将函数作为参数传递给Java

Question

我已经熟悉Android框架和Java，并希望创建一个通用的“ NetworkHelper”类，该类可以处理大多数联网代码，使我能够从中调用网页。

我遵循了来自developer.android.com的这篇文章来创建我的网络类：http : //developer.android.com/training/basics/network-
ops/connecting.html

码：

package com.example.androidapp;

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.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.util.Log;



/**
 * @author tuomas
 * This class provides basic helper functions and features for network communication.
 */


public class NetworkHelper 
{
private Context mContext;


public NetworkHelper(Context mContext)
{
    //get context
    this.mContext = mContext;
}


/**
 * Checks if the network connection is available.
 */
public boolean checkConnection()
{
    //checks if the network connection exists and works as should be
    ConnectivityManager connMgr = (ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();

    if (networkInfo != null && networkInfo.isConnected())
    {
        //network connection works
        Log.v("log", "Network connection works");
        return true;
    }
    else
    {
        //network connection won't work
        Log.v("log", "Network connection won't work");
        return false;
    }

}

public void downloadUrl(String stringUrl)
{
    new DownloadWebpageTask().execute(stringUrl);

}



//actual code to handle download
private class DownloadWebpageTask extends AsyncTask<String, Void, String>
{



    @Override
    protected String doInBackground(String... urls)
    {
        // params comes from the execute() call: params[0] is the url.
        try {
            return downloadUrl(urls[0]);
        } catch (IOException e) {
            return "Unable to retrieve web page. URL may be invalid.";
        }
    }

    // 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 );
            conn.setConnectTimeout(15000);
            conn.setRequestMethod("GET");
            conn.setDoInput(true);
            // Starts the query
            conn.connect();
            int response = conn.getResponseCode();
            Log.d("log", "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);
    }


    // onPostExecute displays the results of the AsyncTask.
    @Override
    protected void onPostExecute(String result) 
    {
        //textView.setText(result);
        Log.v("log", result);

    }

}

}

在活动类中，我以这种方式使用该类：

connHelper = new NetworkHelper(this);

…

if (connHelper.checkConnection())
    {
        //connection ok, download the webpage from provided url
        connHelper.downloadUrl(stringUrl);
    }

我遇到的问题是我应该以某种方式回调该活动，并且应该在“
downloadUrl（）”函数中定义它。例如，下载完成后，将使用加载的字符串作为其参数来调用活动中的公共void“
handleWebpage（String data）”函数。

我进行了一些谷歌搜索，发现我应该以某种方式使用接口来实现此功能。在复习了一些类似的stackoverflow问题/答案之后，我没有使它起作用，并且不确定我是否正确理解了接口：如何在Java中将方法作为参数传递？老实说，使用匿名类对我来说是新手，我不确定在哪里或如何在上述线程中应用示例代码片段。

所以我的问题是如何将回调函数传递给网络类，并在下载完成后调用它？接口声明在哪里，实现关键字等等？请注意，我是Java的初学者（尽管具有其他编程背景），所以请您多加解释:)谢谢！

Answer 1

使用回调接口或带有抽象回调方法的抽象类。

回调接口示例：

public class SampleActivity extends Activity {

    //define callback interface
    interface MyCallbackInterface {

        void onDownloadFinished(String result);
    }

    //your method slightly modified to take callback into account 
    public void downloadUrl(String stringUrl, MyCallbackInterface callback) {
        new DownloadWebpageTask(callback).execute(stringUrl);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //example to modified downloadUrl method
        downloadUrl("http://google.com", new MyCallbackInterface() {

            @Override
            public void onDownloadFinished(String result) {
                // Do something when download finished
            }
        });
    }

    //your async task class
    private class DownloadWebpageTask extends AsyncTask<String, Void, String> {

        final MyCallbackInterface callback;

        DownloadWebpageTask(MyCallbackInterface callback) {
            this.callback = callback;
        }

        @Override
        protected void onPostExecute(String result) {
            callback.onDownloadFinished(result);
        }

        //except for this leave your code for this class untouched...
    }
}

第二种选择更加简洁。您甚至onPostExecute不必完全按照需要定义“
onDownloaded事件”的抽象方法。只需DownloadWebpageTask在您的downloadUrl方法内部添加一个匿名内联类即可。

    //your method slightly modified to take callback into account 
    public void downloadUrl(String stringUrl, final MyCallbackInterface callback) {
        new DownloadWebpageTask() {

            @Override
            protected void onPostExecute(String result) {
                super.onPostExecute(result);
                callback.onDownloadFinished(result);
            }
        }.execute(stringUrl);
    }

    //...