最近有一个功能机项目(不支持触屏)需要实现ListView选中项停在中间放大的效果,网络上的大多是用手滑动屏幕的效果,只能自己写一个。
效果实现了,但是还有小问题。实现原理主要是
1调用setSelection方法 使item项向上一项或向下一项
2调用setSelectionFromTop使选中项居于屏幕中间
这样有几个问题:
1首位几项无法居中(可以使用填充空白项的方案解决)
2按上下键移动item时,会看到listview轻微抖动(原因是setSelection会将选中项挪到listview可见部分的最顶部,之后setSelectionFromTop又让item项向下移动,该问题暂时没有解决思路,有能解决的大侠请多多指教)
下面上demo
1.item布局
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:id="@+id/text"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</LinearLayout>
2.页面布局
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="${relativePackage}.${activityClass}" >
<ListView
android:id="@android:id/list"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</ListView>
</LinearLayout>
3.Adapter
/**
* Created by caihuijian on 18-2-7.
*/
public class ReportSpinnerAdapter extends BaseAdapter {
private static String TAG = "ReportSpinnerAdapter";
private Context context;
private List<String> str;
private ListView list;
public static int select_item = -1;
//隐藏条目和偏移量的基数
private int n = 1;
public ReportSpinnerAdapter(Context context, List<String> str, ListView list) {
this.context = context;
this.str = str;
this.list = list;
}
@Override
public int getCount() {
return str.size();
}
@Override
public Object getItem(int position) {
return str.get(position);
}
@Override
public long getItemId(int position) {
return position;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder hold;
if (convertView == null) {
convertView = LayoutInflater.from(context).inflate(R.layout.item,
parent, false);
hold = new ViewHolder();
hold.textView = (TextView) convertView.findViewById(R.id.text);
convertView.setTag(hold);
} else {
hold = (ViewHolder) convertView.getTag();
}
hold.textView.setText(str.get(position));
Log.v(TAG, "MainActivity.select_item " + MainActivity.select_item
+ " position " + position);
// 选中项放大
if (MainActivity.select_item == position) {
hold.textView.setTextSize(17); // 选中的Item字体大小33
hold.textView.setAlpha(0.9f);
} else {
// 非选中项缩小 透明化
hold.textView.setTextSize(15); // 未选中的Item字体大小30
hold.textView.setAlpha(0.5f);
}
if (n <= 1) {
n = getVisibleCount() / 2;
}
if (position <= n || (position >= (str.size() - n - 1))) {
// 需要隐藏首尾的(visibleCount+1/2)个条目
Log.v(TAG, "position " + position + " str.size() " + str.size()
+ " n " + n);
hold.textView.setAlpha(0.0f);
}
return convertView;
}
static class ViewHolder {
TextView textView;
}
public int getN() {
return n;
}
public int getVisibleCount() {
int tempCount = (list.getLastVisiblePosition())
- list.getFirstVisiblePosition();
return tempCount;
}
}
4.主界面
public class MainActivity extends ListActivity {
private static String TAG = "MainActivity";
ReportSpinnerAdapter adapter;
ListView listView;
public static int select_item = 1;
// TODO int n//n为不显示数目
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 模拟数据初始化
List<String> datas = new ArrayList<String>();
for (int i = 0; i < 20; i++) {
datas.add("test " + i);
}
adapter = new ReportSpinnerAdapter(this, datas, getListView());
listView = getListView();
listView.setAdapter(adapter);
listView.setOnItemSelectedListener(new OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> arg0, View arg1,
int arg2, long arg3) {
Log.v(TAG, "onItemSelected");
// 存储选中项位置 在adapter使用
select_item = arg2;
// 选中后将选中项挪到屏幕中间
getListView().smoothScrollToPositionFromTop(select_item,
(int) getCenterY(), 1);
adapter.notifyDataSetChanged();
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
}
});
}
@Override
protected void onResume() {
// 初始化默认选择第2项,位置在屏幕中间
getListView().setSelectionFromTop(3, (int) getCenterY());
super.onResume();
}
@Override
public boolean dispatchKeyEvent(KeyEvent event) {
final int keyCode = event.getKeyCode();
final int action = event.getAction();
int adapterCount = adapter.getCount();
// 屏蔽连续按下事件,只监听up事件
if (action != KeyEvent.ACTION_UP && keyCode != KeyEvent.KEYCODE_BACK) {
return false;
} else {
switch (keyCode) {
case KeyEvent.KEYCODE_DPAD_DOWN:
Log.v(TAG, "down break select_item" + select_item
+ " adapterCount " + adapterCount);
// 移动到倒数第n项 屏蔽KEYCODE_DPAD_DOWN事件
if (select_item == adapterCount - adapter.getN() - 2) {
return false;
}
// 移到下一项
getListView().setSelection(select_item + 1);
break;
case KeyEvent.KEYCODE_DPAD_UP:
Log.v(TAG, "up break select_item" + select_item);
// 移动到第n项 屏蔽KEYCODE_DPAD_UP事件
if (select_item == adapter.getN() + 1) {
return false;
}
// 移到上一项
getListView().setSelection(select_item - 1);
break;
case KeyEvent.KEYCODE_DPAD_CENTER:
return true;
default:
return super.dispatchKeyEvent(event);
}
}
return super.dispatchKeyEvent(event);
}
private double getCenterY() {
// 计算行高
View listItem = adapter.getView(0, null, listView);
listItem.measure(0, 0);
int height = listItem.getMeasuredHeight() + listView.getDividerHeight();
// 计算显示条目
// 计算偏移量
return (adapter.getN() * height);
}
}