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我的演示程序有一些問題。我創建了一個工作正常的新聞閱讀器應用程序。這個應用程序正在使用黑客新聞API下載新聞。Android：ProgressBar不能正常工作或顯示
當我試圖添加ProgressBar時，直到我的ListView加載，不幸的是它失敗了。它不工作，甚至沒有顯示。
我已經發布了所有我的工作應用程序，但是，只有MainActivity.java和activity_main.xml需要審查。
我盡力找到答案但徒勞無功。請幫我查看我的問題。以下是我的代碼。在此先感謝所有願意幫助的人。
activity_main.xml中
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.xpertwireless.newsreader.MainActivity">
<ListView
android:id="@+id/listView"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="1.0"
tools:layout_constraintBottom_creator="1"
tools:layout_constraintLeft_creator="1"
tools:layout_constraintRight_creator="1"
tools:layout_constraintTop_creator="1" />
<ProgressBar
android:id="@+id/progressBar"
style="?android:attr/progressBarStyle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginTop="8dp"
android:indeterminate="true"
android:visibility="visible"
app:layout_constraintBottom_toBottomOf="@+id/listView"
app:layout_constraintTop_toTopOf="@+id/listView"
tools:layout_editor_absoluteX="168dp"
tools:visibility="visible" />
</android.support.constraint.ConstraintLayout>
MainActivity.java
public class MainActivity extends AppCompatActivity {
ListView listView;
ArrayList<String> newsTitle;
ArrayAdapter<String> arrayAdapter;
SQLiteDatabase newsReaderDB;
ProgressBar progressBar;
public void initializeVar()
{
progressBar = (ProgressBar) findViewById(R.id.progressBar);
listView = (ListView) findViewById(R.id.listView);
newsTitle = new ArrayList<>();
newsTitle.clear();
// initializing the ArrayList and Attaching it to the adapter
arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, newsTitle);
// attached
listView.setAdapter(arrayAdapter); // setting the adapter
}
// Setting up the DownloadTask extending AsyncTask class
// Which will download the data in the background
// in this case, JSON data
public class DownloadTask extends AsyncTask<String,Void,String> {
@Override
protected void onPreExecute() {
super.onPreExecute();
Toast.makeText(getApplicationContext(),"PRE-EXEC Working", Toast.LENGTH_SHORT).show();
progressBar.setVisibility(View.VISIBLE);
}
@Override
protected void onProgressUpdate(Void... values) {
super.onProgressUpdate(values);
Toast.makeText(getApplicationContext(),"ON-PROGRESS Working", Toast.LENGTH_SHORT).show();
progressBar.setVisibility(View.VISIBLE);
}
@Override
protected void onPostExecute(String s) {
super.onPreExecute();
Toast.makeText(getApplicationContext(),"POST-EXEC Working", Toast.LENGTH_SHORT).show();
progressBar.setVisibility(View.GONE);
}
// Setting up the method which will execute in the background
// "varargs" (params ...) in below method is actually alternate way of saying that
// method will accept the particular type of arguments as an array
// in this case, array of String, which will contain urls
@Override
protected String doInBackground(String... urls) {
URL url; // Declaring the URL variable
HttpURLConnection httpURLConnection = null; // declaring and initializing the HttpURLConnection var
InputStream inputStream; // declaring the InputStream reader var, which will hold the incoming data stream
InputStreamReader inputStreamReader; // declaring and initializing the InputStreamReader var to read the incoming stream
int data; // this variable will actually hold the data
String result = ""; // this var will combine all the incoming data in the string and will be returned
try {
url = new URL(urls[0]); // initializing the URL var with the supplied url
httpURLConnection = (HttpURLConnection) url.openConnection(); //opening up the connection to that url
inputStream = httpURLConnection.getInputStream(); // getting the data using the above connection
inputStreamReader = new InputStreamReader(inputStream); // attaching the reader to the data
data = inputStreamReader.read(); // reading data, character by character
while (data != -1) // data will only be -1 when all the received data is read and reading cursor is at EOF
{
char current = (char) data; // this var will hold each character previously retrieved
result += current; // appending the read char to the final var
data = inputStreamReader.read(); // reading the next character
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return result; // returning the final result
