将函数作为参数传递给java

Question

我熟悉Android框架和Java,并希望创建一个通用的"NetworkHelper"类,它可以处理大多数网络代码,使我能够从中调用网页.

我按照developer.android.com上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);

    }

} 

}

在我的活动类中,我使用这种方式:

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活动,它应该可以在"downloadUrl()"函数中定义.例如,当下载完成时,将调用活动中的public void"handleWebpage(String data)"函数,并将加载的字符串作为其参数.

我做了一些谷歌搜索,发现我应该以某种方式使用接口来实现这个功能.在查看了几个类似的stackoverflow问题/答案后,我没有得到它的工作,我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？说实话,使用匿名类对我来说是新的,我不确定我应该在提到的线程中应用示例代码片段的地方或方式.

所以我的问题是如何将回调函数传递给我的网络类并在下载完成后调用它？接口声明的位置,实现关键字等等？请注意我是Java的初学者(虽然有其他编程背景)所以我很感激整个解释:)谢谢!

Answer 1

使用带有抽象回调方法的回调接口或抽象类.

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...
    }
}

第二种选择更简洁.您甚至不必为"onDownloaded事件"定义抽象方法,onPostExecute正如所需要的那样.只需DownloadWebpageTask在downloadUrl方法中使用匿名内联类扩展您的内容.

    //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);
    }

    //...

Answer 2

没有接口,没有lib,没有Java 8需要!

只是使用Callable<V>来自java.util.concurrent

public static void superMethod(String simpleParam, Callable<Void> methodParam) {

    //your logic code [...]

    //call methodParam
    try {
        methodParam.call();

    } catch (Exception e) {
        e.printStackTrace();
    }
}

如何使用它:

 superMethod("Hello world", new Callable<Void>() {
                public Void call() {
                    myParamMethod();
                    return null;
                }
            }
    );

myParamMethod()我们传递的方法作为参数在哪里(在这种情况下methodParam).

Answer 3

是的,界面是恕我直言的最佳方式.例如,GWT使用带有如下界面的命令模式:

public interface Command{
    void execute();
}

通过这种方式,您可以将函数从方法传递到另一个方法

public void foo(Command cmd){
  ...
  cmd.execute();
}

public void bar(){
  foo(new Command(){
     void execute(){
        //do something
     }
  });
}

Answer 4

开箱即用的解决方案是Java中不可能实现这一点.Java不接受高阶函数.它可以通过一些"技巧"来实现.通常,界面是您看到的那个.请看这里了解更多信息.您也可以使用反射来实现它,但这很容易出错.

Answer 5

使用接口可能是 Java 编码架构中最好的方式。

但是，传递一个 Runnable 对象也可以工作，而且我认为它会更加实用和灵活。

 SomeProcess sp;

 public void initSomeProcess(Runnable callbackProcessOnFailed) {
     final Runnable runOnFailed = callbackProcessOnFailed; 
     sp = new SomeProcess();
     sp.settingSomeVars = someVars;
     sp.setProcessListener = new SomeProcessListener() {
          public void OnDone() {
             Log.d(TAG,"done");
          }
          public void OnFailed(){
             Log.d(TAG,"failed");
             //call callback if it is set
             if (runOnFailed!=null) {
               Handler h = new Handler();
               h.post(runOnFailed);
             }
          }               
     };
}

/****/

initSomeProcess(new Runnable() {
   @Override
   public void run() {
       /* callback routines here */
   }
});