给ListView设置emptyView

给ListView设置emptyView

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使用ListView和GridView时，当列表为空时，默认是不显示任何内容的，这样对用户非常不友好，这时我们就需要通过setEmptyView(View emptyView)来设置当列表为空时的提示。

emptyView简单使用Demo

下面展示emptyView使用的简单Demo。

activity_empty_list_view.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.EmptyListViewActivity">

    <ListView
        android:id="@+id/lv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"></ListView>

    <!-- ListView的emptyView,默认不需设置其visiblity属性设置为GONE -->
    <!-- 当ListView为空时,emptyView会自动被设置为GONE -->
    <!-- 当ListView不为空时,emptyView会自动被设置为VIBISIBLE -->
    <LinearLayout
        android:id="@android:id/empty"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:orientation="vertical">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:gravity="center"
            android:text="Empty" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:gravity="center"
            android:text="EmptyView可以是ViewGroup" />
    </LinearLayout>

</LinearLayout>

EmptyListViewActivity.java:

public class EmptyListViewActivity extends Activity {

    private ListView mLv;

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

        mLv = (ListView) findViewById(R.id.lv);
        mLv.setAdapter(new ArrayAdapter<>(this,
                android.R.layout.simple_list_item_1, generateString(0)));
        mLv.setEmptyView(findViewById(android.R.id.empty));
    }

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

}

运行结果如下：


emptyView复杂使用Demo

前一个Demo讲了emptyView的简单使用方法，那如果我们要自定义emptyView或者emptyView的布局跟ListView不在同一个布局文件中呢？恩，你可能会说：直接调用setEmptyView(View emptyView)不行嘛，难道有啥不一样？
看来，你还是too young too simple。直接调用的结果就是：emptyView根本就不会显示！！！
新建一个ComplexEmptyListViewActivity.java:

public class ComplexEmptyListViewActivity extends Activity {

    private ListView mLv;
    private EmptyView mEmptyView;

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

        mLv = (ListView) findViewById(R.id.lv);
        mLv.setAdapter(new ArrayAdapter<>(this,
                android.R.layout.simple_list_item_1, Utils.generateString(0)));

        mEmptyView = new EmptyView(this);
        //此处是重点,如果不将mEmptyView添加进当前的ViewGroup,mEmptyView都不可见
        //((ViewGroup) mLv.getParent()).addView(mEmptyView);
        mLv.setEmptyView(mEmptyView);
    }
}

布局文件activity_complex_empty_list_view.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.EmptyListViewActivity">

    <ListView
        android:id="@+id/lv"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"></ListView>

</LinearLayout>

EmptyView:

public class EmptyView extends LinearLayout {

    public EmptyView(Context context) {
        super(context);
        init(context);
    }

    public EmptyView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context);
    }

    private void init(Context context) {
        View view = LayoutInflater.from(context).inflate(R.layout.complex_empty_view, this);
    }
}

complex_empty_view.xml:

<?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:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="假装我是复杂的emptyView" />

</LinearLayout>

运行demo，你会发现ListView为空时，根本就没显示emptyView。这是为什么呢？因为emptyView根本就没添加到当前Activity的布局中，所以即使设置了emptyView也不会显示，解决方法就是把emptyView添加到当前Activity的布局中（ComplexEmptyListViewActivity注释的内容）。

补充

1. 细心的可能已经发现，在两个demo中，我们都没去设置emptyView的visiblity属性，这是为什么呢？其实看下源码就知道了~

   当调用setEmptyView(View emptyView)方法时，ListView会调用到updateEmptyStatus(boolean empty)，在这里系统会跟empty去判断是否显示emptyView。恩，就这么简单~

   /**
    * Sets the view to show if the adapter is empty
    */
   @android.view.RemotableViewMethod
   public void setEmptyView(View emptyView) {
       mEmptyView = emptyView;
   
       // If not explicitly specified this view is important for accessibility.
       if (emptyView != null
               && emptyView.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
           emptyView.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
       }
   
       final T adapter = getAdapter();
       final boolean empty = ((adapter == null) || adapter.isEmpty());
       updateEmptyStatus(empty);
   }
   
   /**
    * Update the status of the list based on the empty parameter.  If empty is true and
    * we have an empty view, display it.  In all the other cases, make sure that the listview
    * is VISIBLE and that the empty view is GONE (if it's not null).
    */
   private void updateEmptyStatus(boolean empty) {
       if (isInFilterMode()) {
           empty = false;
       }
   
       if (empty) {
           if (mEmptyView != null) {
               mEmptyView.setVisibility(View.VISIBLE);
               setVisibility(View.GONE);
           } else {
               // If the caller just removed our empty view, make sure the list view is visible
               setVisibility(View.VISIBLE);
           }
   
           // We are now GONE, so pending layouts will not be dispatched.
           // Force one here to make sure that the state of the list matches
           // the state of the adapter.
           if (mDataChanged) {
               this.onLayout(false, mLeft, mTop, mRight, mBottom);
           }
       } else {
           if (mEmptyView != null) mEmptyView.setVisibility(View.GONE);
           setVisibility(View.VISIBLE);
       }
   }