Question

我试图放大中心列表查看项目。（最后，我希望将其扩展为一种简单而轻盈的3D效果，在屏幕中间放置元素时会放大，当它们位于下方时缩小，总放大的黑色背景和文本项目放大和缩小......）

所以，我做了一个虚拟列表视图，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Adapter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的代码，我已经跳过了进口:(它不是那么大！......大部分是非常普遍的）

MainActivity.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，在“row.xml”中我只有一个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。

为什么最重要的元素是大而不是中间元素，我特意将它设置为中间元素？

这是我当前代码给出的屏幕：

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，仍然提供null异常。

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

Answer 1

需要解决一些问题。

您需要使用ViewHolder模式。那些以及您真正需要了解的关于ListView的内容都在GoogleIO 2010 - The World of ListView视频中。
调整getView中项目的布局并不是一个好主意，因为这会对ListView使用的视图回收器产生负面影响。那也是在视频中。我真的鼓励你尝试找到一种不同的方式来显示你想要显示的内容。

设置文字

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。如果传入正确的参数，ArrayAdapter将为您设置文本。有关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);

如果我是你，我只是让适配器设置文本并通过调用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
}

如果你真的想自己动手，那么它采用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.

要在屏幕中间找到该项目，您只需要将您的可见项目计数并减去该计数的一半，如上所述。

设置高度实施

您需要在ListView上设置AbsListView.OnScrollListener，当滚动停止时，将调整应用于中间项。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)); 

        }
});

我已经对ListView和ExpandableListView做了一些tangentially related blog posts，它们有关于如何刷新使用childAt策略的可见项（示例代码只是更改项目的文本/背景颜色）的完整代码示例。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

只要您使用TextView作为项目布局（android.R.simple_list_item_1是TextView），这就完全符合您的要求。如果您决定使用ViewGroup布局，则需要使用LayoutParams方法替换item.setHeight。

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--;
              }
        }
    }
});

Answer 2

第一个没有文字出现

在MySimpleArrayAdapter上，使用getView方法。你忘了为textView设置textTxt。这就是没有文字显示的原因。

第二个让中间项更大

在设置listview适配器之前尝试getLastVisiblePosition。我猜v的值必须为0，位置== 0.这就是为什么topper项目变大了。如果您尝试将其更改为位置== 1或2，则第二个或第三个项目可能会变大。

为了使中间项变大，setOnScrollLinstener为listview

public int currentLargedPosition = 0 // global value

listView.setOnScrollListener(new OnScrollListener(){

    @Override
    public void onScroll(AbsListView arg0, int firstVisibleItem, int visibleItemCount, int totalItemCount){
        Log.i("Scroll","first "+firstVisibleItem+", visibleItemCount "+visibleItemCount+",totalCount "+totalItemCount); 
        int center = (visibleItemCount)/2 + firstVisibleItem;
        if(currentLargedPosition != center){
            enlargeMiddleView(currentLargedPosition-firstVisibleItem, center-firstVisibleItem);
            currentLargedPosition = center;
        }

    }

    @Override
    public void onScrollStateChanged(AbsListView arg0, int arg1) {
        Log.i("scroll","onScrollStateChanged");             
    }
});

void enlargeMiddleView(int oldPosition, int newPosition){       

    // get enlarged view and make it return default size
    TextView newTextView = (TextView)listView.getChildAt(oldPosition).findViewById(R.id.text);
    newTextView.setHeight(50);

    // get the current center view and make it bigger
    TextView oldTextView = (TextView)listView.getChildAt(newPosition).findViewById(R.id.text);
    oldTextView.setHeight(300);     
}

扩大listview中的中间元素 - android？

2 个答案:

设置文字

关于getXVisibleItem方法

设置高度实施

编辑2：对于android.R.simple_list_item_1