Question

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

我正在关注Google指南http://developer.android.com/training/basics/network-ops/connecting.html

这是我的主要活动

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);
    }
}
}

正如您所看到的，我有一个使用DisplayMessageActivity的sendMessage方法我也有getUpdate方法，它调用了一个单独的.java文件中的DownloadUpdate类

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);
}
}

从这里调用DisplayMessageActivity时，我不明白我必须填写上下文

要完成，这是m清单文件

<?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>

如果我能提供任何其他内容，请告诉我

Answer 1

将上下文作为参数传递，如下所示：

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

在DownloadUpdate中添加构造函数：

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

然后使用DownloadUpdate类中的上下文作为全局参数，并使用此

调用intent

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

Answer 2

这样打电话：

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

Answer 3

我认为你从另一个班级或活动中呼叫DownloadUpdate：

new DownloadUpdate().execute(params);

解决DownloadUpdate中的上下文问题，创建一个类似的构造函数：

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

此处this.context表示Context类中全局声明的DownloadUpdate变量。然后将DownloadUpdate称为：

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。像：

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

从MainActivity以外的类调用intent

3 个答案: