作者:Vamei 出处:http://www.cnblogs.com/vamei 欢迎转载,也请保留这段声明。谢谢!
上一讲介绍了用WebView来抓取一个网页内容。这一讲我将介绍如何在安卓内部直接进行HTTP通信。
《亚当的诞生》,西斯廷礼拜堂的吊顶画,米开朗基罗之作。当时的教皇强迫沉迷于雕塑的米开朗基罗画巨幅壁画。米开朗基罗认为这是在浪费自己的才华,充满愤怒的作画。当然,他又成功了。
描述
这一讲中,我将使用JSON,将数据库备份到远程的数据库中。反过来我也将从远程数据库中抓取条目,并放入安卓的数据库。相关的安卓知识点包括:
- 线程
- HTTP通信
- JSON
增加ActionBar
我首先在首页上增加一菜单,用于触发下载和上传功能。这是通过ActionBar实现的。ActionBar在页面的顶端增加一个横幅。这个横幅上可以有应用的图标、文字信息和选项菜单(OptionMenu)。
我在布局文件res/menu/main.xml中定义ActionBar的菜单:
<menu xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" > <item android:id="@+id/action_upload" android:orderInCategory="100" android:title="Upload" app:showAsAction="never"/> <item android:id="@+id/action_download" android:orderInCategory="100" android:title="Download" app:showAsAction="never"/> </menu>
现在,修改之前的MainActivity.java。MainActivity将继承ActionBarActivity。这样,MainActivity页面的顶端将增加一条ActionBar。接下来,我要覆盖ActionBarActivity的两个方法。一个是onCreateOptionsMenu()方法。在这个方法中,我绑定上面的视图文件到ActionBar上。另一个方法onOptionsItemSelected(),主要用于说明菜单各个选项被点击后的动作。代码如下:
package me.vamei.vamei;import android.content.Context; import android.content.Intent; import android.content.SharedPreferences; import android.os.Bundle; import android.support.v7.app.ActionBarActivity;import android.view.Menu; import android.view.MenuItem; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; import android.widget.TextView;public class MainActivity extends ActionBarActivity implements OnClickListener { private SharedPreferences sharedPref; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); sharedPref = this.getSharedPreferences("me.vamei.vamei", Context.MODE_PRIVATE); Button btn1 = (Button) findViewById(R.id.author); btn1.setOnClickListener(this); Button btn2 = (Button) findViewById(R.id.category); btn2.setOnClickListener(this); } @Override protected void onResume() { super.onResume(); TextView nameView = (TextView) findViewById(R.id.welcome); // retrieve content from shared preference, with key "name" String welcome = "Welcome, " + sharedPref.getString("name", "unknown") + "!"; nameView.setText(welcome); } // method for interface OnClickListener @Override public void onClick(View v) { Intent intent; // Routing to different view elements switch(v.getId()) { case R.id.author: intent = new Intent(this, SelfEditActivity.class); startActivity(intent); break; case R.id.category: intent = new Intent(this, CategoryActivity.class); startActivity(intent); break; } } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } @Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); switch (id) { case R.id.action_download: return true; case R.id.action_upload: return true; } return super.onOptionsItemSelected(item); } }
在上面,我的onOptionsItemSelected()方法还没有包含具体的功能。我将在下一部分为该方法增加功能。 运行应用后效果如下:
HTTP获得数据
下一步,我将增加"Download"按钮点击后的功能。按钮点击后,应用将访问互联网,并获得URL指向的.js文件。获得.js文件后,我从该文件中提取JSON对象,这个对象中包含一些新的Category名字。最后,我在数据库中增加Category条目:
@Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); switch (id) { case R.id.action_download: Thread thread = new Thread() { @Override public void run(){ try{ // Http Get InputStream content; HttpClient httpclient = new DefaultHttpClient(); HttpResponse response = httpclient.execute( new HttpGet("http://files.cnblogs.com/vamei/android_contact.js")); content = response.getEntity().getContent(); BufferedReader reader = new BufferedReader(new InputStreamReader(content)); final StringBuilder sb = new StringBuilder(); String line = null; while ((line = reader.readLine()) != null) { sb.append(line); } content.close(); // Parse JSON Object and Save to DB JSONObject receivedObject = new JSONObject(sb.toString()); JSONArray categoryObjects = receivedObject.getJSONArray("category"); ContactsManager cm = new ContactsManager(getApplicationContext()); JSONObject categoryObject; for (int i=0; i< categoryObjects.length(); i++) { categoryObject = categoryObjects.getJSONObject(i); String name = categoryObject.getString("name"); Category category = new Category(name); cm.createCategory(category); } } catch (Exception e) { Log.i("Http Error", e.getMessage().toString()); } } }; thread.start(); break; } return super.onOptionsItemSelected(item); }
