处理列表的空状态

app/src/main/java/com/example/quakereport/EarthquakeActivity.java

@@ -22,6 +22,7 @@
 import android.view.View;
 import android.widget.AdapterView;
 import android.widget.ListView;
+import android.widget.TextView;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
@@ -48,12 +49,18 @@ public class EarthquakeActivity extends AppCompatActivity implements LoaderManag
      * URL for earthquake data from the USGS dataset
      */
     private static final String USGS_REQUEST_URL =
-            "https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&orderby=time&minmag=5&limit=20";
+            "https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&orderby=time&minmag=5&limit=0";
+
     /**
      * 地震列表的适配器
      */
     private EarthquakeAdapter earthquakeAdapter;
 
+    /**
+     * 列表为空时显示的 空视图
+     */
+    private TextView emptyView;
+
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         Log.i(LOG_TAG, "TEST: onCreate() called ...");
@@ -64,6 +71,10 @@ protected void onCreate(Bundle savedInstanceState) {
         // 在布局中查找 {@link ListView} 的引用
         ListView earthquakeListView = (ListView) findViewById(R.id.list);
 
+        // 为 ListView 绑定空视图，在无数据时显示
+        emptyView = (TextView) findViewById(R.id.empty_view);
+        earthquakeListView.setEmptyView(emptyView);
+
         // 创建新适配器，将空地震列表作为输入
         earthquakeAdapter = new EarthquakeAdapter(this, new ArrayList<Earthquake>());
 
@@ -113,6 +124,9 @@ public Loader<List<Earthquake>> onCreateLoader(int id, @Nullable Bundle args) {
     public void onLoadFinished(@NonNull Loader<List<Earthquake>> loader, List<Earthquake> earthquakes) {
         Log.i(LOG_TAG, "TEST: onLoadFinished() called ...");
 
+        // Set empty state text to display "No earthquakes found."
+        emptyView.setText(R.string.no_earthquakes);
+
         // 清除之前地震数据的适配器
         earthquakeAdapter.clear();
 

app/src/main/res/layout/earthquake_activity.xml

@@ -1,24 +1,22 @@
 <?xml version="1.0" encoding="utf-8"?>
-<!-- Copyright (C) 2016 The Android Open Source Project
-
-     Licensed under the Apache License, Version 2.0 (the "License");
-     you may not use this file except in compliance with the License.
-     You may obtain a copy of the License at
+<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent">
 
-          http://www.apache.org/licenses/LICENSE-2.0
+    <!-- Layout for a list of earthquakes -->
+    <ListView
+        android:id="@+id/list"
+        android:layout_width="match_parent"
+        android:layout_height="match_parent"
+        android:divider="@null"
+        android:dividerHeight="0dp"
+        android:orientation="vertical" />
 
-     Unless required by applicable law or agreed to in writing, software
-     distributed under the License is distributed on an "AS IS" BASIS,
-     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-     See the License for the specific language governing permissions and
-     limitations under the License.
--->
-<!-- Layout for a list of earthquakes -->
-<ListView
-    xmlns:android="http://schemas.android.com/apk/res/android"
-    android:id="@+id/list"
-    android:orientation="vertical"
-    android:layout_width="match_parent"
-    android:layout_height="match_parent"
-    android:divider="@null"
-    android:dividerHeight="0dp"/>
+    <!-- Empty view is only visible when the list has no items. -->
+    <TextView
+        android:id="@+id/empty_view"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_centerInParent="true"
+        android:textAppearance="?android:textAppearanceMedium" />
+</RelativeLayout>

app/src/main/res/values/strings.xml

@@ -1,6 +1,11 @@
 <resources>
     <string name="app_name">Quake Report</string>
+
     <!-- Default text to show with the earthquake location ("Near the Pacific-Antarctic Ridge")
     if no specific kilometer distance from the primary location is given [CHAR LIMIT=30] -->
     <string name="near_the">Near the</string>
+
+    <!-- Text to display in the list when there are no earthquakes [CHAR LIMIT=NONE] -->
+    <string name="no_earthquakes">No earthquakes found.</string>
+
 </resources>