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Question

我是 Java 和 Android 开发的新手。

坚持在 android 中实施 "SWIPE TO REFRESH"。我正在尝试访问其他 class 中的方法。我将突出显示 android studio 在 blockquote 中给出的错误。非常感谢帮助。

MainActivity.java

import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.widget.SwipeRefreshLayout;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends AppCompatActivity{

private String TAG = MainActivity.class.getSimpleName();

private SwipeRefreshLayout swipeRefreshLayout;
private ProgressDialog pDialog;
private ListView lv;

// URL to get contacts JSON
private static String url = "http://hooman.com/hoo.php";

ArrayList<HashMap<String, String>> contactList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    contactList = new ArrayList<>();
    swipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swiperefresh);


    lv = (ListView) findViewById(R.id.list);

    swipeRefreshLayout.setOnRefreshListener(this);
    swipeRefreshLayout.post(new Runnable() {
                                @Override
                                public void run() {
                                    swipeRefreshLayout.setRefreshing(true);
                                    doInBackground();
                                }
                            }
    );
    new GetContacts().execute();
}
@Override
public void onRefresh() {
    doInBackground();
}

/**
 * Async task class to get json by making HTTP call
 */
private class GetContacts extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        pDialog = new ProgressDialog(MainActivity.this);
        pDialog.setMessage("Please wait...");
        pDialog.setCancelable(false);
        pDialog.show();

    }

    @Override
    protected Void doInBackground(Void... arg0) {
        HttpHandler sh = new HttpHandler();
        swipeRefreshLayout.setRefreshing(true);
        // Making a request to url and getting response
        String jsonStr = sh.makeServiceCall(url);

        Log.e(TAG, "Response from url: " + jsonStr);

        if (jsonStr != null) {
            try {

                JSONArray contacts = new JSONArray(jsonStr);

                // looping through All Contacts
                for (int i = 0; i < contacts.length(); i++) {
                    JSONObject c = contacts.getJSONObject(0);

                    String kw = c.getString("kw");
                    String kva = c.getString("kva");
                    String pf = c.getString("pf");

                    // tmp hash map for single contact
                    HashMap<String, String> contact = new HashMap<>();

                    // adding each child node to HashMap key => value
                    contact.put("kw", kw);
                    contact.put("kva", kva);
                    contact.put("pf", pf);

                    // adding contact to contact list
                    contactList.add(contact);

                    // stopping swipe refresh
                    swipeRefreshLayout.setRefreshing(false);
                }
            } catch (final JSONException e) {
                Log.e(TAG, "Json parsing error: " + e.getMessage());
                Log.e("JSON Parser", "Error parsing data " + e.toString());
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(),
                                "Json parsing error: " + e.getMessage(),
                                  Toast.LENGTH_LONG)
                                .show();
                    }
                });

            }
        } else {
            Log.e(TAG, "Couldn't get json from server.");
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    Toast.makeText(getApplicationContext(),
                            "Couldn't get json from server. Check LogCat for possible errors!",
                            Toast.LENGTH_LONG)
                            .show();
                    // stopping swipe refresh
                    swipeRefreshLayout.setRefreshing(false);
                }
            });

        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (pDialog.isShowing())
            pDialog.dismiss();
        /**
         * Updating parsed JSON data into ListView
         * */
        ListAdapter adapter = new SimpleAdapter(
                MainActivity.this, contactList,
                R.layout.list_item, new String[]{"kw", "kva",
                "pf"}, new int[]{R.id.kw,
                R.id.kva, R.id.pf});

        lv.setAdapter(adapter);
    }

}
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.SwipeRefreshLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/swiperefresh"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

<ListView
    android:id="@+id/list"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" />
</android.support.v4.widget.SwipeRefreshLayout>

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:padding="10dp">

    <TextView
        android:id="@+id/kw"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="2dip"
        android:paddingTop="6dip"
        android:textColor="@color/colorPrimaryDark"
        android:textSize="16sp"
        android:textStyle="bold" />

    <TextView
        android:id="@+id/kva"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:paddingBottom="2dip"
        android:textColor="@color/colorAccent" />

    <TextView
        android:id="@+id/pf"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="#5d5d5d"
        android:textStyle="bold" />
</LinearLayout>

提前致谢。

Answer 1

首先，不要从 doInBackground() 方法调用 UI 视图。

您可以从 onPreExcute() 方法或 onPostExecute 方法调用它们。

但是由于AsyncTask在其他文件中，你也不能直接调用它们。

因为您想在刷新 SwipeRefreshLayout 时执行 AsyncTask 代码。

有两种方法：

1st：使 AsyncTask class 成为调用 activity 的内部 class。（仅当您打算仅在此 activity.

中使用此 Asynctask 时才有用

第二：在 AsyncTask 中实现回调 How to get the result of OnPostExecute() to main activity because AsyncTask is a separate class?

3：使用 Volley / Retrofit（库）。（由于您只调用 API 并获取 JSON 数据，它们默认使用 "Other thread than main threads" 并且也内置了 JSON 支持。

Answer 2

首先，你有循环依赖。 AsyncTask 无法引用 Activity 中的 swipeRefreshLayout 视图，并且 Activity 需要一个任务实例才能调用 task.doInBackground()（您应该 永远不要 任何方式）。只需要使用 execute() 很少使用 execute().get()

您可以做的是覆盖 activity

中的 postExecute 函数

swipeRefreshLayout.post(new Runnable() {
        @Override
        public void run() {
            swipeRefreshLayout.setRefreshing(true);

            // Showing progress dialog
            final ProgressDialog pDialog = new ProgressDialog(MainActivity.this);
            pDialog.setMessage("Please wait...");
            pDialog.setCancelable(false);
            pDialog.show();

            contactList.clear();

            new GetContacts() {

              @Override
              protected void onPostExecute(Void result) {
                  // stopping swipe refresh
                  swipeRefreshLayout.setRefreshing(false);

                  // Dismiss the progress dialog
                  if (pDialog.isShowing())
                      pDialog.dismiss();

                  ListAdapter adapter = new SimpleAdapter(
                  MainActivity.this, contactList, R.layout.list_item, 
                  new String[]{"kw", "kva", "pf"}, 
                  new int[]{R.id.kw,R.id.kva, R.id.pf});

                  lv.setAdapter(adapter);  

                }
            }.execute();
        }
    }
);

滑动刷新

Swipe to refresh

java

android

pull-to-refresh

android-swipe