注意到,上面的网络访问部分启动了一个新线程Thread。为了确保界面的流畅,安卓规定网络访问不能在负责图画界面的主线程中进行。所以,我们必须把网络访问放在一个新的线程中。我们通过异步的方式进行网络访问,将在下一部分介绍。
程序中的JSONObject和JSONArray用于解析接收到的JSON字符串。
使用AsyncTask
AsyncTask在背景进程中工作。AsyncTask分为工作准备、工作进行和工作完成三个部分。AsyncTask有三个方法,onPreExecute(), doInBackground(), onPostExecute()分别代表这三个部分的任务。其中,doInBackground在背景进程中进行,因此可以把网络访问放入其中。此外,在doInBackground中,可以通过调用publishProgress(),来更新任务的进度。进度更新后,AsyncTask将调用onProgressUpdate()方法。
AsyncTask有三个类型<X, Y, Z>。它们分别是doInBackground(X), onProgressUpdate(Y)和onPostExecute(Z)的参数类型。此外,doInBackground()方法的返回值将成为onPostExecute()的参数,因此doInBackground()方法的返回值类型也是Z。
当工作完成,AsyncTask会通知主线程。AsyncTask与Thread的目的相同,但它异步的调用方式更方便编写,也更容易实现主线程和背景线程之间的信息传递。我下面会实现Upload的对应功能,即把Category表转成JSON,再把该JSON字符串发送到特定的URL。
@Override public boolean onOptionsItemSelected(MenuItem item) { // Handle action bar item clicks here. The action bar will // automatically handle clicks on the Home/Up button, so long // as you specify a parent activity in AndroidManifest.xml. int id = item.getItemId(); switch (id) { case R.id.action_download: Thread thread = new Thread() { @Override public void run(){ try{ InputStream content; // Http Get HttpClient httpclient = new DefaultHttpClient(); HttpResponse response = httpclient.execute( new HttpGet("http://files.cnblogs.com/vamei/android_contact.js")); content = response.getEntity().getContent(); BufferedReader reader = new BufferedReader(new InputStreamReader(content)); StringBuilder sb = new StringBuilder(); String line = null; while ((line = reader.readLine()) != null) { sb.append(line); } content.close(); // Parse JSON Object and Save to DB JSONObject receivedObject = new JSONObject(sb.toString()); JSONArray categoryObjects = receivedObject.getJSONArray("category"); ContactsManager cm = new ContactsManager(getApplicationContext()); JSONObject categoryObject; for (int i=0; i< categoryObjects.length(); i++) { categoryObject = categoryObjects.getJSONObject(i); String name = categoryObject.getString("name"); Category category = new Category(name); cm.createCategory(category); } } catch (Exception e) { Log.i("Http Error", e.getMessage().toString()); } } }; thread.start(); break; // upload action case R.id.action_upload: UploadTask newTask = new UploadTask(); newTask.execute("http://files.cnblogs.com/"); break; } return super.onOptionsItemSelected(item); } private class UploadTask extends AsyncTask <String, String, String> { /*main worker*/ @Override protected String doInBackground(String...params) { ContactsManager cm = new ContactsManager(getApplicationContext()); List<Category> categories = cm.getAllCategories(); JSONObject sendObject = new JSONObject(); JSONArray categoryObjects = new JSONArray(); try { for (int i=0; i<categories.size(); i++) { JSONObject categoryObject = new JSONObject(); categoryObject.put("name", categories.get(i).getName()); categoryObjects.put(categoryObject); } sendObject.put("category", categoryObjects); // update progress once publishProgress("JSON DONE"); // posting to URL HttpClient httpClient = new DefaultHttpClient(); HttpPost httpPost = new HttpPost(params[0]); StringEntity se = new StringEntity(sendObject.toString()); se.setContentEncoding(new BasicHeader(HTTP.CONTENT_TYPE, "application/json")); httpPost.setEntity(se); HttpResponse httpResponse = httpClient.execute(httpPost); // update progress again publishProgress("NETWORK DONE"); return httpResponse.getStatusLine().toString(); } catch (Exception e) { e.printStackTrace(); return "Crashed"; } } /*after background work is done*/ @Override protected void onPostExecute(String result) { Toast.makeText(MainActivity.this, result, Toast.LENGTH_LONG).show(); } /*when progress is updated*/ @Override protected void onProgressUpdate(String...params) { Toast.makeText(MainActivity.this, params[0], Toast.LENGTH_SHORT).show(); } }
这里的URL并不能处理POST方法。如果有兴趣,可以使用上一部分的Play框架,自制接受POST的服务器,并处理这里上传的JSON。
总结
ActionBarActivity
Thread, AsyncTask
JSONObject, JSONArray
Http, get and post