JSON


1   /*
2    * Copyright 2006 Google Inc.
3    * 
4    * Licensed under the Apache License, Version 2.0 (the "License"); you may not
5    * use this file except in compliance with the License. You may obtain a copy of
6    * the License at
7    * 
8    * http://www.apache.org/licenses/LICENSE-2.0
9    * 
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13   * License for the specific language governing permissions and limitations under
14   * the License.
15   */
16  package com.google.gwt.sample.json.client;
17  
18  import com.google.gwt.json.client.JSONArray;
19  import com.google.gwt.json.client.JSONException;
20  import com.google.gwt.json.client.JSONObject;
21  import com.google.gwt.json.client.JSONParser;
22  import com.google.gwt.json.client.JSONString;
23  import com.google.gwt.json.client.JSONValue;
24  import com.google.gwt.user.client.HTTPRequest;
25  import com.google.gwt.user.client.ResponseTextHandler;
26  import com.google.gwt.user.client.Window;
27  import com.google.gwt.user.client.ui.Button;
28  import com.google.gwt.user.client.ui.ClickListener;
29  import com.google.gwt.user.client.ui.RootPanel;
30  import com.google.gwt.user.client.ui.Tree;
31  import com.google.gwt.user.client.ui.TreeItem;
32  import com.google.gwt.user.client.ui.Widget;
33  
34  import java.util.Iterator  ;
35  import java.util.Set  ;
36  
37  /**
38   * Class that acts as a client to a JSON service. Currently, this client just
39   * requests a text which contains a JSON encoding of a search result set from
40   * yahoo. We use a text file to demonstrate how the pieces work without tripping
41   * on cross-site scripting issues.
42   * 
43   * If you would like to make this a more dynamic example, you can associate a
44   * servlet with this example and simply have it hit the yahoo service and return
45   * the results.
46   */
47  public class JSON {
48    /**
49     * Class for handling the response text associated with a request for a JSON
50     * object.
51     * 
52     */
53    private class JSONResponseTextHandler implements ResponseTextHandler {
54      public void onCompletion(String   responseText) {
55        try {
56          JSONValue jsonValue = JSONParser.parse(responseText);
57          displayJSONObject(jsonValue);
58        } catch (JSONException e) {
59          displayError(responseText);
60        }
61        searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
62      }
63    }
64  
65    /*
66     * Class for handling the fetch button's click event.
67     */
68    private class SearchButtonClickListener implements ClickListener {
69      public void onClick(Widget sender) {
70        jsonTree.setVisible(false);
71        doFetchURL();
72      }
73    }
74  
75    /*
76     * Default URL to use to fetch JSON objects. Note that the contents of this
77     * JSON result were as a result of requesting the following URL:
78     * 
79     * http://api.search.yahoo.com/ImageSearchService/V1/imageSearch?appid=YahooDemo&query=potato&results=2&output=json
80     * 
81     */
82    private static final String   DEFAULT_SEARCH_URL = "search-results.js";
83  
84    /*
85     * Text displayed on the fetch button when we are in a default state.
86     */
87    private static final String   SEARCH_BUTTON_DEFAULT_TEXT = "Search";
88  
89    /*
90     * Text displayed on the fetch button when we are waiting for a JSON reply.
91     */
92    private static final String   SEARCH_BUTTON_WAITING_TEXT = "Waiting for JSON Response...";
93  
94    private Tree jsonTree = new Tree();
95  
96    private Button searchButton = new Button();
97  
98    /**
99     * Entry point for this simple application. Currently, we build the
100    * application's form and wait for events.
101    */
102   public void onModuleLoad() {
103     initializeMainForm();
104   }
105 
106   /*
107    * Add the object presented by the JSONValue as a children to the requested
108    * TreeItem.
109    */
110   private void addChildren(TreeItem treeItem, JSONValue jsonValue) {
111     JSONArray jsonArray;
112     JSONObject jsonObject;
113     JSONString jsonString;
114 
115     if ((jsonArray = jsonValue.isArray()) != null) {
116       for (int i = 0; i < jsonArray.size(); ++i) {
117         TreeItem child = treeItem.addItem(getChildText("["
118             + Integer.toString(i) + "]"));
119         addChildren(child, jsonArray.get(i));
120       }
121     } else if ((jsonObject = jsonValue.isObject()) != null) {
122       Set   keys = jsonObject.keySet();
123       for (Iterator   iter = keys.iterator(); iter.hasNext();) {
124         String   key = (String  ) iter.next();
125         TreeItem child = treeItem.addItem(getChildText(key));
126         addChildren(child, jsonObject.get(key));
127       }
128     } else if ((jsonString = jsonValue.isString()) != null) {
129       // Use stringValue instead of toString() because we don't want escaping
130       treeItem.addItem(jsonString.stringValue());
131     } else {
132       // JSONBoolean, JSONNumber, and JSONNull work well with toString().
133       treeItem.addItem(getChildText(jsonValue.toString()));
134     }
135   }
136 
137   private void displayError(String   responseText) {
138     jsonTree.removeItems();
139     jsonTree.setVisible(true);
140     TreeItem treeItem = jsonTree.addItem("Failed to parse JSON response");
141     treeItem.addItem(responseText);
142     treeItem.setStyleName("JSON-JSONResponseObject");
143     treeItem.setState(true);
144   }
145 
146   /*
147    * Update the treeview of a JSON object.
148    */
149   private void displayJSONObject(JSONValue jsonValue) {
150     jsonTree.removeItems();
151     jsonTree.setVisible(true);
152     TreeItem treeItem = jsonTree.addItem("JSON Response");
153     addChildren(treeItem, jsonValue);
154     treeItem.setStyleName("JSON-JSONResponseObject");
155     treeItem.setState(true);
156   }
157 
158   /*
159    * Fetch the requested URL.
160    */
161   private void doFetchURL() {
162     searchButton.setText(SEARCH_BUTTON_WAITING_TEXT);
163     if (!HTTPRequest.asyncGet(DEFAULT_SEARCH_URL, new JSONResponseTextHandler())) {
164       // Reset the caption.
165       //
166       searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
167     }
168   }
169 
170   /*
171    * Causes the text of child elements to wrap.
172    */
173   private String   getChildText(String   text) {
174     return "<span style='white-space:normal'>" + text + "</span>";
175   }
176 
177   /**
178    * Initialize the main form's layout and content.
179    */
180   private void initializeMainForm() {
181     searchButton.setStyleName("JSON-SearchButton");
182     searchButton.setText(SEARCH_BUTTON_DEFAULT_TEXT);
183     searchButton.addClickListener(new SearchButtonClickListener());
184 
185     // Avoids showing an "empty" cell
186     jsonTree.setVisible(false);
187 
188     // Find out where the host page wants the button.
189     //
190     RootPanel searchButtonSlot = RootPanel.get("search");
191     if (searchButtonSlot == null) {
192       Window.alert("Please define a container element whose id is 'search'");
193       return;
194     }
195 
196     // Find out where the host page wants the tree view.
197     //
198     RootPanel treeViewSlot = RootPanel.get("tree");
199     if (treeViewSlot == null) {
200       Window.alert("Please define a container element whose id is 'tree'");
201       return;
202     }
203 
204     // Add both widgets.
205     //
206     searchButtonSlot.add(searchButton);
207     treeViewSlot.add(jsonTree);
208   }
209 }
210
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