从MainActivity以外的类调用intent

Question

首先让我指出，这是我第一次编写应用程序，甚至是Java。我习惯编写PHP和JS，所以请使用

我遵循谷歌指南http://developer.android.com/training/basics/network-ops/connecting.html

这是我的Main Activity

package com.example.eerstetest;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
public class MainActivity extends Activity {
public final static String EXTRA_MESSAGE = "com.example.eerstetest.MESSAGE";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
@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;
}
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class); 
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}
public void getUpdate(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class); 
    // see if there is a data connection
    ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
    if (networkInfo != null && networkInfo.isConnected()) {
        // fetch data
        //textView.setText("Network connection available.");
        String dataUrl = "http://www.myurl.com";
        new DownloadUpdate().execute(dataUrl);
    } else {
        // display error
        //textView.setText("No network connection available.");
        String message = "No network connection available.";
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}
}

正如你所看到的，我有一个sendMessage方法的工作，使用DisplayMessageActivity我还有getuupdate方法，它调用DownloadUpdate类，该类位于单独的.java文件

中。

package com.example.eerstetest;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.io.UnsupportedEncodingException;
import java.net.HttpURLConnection;
import java.net.URL;
import android.content.Context;
import android.content.Intent;
import android.os.AsyncTask;
import android.util.Log;
// Uses AsyncTask to create a task away from the main UI thread. This task takes a 
// URL string and uses it to create an HttpUrlConnection. Once the connection
// has been established, the AsyncTask downloads the contents of the webpage as
// an InputStream. Finally, the InputStream is converted into a string, which is
// displayed in the UI by the AsyncTask's onPostExecute method.
public class DownloadUpdate extends AsyncTask<String, Integer, String> {
private static final String DEBUG_TAG = "HttpExample";
//private TextView textView;
public final static String EXTRA_MESSAGE = "com.example.eerstetest.MESSAGE";
@Override
protected String doInBackground(String... url) {
   // params comes from the execute() call: params[0] is the url.
   try {
       return downloadUrl(url[0]);
   } catch (IOException e) {
       return "Unable to retrieve web page. URL may be invalid.";
   }
}
// onPostExecute displays the results of the AsyncTask.
@Override
protected void onPostExecute(String result) {
   //textView.setText(result);
    Intent intent = new Intent(this, DisplayMessageActivity.class); 
    intent.putExtra(EXTRA_MESSAGE, result);
    startActivity(intent);
}
// Given a URL, establishes an HttpUrlConnection and retrieves
// the web page content as a InputStream, which it returns as
// a string.
private String downloadUrl(String myurl) throws IOException {
    InputStream is = null;
    // Only display the first 500 characters of the retrieved
    // web page content.
    int len = 500;
    try {
        URL url = new URL(myurl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(10000 /* milliseconds */);
        conn.setConnectTimeout(15000 /* milliseconds */);
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        // Starts the query
        conn.connect();
        int response = conn.getResponseCode();
        Log.d(DEBUG_TAG, "The response is: " + response);
        is = conn.getInputStream();
        // Convert the InputStream into a string
        String contentAsString = readIt(is, len);
        return contentAsString;
    // Makes sure that the InputStream is closed after the app is
    // finished using it.
    } finally {
        if (is != null) {
            is.close();
        } 
    }
}
// Reads an InputStream and converts it to a String.
public String readIt(InputStream stream, int len) throws IOException, UnsupportedEncodingException {
    Reader reader = null;
    reader = new InputStreamReader(stream, "UTF-8");        
    char[] buffer = new char[len];
    reader.read(buffer);
    return new String(buffer);
}
}

我不明白从这里调用DisplayMessageActivity时我必须为上下文填写什么

为了完整，这里是我的manifest文件

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.eerstetest"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="17" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.example.eerstetest.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name="com.example.eerstetest.DisplayMessageActivity"
        android:label="@string/title_activity_display_message"
        android:parentActivityName="com.example.eerstetest.MainActivity" >
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.example.eerstetest.MainActivity" />
    </activity>
    <activity
        android:name="com.example.eerstetest.DownloadUpdate"
        android:label="@string/title_activity_download_update" >
    </activity>
</application>
</manifest>

如果还有什么需要我提供的，请告诉我

Answer 1

像这样传递上下文作为参数:

new DownloadUpdate(MainActivity.this).execute(dataUrl);

在DownloadUpdate:

中添加构造函数

Context context;
public DownloadUpdate(Context context){
    this.context = context
}

，然后在DownloadUpdate类中使用context作为全局参数，并使用this

调用intent

Intent intent = new Intent(DownloadUpdate.this, DisplayMessageActivity.class); 
intent.putExtra(EXTRA_MESSAGE, result);
context.startActivity(intent);

Answer 2

这样调用:

Intent intent = new Intent(DownloadUpdate.this, DisplayMessageActivity.class); 
intent.putExtra(EXTRA_MESSAGE, result);
startActivity(intent);

Answer 3

我认为你是从另一个类或活动调用DownloadUpdate:

new DownloadUpdate().execute(params);

解决DownloadUpdate中的上下文问题，创建如下构造函数:

public DownloadUpdate(Context context){
    this.context = context;
}

这里的this.context是指在DownloadUpdate类中全局声明的Context变量。然后调用DownloadUpdate为:

new DownloadUpdate(your_context).execute(params);

然后使用您在DownloadUpdate中获得的上下文来创建您的意图。如:

@Override
protected void onPostExecute(String result) {
   //textView.setText(result);
    Intent intent = new Intent(context, DisplayMessageActivity.class); 
    intent.putExtra(EXTRA_MESSAGE, result);
    startActivity(intent);
}