ListView中item定位

ListView 实现定位特定 item

最近在项目中需要使 ListView 能跳转到特定的 item，查阅文档后，发现 ListView 有以下几种方法可供使用：

• smoothScrollToPosition(int position):平滑滚动到指定位置。如果 position 为负，则滚动到第一条；如果 position 大于 ListView item 的最大值，则滚动到最后一条。
• smoothScrollByOffset(int offset):平滑滚动偏移量 offset 指定的 item。如果 offset 大于 0，则表示下移；如果 offset 小于 0，则表示上移。当 offset 超过可以移动的最大值时，默认移动到顶部或尾部。
• setSelection(int position):直接跳转到指定的位置，没有任何滚动效果。
• setSelectionFromTop(int position,int offset):直接跳转到指定的位置，并在Y轴设置一个偏移量。该偏移量是个 padding 值，可以用来实现类似 QQ 查看历史消息的功能。可参考ListView的setSelection()和setSelectionFromTop()联系

注：通过查看源代码，其实可以发现 setSelection(int position) 其实调用的是 setSelectionFromTop(int position,int offset) 方法，只不过 offset = 0 而已。

源码：

/**
* Sets the currently selected item. If in touch mode, the item will not be selected
* but it will still be positioned appropriately. If the specified selection position
* is less than 0, then the item at position 0 will be selected.
*
* @param position Index (starting at 0) of the data item to be selected.
*/
@Override
public void setSelection(int position) {
   setSelectionFromTop(position, 0);
}
    
/**
* Sets the selected item and positions the selection y pixels from the top edge
* of the ListView. (If in touch mode, the item will not be selected but it will
* still be positioned appropriately.)
*
* @param position Index (starting at 0) of the data item to be selected.
* @param y The distance from the top edge of the ListView (plus padding) that the
*        item will be positioned.
*/
public void setSelectionFromTop(int position, int y) {
   if (mAdapter == null) {
       return;
   }

   if (!isInTouchMode()) {
       position = lookForSelectablePosition(position, true);
       if (position >= 0) {
           setNextSelectedPositionInt(position);
       }
   } else {
       mResurrectToPosition = position;
   }

   if (position >= 0) {
       mLayoutMode = LAYOUT_SPECIFIC;
       mSpecificTop = mListPadding.top + y;

       if (mNeedSync) {
           mSyncPosition = position;
           mSyncRowId = mAdapter.getItemId(position);
       }

       if (mPositionScroller != null) {
           mPositionScroller.stop();
       }
       requestLayout();
   }
} 

简单小Demo

MainActivity.java:

public class MainActivity extends Activity implements View.OnClickListener {

    private EditText mEdt;
    private Button mBtnJump, mBtnMoveUp, mBtnMoveDown;
    private Button mBtnSelect;
    private Button mBtnSelectPositive,mBtnSelectNagetive;
    private ListView mLv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mEdt = (EditText) findViewById(R.id.edt);
        mBtnJump = (Button) findViewById(R.id.btn_jump);
        mBtnMoveUp = (Button) findViewById(R.id.btn_move_up);
        mBtnMoveDown = (Button) findViewById(R.id.btn_move_down);
        mBtnSelect = (Button) findViewById(R.id.btn_select);
        mBtnSelectPositive = (Button) findViewById(R.id.btn_select_from_top_positive_offset);
        mBtnSelectNagetive = (Button) findViewById(R.id.btn_select_from_top_nagetive_offset);
        mLv = (ListView) findViewById(R.id.lv);
        mLv.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, generateString()));

        mBtnJump.setOnClickListener(this);
        mBtnMoveUp.setOnClickListener(this);
        mBtnMoveDown.setOnClickListener(this);
        mBtnSelect.setOnClickListener(this);
        mBtnSelectPositive.setOnClickListener(this);
        mBtnSelectNagetive.setOnClickListener(this);
    }

    private List<String> generateString() {
        List<String> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            list.add("item " + i);
        }
        return list;
    }

    @Override
    public void onClick(View v) {
        String input = mEdt.getText().toString();
        int position = Integer.valueOf(input);
        switch (v.getId()) {
            case R.id.btn_jump:
                //平滑滚动到指定位置
                mLv.smoothScrollToPosition(position);
                break;
            case R.id.btn_move_up:
                //平滑上移指定数量的item
                mLv.smoothScrollByOffset(-position);
                break;
            case R.id.btn_move_down:
                //平滑下移指定数量的item
                mLv.smoothScrollByOffset(position);
                break;
            case R.id.btn_select:
                //跳转到指定的位置,不平移
                mLv.setSelection(position);
                break;
            case R.id.btn_select_from_top_positive_offset:
                //跳转到指定的位置,第二个参数表示Y轴偏移量
                mLv.setSelectionFromTop(position, 100);
                break;
            case R.id.btn_select_from_top_nagetive_offset:
                //跳转到指定的位置,第二个参数表示Y轴偏移量
                mLv.setSelectionFromTop(position, -100);
                break;
        }
    }
}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<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="com.littlejie.listview.MainActivity">

    <EditText
        android:id="@+id/edt"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入你要跳转的行数(0~9999)"
        android:inputType="number" />

    <Button
        android:id="@+id/btn_jump"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="点我跳转(smoothScrollToPosition)" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/btn_move_up"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="上移指定偏移量" />

        <Button
            android:id="@+id/btn_move_down"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="下移指定偏移量" />
    </LinearLayout>

    <Button
        android:id="@+id/btn_select"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="直接跳转到position" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/btn_select_from_top_positive_offset"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="setSelectionFromTop(+)" />

        <Button
            android:id="@+id/btn_select_from_top_nagetive_offset"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="setSelectionFromTop(-)" />
    </LinearLayout>

    <ListView
        android:id="@+id/lv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Hello World!" />
</LinearLayout>