Andorra",
        "Angola", "Anguilla", "Antarctica", "Antigua and Barbuda", "Argentina",
        "Armenia", "Aruba", "Australia", "Austria", "Azerbaijan",
        "Bahrain", "Bangladesh", "Barbados", "Belarus", "Belgium",
        "Belize", "Benin", "Bermuda", "Bhutan", "Bolivia",
        "Bosnia and Herzegovina", "Botswana", "Bouvet Island", "Brazil", "British Indian Ocean Territory",
        "British Virgin Islands", "Brunei", "Bulgaria", "Burkina Faso", "Burundi",
        "Cote d'Ivoire", "Cambodia", "Cameroon", "Canada", "Cape Verde",
        "Cayman Islands", "Central African Republic", "Chad", "Chile"};

    @Override
    public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.main);

      cpt = 0;
      setListAdapter(new CustomArrayAdapter(this, R.layout.list_item, COUNTRIES));  
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d("AdapterTest", "onDestroy()");
    }

    private class CustomArrayAdapter extends ArrayAdapter<String>{
        public CustomArrayAdapter(Context context, int textViewResourceId, String[] countries) {
            super(context, textViewResourceId, countries);
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            Log.d("AdapterTest", " cpt:"+ ++cpt);
            return super.getView(position, convertView, parent);
        }
}
}

所以我的解决方案是简单地在fill_parent上设置layout_width,并修改listview背景的图像以添加一些透明边框以适应屏幕大小.

但我想知道为什么这个问题出现在listview上的wrap_content,以及为什么它不在文档中,如果它是一个已知问题...

Answer 1

通过将宽度设置为"wrap_content",您将告诉ListView与其最宽的子项一样宽.因此,ListView必须测量其项目并获取必须在适配器上调用getView()的项目.这可能会发生几次,具体取决于布局传递的数量,父布局的行为等.

请记住,不能保证在适配器上调用getView()的次数,也不保证调用的顺序.因此,您必须实现getView()尽可能高效.最重要的是,这不是问题,这是在这种情况下的预期行为.

Answer 2

答案很简单:如果你的布局宽度或布局高度设置ListView到wrap_contentListView控件将尝试测量每个附加到它单一视图-这绝对不是你想要的.

一般来说,请wrap_content始终避免设置ListViews或GridViews.