}
}
// This method will download the IDs for top 500 news
public void getNewsID()
{
String rawData = "";
DownloadTask downloader = new DownloadTask();
try {
// this will download the JSON data for news IDs (only top 500 stories will be downloaded)
rawData = downloader.execute("https://hacker-news.firebaseio.com/v0/topstories.json").get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
parseNewsID(rawData);
}
// This method will extract individual news ID from the JSON Array received
public void parseNewsID(String rawData){
try {
JSONArray jsonArray = new JSONArray(rawData);
for(int i = 0; i < jsonArray.length(); ++i)
{
int newsID = jsonArray.getInt(i);
if (!isNewsExist(newsID)) { // check if news ID exist in the db
getNewsJSON(newsID);
}
}
} catch (JSONException e) {
e.printStackTrace();
}
}
// This method will get the JSON of individual news based on its ID
public void getNewsJSON(int newsID)
{
String newsJSON = "";
DownloadTask downloadTask = new DownloadTask();
try {
newsJSON = downloadTask.execute("https://hacker-news.firebaseio.com/v0/item/" + newsID + ".json").get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
parseNews(newsJSON);
}
// This method will return the individual news JSON object
public void parseNews(String newsJSON)
{
JSONObject jsonObject = null;
try {
jsonObject = new JSONObject(newsJSON);
} catch (JSONException e) {
e.printStackTrace();
}
downloadNewsInfo(jsonObject);
}
// This method will download the News ID, Title and URL as a string
public void downloadNewsInfo(JSONObject jsonObject)
{
int newsID = 0;
String newsTitle = "";
String newsURL = "";
try {
if (!jsonObject.isNull("title") && !jsonObject.isNull("url")) { // check if title and url exist in url
newsID = jsonObject.getInt("id");
newsTitle = jsonObject.getString("title");
newsURL = jsonObject.getString("url");
}
} catch (JSONException e) {
e.printStackTrace();
}
if (newsID != 0) {
insertNewsIntoDB(newsID, newsTitle, newsURL);
}
}
public void retrieveAndDisplayTile()
{
Cursor newsCursor = newsReaderDB.rawQuery("SELECT title FROM news", null);
int titleIndex = newsCursor.getColumnIndex("title");
newsCursor.moveToFirst();
if (newsCursor.moveToFirst())
{
newsTitle.add(newsCursor.getString(titleIndex));
}
while (newsCursor.moveToNext())
{
newsTitle.add(newsCursor.getString(titleIndex));
newsCursor.moveToNext();
}
updateListView();
}
// this method will retrieve the url corresponding to the newsTitle
public String retrieveURL(String newsTitle)
{
String newsURL = "";
Cursor urlCursor = newsReaderDB.rawQuery("SELECT url FROM news WHERE title = '" + newsTitle + "' ",null);
int urlIndex = urlCursor.getColumnIndex("url");
urlCursor.moveToFirst();
if (urlCursor.moveToFirst())
{
try {
newsURL = (String) ObjectSerializer.deserialize(urlCursor.getString(urlIndex));
} catch (IOException e) {
e.printStackTrace();
}
}
return newsURL;
}
// Check in the database if the news exist
public boolean isNewsExist(int newsID)
{
Cursor c = newsReaderDB.rawQuery("SELECT id FROM news", null); // start the cursor
int idIndex = c.getColumnIndex("id"); // get column index
c.moveToFirst(); // move cursor to the first entry
if (c.moveToFirst())
{
if (newsID == c.getInt(idIndex)) // check if newsID is match with the ID in db
{
return true; // return true if yes
}
}
while (c.moveToNext())
{
if (newsID == c.getInt(idIndex)) // check if newsID is match with the ID in db
{
return true; // return true if yes
}
c.moveToNext(); // if not, check remaining
}
return false; // return false if newsID doesn't exist in the db
}
// This method will update the ArrayAdapter, hence updating ListView with new data
public void updateListView()
{
arrayAdapter.notifyDataSetChanged();
}
// This method will check if internet is available or not
private boolean isInternetAvailable() {
ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
return activeNetworkInfo != null && activeNetworkInfo.isConnectedOrConnecting();
}
// This method will either create or open the db for this app
public void createOpenDB()
{
newsReaderDB = this.openOrCreateDatabase("newsReader", MODE_PRIVATE, null);
}
// This method will insert the downloaded news into the db
public void insertNewsIntoDB(int newsID, String newsTitle, String newsURL)
