android - 扩大 ListView 中的中间元素 - android？

Question

我正在尝试放大最居中的 ListView 项目。 (最终，我希望将其扩展为一种简单轻巧的3D效果，元素在屏幕中间时会放大，在下方时会缩小，全黑背景和文本项放大和缩小……)

所以，我制作了一个虚拟 ListView ，每一行都是一个 TextView 元素。下面，我有一个自定义适配器。

我所做的唯一相关修改是:

在 OnCreate() 中，我将以下变量传递给适配器:

int v = (listview.getLastVisiblePosition() - listview.getFirstVisiblePosition()) + 1;

在适配器的 getView() 中，我得到了这个:

if (position == x/2) {
        textView.setHeight(100);
    }

无论如何，这里是所有代码，我已经跳过了导入:(它不是那么大!......大部分都是很通用的)

主要 Activity .java :

    public class MainActivity extends Activity {

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

        final ListView listview = (ListView) findViewById(R.id.listview);
        String[] values = new String[] { "Android", "iPhone", "WindowsMobile",
                "Blackberry", "WebOS", "Ubuntu", "Windows7","iPhone", "WindowsMobile",
                "Blackberry", "WebOS", "Ubuntu", "Windows7","iPhone", "WindowsMobile",
                "Blackberry", "WebOS", "Ubuntu", "Windows7", "Max OS X" };

        final ArrayList<String> list = new ArrayList<String>();
        for (int i = 0; i < values.length; ++i) {
            list.add(values[i]);
        }
        int v = (listview.getLastVisiblePosition() - listview.getFirstVisiblePosition()) + 1;


        final MySimpleArrayAdapter adapter = new MySimpleArrayAdapter(this, v, R.layout.row, list);
        listview.setAdapter(adapter);

        listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, final View view,
                    int position, long id) {

            }

        });
    }
}

MySimpleArrayAdapter.java

 public class MySimpleArrayAdapter extends ArrayAdapter<String> {
    private final Context context;
    private final int x;

    HashMap<String, Integer> mIdMap = new HashMap<String, Integer>();

    public MySimpleArrayAdapter(Context context, int a, int textViewResourceId,
            List<String> objects) {
        super(context, textViewResourceId, objects);
        this.context = context;
        this.x = a;
        for (int i = 0; i < objects.size(); ++i) {
            mIdMap.put(objects.get(i), i);
        }

    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater inflater = (LayoutInflater) context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View rowView = inflater.inflate(R.layout.row, parent, false);
        TextView textView = (TextView) rowView.findViewById(R.id.textView1);

        if (position == x/2) {
            textView.setHeight(100);
        }

        return rowView;
    }

    @Override
    public long getItemId(int position) {
        String item = getItem(position);
        return mIdMap.get(item);
    }

    @Override
    public boolean hasStableIds() {
        return true;
    }
}

此外，在我的 activity_main.xml 中我只有一个 ListView ，而在“row.xml”中我有一个 TextView ..

问题:为什么没有文字出现？我清楚地将列表值添加到 textview 中。

为什么最上面的元素大而不是中间的元素，我特意设置到中间的？

这是我当前代码给出的屏幕截图:

LOGCAT 错误 - KMDev 的解决方案

10-06 17:54:02.529: E/AndroidRuntime(19596): FATAL EXCEPTION: main
10-06 17:54:02.529: E/AndroidRuntime(19596): java.lang.RuntimeException: Unable to start activity ComponentInfo{mple.finala/mple.finala.MainActivity}: java.lang.NullPointerException
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2077)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2104)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread.access$600(ActivityThread.java:134)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.os.Handler.dispatchMessage(Handler.java:99)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.os.Looper.loop(Looper.java:154)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread.main(ActivityThread.java:4624)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at java.lang.reflect.Method.invokeNative(Native Method)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at java.lang.reflect.Method.invoke(Method.java:511)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:809)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:576)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at dalvik.system.NativeStart.main(Native Method)
10-06 17:54:02.529: E/AndroidRuntime(19596): Caused by: java.lang.NullPointerException
10-06 17:54:02.529: E/AndroidRuntime(19596):    at mple.finala.MainActivity$2.onScroll(MainActivity.java:79)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.widget.AbsListView.invokeOnItemScrollListener(AbsListView.java:1297)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.widget.AbsListView.setOnScrollListener(AbsListView.java:1286)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at mple.finala.MainActivity.onCreate(MainActivity.java:58)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.Activity.performCreate(Activity.java:4479)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
10-06 17:54:02.529: E/AndroidRuntime(19596):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2041)
10-06 17:54:02.529: E/AndroidRuntime(19596):    ... 11 more

编辑 2:

完整代码，仍然给出 null 异常。

http://codeviewer.org/view/code:373d

Answer 1

有几件事需要解决。

1. 您需要使用 ViewHolder 模式。关于 ListView，您真正需要了解的内容和内容都在 GoogleIO 2010 - The World of ListView 中。视频。

2. 在 getView 中调整项目的布局并不是一个好主意，因为这会对 ListView 使用的 View 回收器产生负面影响。视频中也是如此。我真的鼓励您尝试并找到一种不同的方式来显示您想要显示的内容。

设置文本

1. 您将项目所需的 x 维度传递给适配器的构造函数，而不是为其提供实际的项目资源 ID。如果您传入正确的参数，ArrayAdapter 将为您设置文本。对于使用字符串资源和默认提供的列表项布局的简单示例:

