在java中将函数作为参数传递 [英] Passing function as a parameter in java
问题描述
我开始熟悉 Android 框架和 Java，并想创建一个通用的NetworkHelper"类，该类将处理大部分网络代码，使我能够从中调用网页.
I'm getting familiar with Android framework and Java and wanted to create a general "NetworkHelper" class which would handle most of the networking code enabling me to just call web-pages from it.
我按照 developer.android.com 上的这篇文章创建了我的网络课程:http://developer.android.com/training/basics/network-ops/connecting.html
I followed this article from the developer.android.com to create my networking class: http://developer.android.com/training/basics/network-ops/connecting.html
代码:
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，我这样使用类:
In my activity class I use the class this way:
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对活动进行回调，并且它应该可以在downloadUrl()"函数中定义.例如，当下载完成时，活动中的 public void "handleWebpage(String data)" 函数以加载的字符串作为其参数被调用.
Problem I'm having is that I should somehow make a callback back to the activity and it should be definable in "downloadUrl()" function. For example when download finishes, public void "handleWebpage(String data)" function in activity is called with loaded string as its parameter.
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? 老实说，使用匿名类对我来说是新的，我不确定在哪里或我应该如何在提到的线程中应用示例代码片段.
I did some googling and found that I should somehow use interfaces to achieve this functionality. After reviewing few similar stackoverflow questions/answers I didn't get it working and I'm not sure if I understood interfaces properly: How do I pass method as a parameter in Java? To be honest using the anonymous classes is new for me and I'm not really sure where or how I should apply the example code snippets in the mentioned thread.
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?接口声明在哪里，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)，所以我希望能得到完整的解释:) 谢谢！
So my question is how I could pass the callback function to my network class and call it after download finishes? Where the interface declaration goes, implements keyword and so on? Please note that I'm beginner with Java (have other programming background though) so I'd appreciate a throughout explanation :) Thank you!
推荐答案
使用回调接口或带有抽象回调方法的抽象类.
Use a callback interface or an abstract class with abstract callback methods.
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...
}
}
第二个选项更加简洁.您甚至不必为onDownloaded 事件"定义抽象方法，因为 onPostExecute 正是需要的.只需在 downloadUrl 方法中使用匿名内联类扩展您的 DownloadWebpageTask.
The second option is even more concise. You do not even have to define an abstract method for "onDownloaded event" as onPostExecute does exactly what is needed. Simply extend your DownloadWebpageTask with an anonymous inline class inside your downloadUrl method.
//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);
}
//...
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