{
try {
String tempUrlStringObj = ObjectSerializer.serialize(newsURL); // This object serialization of the content will help save the data in the db properly and without any errors
String sqlStatment = "INSERT INTO news (id, title, url) VALUES (?, ? , ?)";
SQLiteStatement statement = newsReaderDB.compileStatement(sqlStatment);
statement.bindString(1, String.valueOf(newsID));
statement.bindString(2, newsTitle);
statement.bindString(3, tempUrlStringObj);
statement.execute();
} catch (Exception e)
{
e.printStackTrace();
}
}
// Create Table
public void createDBTbl()
{
newsReaderDB.execSQL("CREATE TABLE IF NOT EXISTS news (id INT(100000) PRIMARY KEY, title VARCHAR, url VARCHAR)");
}
// Delete the table
public void deleteDBTbl()
{
if (isInternetAvailable()) {
newsReaderDB.execSQL("DROP TABLE IF EXISTS news");
}
}
// This method will start the program
public void fireUp()
{
createOpenDB();
deleteDBTbl();
createDBTbl();
getNewsID();
retrieveAndDisplayTile();
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initializeVar(); // calling the variable initializer fnx
fireUp(); // Start the app
Toast.makeText(getApplicationContext(),"Total number of news items are: " + listView.getAdapter().getCount(),Toast.LENGTH_LONG).show();
// Setting the onClickListener for ListView
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
String title = (String)listView.getItemAtPosition(position);
String url = retrieveURL(title);
// Send URL to the webView Activity
Intent intent = new Intent(getApplicationContext(), Main2Activity.class);
intent.putExtra("URL", url);
startActivity(intent);
}
});
}
}
activity_main2.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.xpertwireless.newsreader.Main2Activity">
<WebView
android:id="@+id/webView"
android:layout_width="368dp"
android:layout_height="495dp"
android:paddingBottom="0dp"
android:paddingEnd="0dp"
android:paddingLeft="0dp"
android:paddingRight="0dp"
android:paddingStart="0dp"
android:paddingTop="0dp"
tools:layout_editor_absoluteX="8dp"
tools:layout_editor_absoluteY="8dp" />
</android.support.constraint.ConstraintLayout>
Main2Activity.java
public class Main2Activity extends AppCompatActivity {
String newsURL = "";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
WebView webView = (WebView) findViewById(R.id.webView);
webView.getSettings().setJavaScriptEnabled(true); // setting javascript enabled
webView.setWebViewClient(new WebViewClient()); // open the web page in the same application
newsURL = getIntent().getStringExtra("URL"); // getting url through intent
webView.loadUrl(newsURL); // loading url
}
}
ObjectSerializer.java
public class ObjectSerializer {
public static String serialize(Serializable obj) throws IOException {
if (obj == null) return "";
try {
ByteArrayOutputStream serialObj = new ByteArrayOutputStream();
ObjectOutputStream objStream = new ObjectOutputStream(serialObj);
objStream.writeObject(obj);
objStream.close();
return encodeBytes(serialObj.toByteArray());
} catch (Exception e) {
throw new RuntimeException(e);
}
}
public static Object deserialize(String str) throws IOException {
if (str == null || str.length() == 0)
{
return null;
}
try {
ByteArrayInputStream serialObj = new ByteArrayInputStream(decodeBytes(str));
ObjectInputStream objStream = new ObjectInputStream(serialObj);
return objStream.readObject();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
public static String encodeBytes(byte[] bytes) {
StringBuffer strBuf = new StringBuffer();
for (int i = 0; i < bytes.length; i++) {
strBuf.append((char) (((bytes[i] >> 4) & 0xF) + ((int) 'a')));
strBuf.append((char) (((bytes[i]) & 0xF) + ((int) 'a')));
}
return strBuf.toString();
}
public static byte[] decodeBytes(String str) {
byte[] bytes = new byte[str.length()/2];
for (int i = 0; i < str.length(); i += 2) {
char c = str.charAt(i);
bytes[i/2] = (byte) ((c - 'a') << 4);
c = str.charAt(i+1);
bytes[i/2] += (c - 'a');
}
return bytes;
}
}
AndroidManifest.xml中
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.xpertwireless.newsreader">
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".Main2Activity"></activity>
</application>
</manifest>
但我甚至無法看到它一秒鐘，因爲我已經在PreExec和PostExec中包含ProgressBar。它不應該顯示至少一秒左右嗎？ –