   String[] mData = getResources().getStringArray(R.array.list_examples);
   ArrayAdapter<String> mBaseAdapter = 
               new ArrayAdapter<String>(context, 
                                        android.R.layout.simple_list_item_1,
                                        android.R.id.text1,
                                        mListData);
   

2. 如果我是你，我会让适配器设置文本并通过像这样调用 super 来获取项目布局:

   @Override
   public View getView(int position, View convertView, ViewGroup viewGroup){
       convertView = super.getView(position,convertView, viewGroup);
       //other code here
   }
   

如果你真的想自己做，这里有 ViewHolder 模式

    //This is the ViewHolder
    static class Holder{
        TextView mText;
    }


    @Override
    public View getView(int position, View convertView, ViewGroup viewGroup){

         convertView = super.getView(position, convertView, viewGroup);

         if (convertView == null){
              // If you reach this, it most likely means you're not supplying the
              // adapter the layout and textview resource it needs. You can
              // just comment out your getView override and see if your text gets
              // displayed.
         }

         Holder mHolder;
         if(convertView.getTag() == null){
            mHolder = new Holder();
            mHolder.mText = (TextView)convertView.findViewById(android.R.id.text1);
            convertView.setTag(mHolder);
         } else {
            mHolder = (Holder) convertView.getTag();
         }

         mHolder.mText.setText(getItem(position).toString());
    }

关于 getXVisibleItem 方法

// These are both implemented in AdapterView which means
// they're flat list positions
int firstVis = mList.getFirstVisiblePosition(); 
int lastVis = mList.getLastVisiblePosition();

/* This is the "conversion" from flat list positions to ViewGroup child positions */
int count = lastVis - firstVis; 

 /*  getChildAt(pos) is implemented in ViewGroup and has a different meaning for 
 *  its position values. ViewGroup tracks visible items as children and is 0 indexed. 
 *  This means you'll have 0 - X positions where X is however many items it takes 
 *  to fill the visible area of your screen; usually less than 10. */
 View listItem = mList.getChildAt(count - ((int)count/2)); // This will get the middle 
                                                        // item for you to adjust as 
                                                        // you want.

要找到屏幕中间的项目，您只需按照上面的说明，将可见项目数减去一半即可。

设置高度实现

您需要设置一个 AbsListView.OnScrollListener在您的 ListView 上，当滚动停止时，将您的调整应用于中间项目。例如:

mList.setOnScrollListener(new OnScrollListener(){
    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState){

    }       

    @Override
    public void onScroll(AbsListView view, 
                         int firstVisibleItem, 
                         int visibleItemCount,
                         int totalItemCount){

        //Note that this is called when the scroll completes. 
        //It gives us the positions we want so just calculate
        //the middle item, grab it and adjust the height directly
        //or by increasing margins if the item layout_height is 
        //wrap_content. I'm not positive the margins do what you
        // need, but setting the height directly should.
        LinearLayout listItem = mList // <-- Replace LinearLayout with your layout
                      .getChildAt(visibleItemCount - ((int)visibleItemCount/2));

        //Make sure you use the right LayoutParams for the listItem Layout
        //If your listItem is a RelativeLayout for instance, you would use:
        // listItem.setLayoutParams(
        //      new RelativeLayout.LayoutParams(LayoutParams.FILL_PARENT,
        //                                      theHeightYouWantHere));
        listItem.setLayoutParams(
                new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,
                                              theHeightYouWantHere)); 

        }
});

我做了一些 tangentially related blog posts关于 ListView 和 ExpandableListView，它们有关于如何刷新可见项目的完整代码示例(示例代码只是更改项目的文本/背景颜色)，它们使用 childAt 策略。请原谅个人博客链接及其样式(它是新的，需要更多工作:)，但代码运行良好。

编辑 2:对于 android.R.simple_list_item_1

只要您将 TextView 用于项目布局(android.R.simple_list_item_1 是一个 TextView)，这就完全符合您的要求。如果您决定使用 ViewGroup 布局，则需要将 item.setHeight 替换为 LayoutParams 方法。

mList.setOnScrollListener(new OnScrollListener(){
    @Override
    public void onScrollStateChanged(final AbsListView view, final int scrollState){

    }

    @Override
    public void onScroll(final AbsListView view, 
                         final int firstVisibleItem,
                         final int visibleItemCount,
                         final int totalItemCount){
        if (visibleItemCount != 0){

             Log.i( TAG, "firstVisibleItem: "
                         + firstVisibleItem
                         + "\nvisibleItemCount: "
                         + visibleItemCount);

             final int midPosition = visibleItemCount - (visibleItemCount / 2); 
             final TextView listItem = (TextView) mList.getChildAt(midPosition);
             listItem.setHeight(500); //Only works if you're using
                                      //android.R.simple_list_item_1 since it's
                                      //not a ViewGroup, but rather a TextView

             int count = visibleItemCount;
             while (count >= 0){ // Here we need to loop through and make sure
                                 // all recycled items are returned to their 
                                 // original height. 
                 final TextView item = (TextView) mList.getChildAt(count);
                 if (item != null && count != midPosition){
                     item.setHeight(100);
                 }
                 count--;
              }
        }
    }
});