传递函数为Java中的一个参数 [英] Passing function as a parameter in java
问题描述
我越来越熟悉了Android框架和Java，并希望创建一个一般的NetworkHelper类，它会处理大部分的网络code使我只是调用网页都如此。
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.
我跟着这篇文章从developer.android.com创建我的网络类：<一href="http://developer.android.com/training/basics/network-ops/connecting.html">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
code：
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);
}
}
}
在我的活动课我使用类是这样的：
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);
}
问题我有是，我应该以某种方式使返回活动回调，它应该是定义在downloadUrl（）函数。例如，当下载完成，公共无效handleWebpage（字符串数据）的活动函数调用加载的字符串作为它的参数。
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一功能。回顾几个类似计算器问题之后/我没有得到答案，它的工作，我不知道如果我理解正确的接口：的如何传递方法，在Java中的参数？以使用匿名类是新的对我说实话，我真的不知道在哪里，我应该如何应用的例子code段中提到的主题。
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.
所以我的问题是我怎么能传递回调函数，以我的网络级和下载完成后调用它呢？凡接口声明的推移，implements关键字等等？ 请注意，我是初学者使用Java（有其他的编程背景，虽然），所以我最好的AP preciate一个整个的解释:)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
使用一个回调接口或抽象类抽象的回调方法。
Use a callback interface or an abstract class with abstract callback methods.
回调接口，例如：
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...
}
}
第二个选项是更简洁。你甚至不必定义为 onPostExecute 的抽象方法onDownloaded事件确实是需要什么。简单地延长你的 DownloadWebpageTask 与匿名内嵌类的 downloadUrl 方法内。
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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