在下面的代码中，displayList()方法负责使用数据库表中的内容填充ArrayList，具体取决于查看的目的，然后在ListView中显示。
但是，第一次运行新版本的数据库或第一次运行该程序时，当创建一个新的Database Manipulator类实例（在本例中为ManipulateDatabase）时，AsyncTask运行到将初步数据导入数据库，这会中断正在填充的ArrayList并显示ListView。因此，在AsyncTask完成后，ListView显示为空。
我的问题是，如何解决问题，以便在AsyncTask完成后，ListView会显示正确的内容？我非常感谢有用的回应。
相关代码如下所示。
displayList()
private void displayList() {
try {
System.out.println("DISPLAYING HERE");
md = new ManipulateDatabase(this, new ProgressDialog(this));
if (purpose.equals("ViewNovel")) {
setTitle("Novels");
listContents = md.getPieces("0");
System.out.println(purpose);
} else if (purpose.equals("TestNovel")) {
setTitle("Novels");
listContents = md.getPieces("0");
System.out.println(purpose);
} else if (purpose.equals("TestPlay")) {
setTitle("Plays");
listContents = md.getPieces("1");
System.out.println(purpose);
} else if (purpose.equals("ViewPlay")) {
setTitle("Plays");
listContents = md.getPieces("1");
System.out.println(purpose);
} else if (purpose.equals("ViewStats")) {
setTitle("Statistics");
listContents = md.getPieces("0");
listContents.addAll(md.getPieces("1"));
System.out.println(purpose);
} else if (purpose.endsWith("/Char")) {
int point = purpose.indexOf("/");
String rest = purpose.substring(point + 1, purpose.length());
point = rest.indexOf("/");
String title = rest.substring(0, point);
listContents = md.getElements(title, "1", "");
setTitle(title + " - Characters");
System.out
.println("Tried to get Char elements and put in array");
System.out.println(purpose);
}
else if (purpose.endsWith("/Theme")) {
int point = purpose.indexOf("/");
String rest = purpose.substring(point + 1, purpose.length());
point = rest.indexOf("/");
String title = rest.substring(0, point);
listContents = md.getElements(title, "0", "");
setTitle(title + " - Themes");
System.out.println(purpose);
} else if (purpose.contains("*")) {
System.out.println("Trying to get quotes here guys!");
int lastSlash = purpose.lastIndexOf("*");
String extract = purpose.substring(lastSlash + 1,
purpose.length());
System.out.println(extract);
listContents.clear();
listContents = md.getQuotes(extract);
setTitle("Quotes");
} else if (purpose.contains("/Char/")
|| purpose.contains("/Theme/")) {
int lastSlash = purpose.lastIndexOf("/");
String extract = purpose.substring(lastSlash + 1,
purpose.length());
System.out.println(purpose);
listContents = md.getPoints(extract, "");
setTitle(extract);
}
} catch (IOException e) {
System.out.println("Could not create new MD object");
e.printStackTrace();
}
setContentView(R.layout.genlist_layout);
setListAdapter(new ArrayAdapter<String>(this,
android.R.layout.simple_list_item_1, listContents));
}
AsyncTask
private class ImportLitData extends AsyncTask<String, Void, Boolean> {
/**
* ProgressDialog in which progress is shown
*/
private ProgressDialog progressDialog;
/**
* Context of class from which the TXTs will need to be imported -
* Context of the AsyncTask
*/
private Activity context;
/**
* Boolean to store whether import of data from text file was successful
* or not
*/
private Boolean success;
/**
* Constructor method
*
* @param dialogIn
* Progress Dialog which will be updated
* @param contextIn
* context of Class
*/
public ImportLitData(ProgressDialog dialogIn, Activity contextIn) {
// progressDialog = dialogIn;
context = contextIn;
setUpDialog();
}
/**
* Sets up the download progress dialog
*/
private void setUpDialog() {
progressDialog = new ProgressDialog(context);
progressDialog.setMessage("Importing initial data");
progressDialog.setIndeterminate(true);
progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
progressDialog.setCancelable(false);
// progressDialog.show();
}
protected Boolean doInBackground(String... params) {
System.out.println("do in background!");
BufferedReader pieceScan;
BufferedReader specificScan;
BufferedReader elementScan;
BufferedReader quoteScan;
try {
pieceScan = new BufferedReader(new InputStreamReader(
getPieceTextFile(context), "UTF-8"));
specificScan = new BufferedReader(new InputStreamReader(
getSpecificTextFile(context), "UTF-8"));
elementScan = new BufferedReader(new InputStreamReader(
getElementTextFile(context), "UTF-8"));
quoteScan = new BufferedReader(new InputStreamReader(
getQuoteTextFile(context), "UTF-8"));
openSesame();
String line;
String pieceName;
String isDrama;
String point;
String element;
String quote;
String isCharacter;
int commaPoint;
int commaPoint2;
System.out.println("INITIAL LAUNCH BLOCK INITIATED");
while ((line = pieceScan.readLine()) != null) {
System.out.println("PieceInsertion: ");
commaPoint = line.indexOf(",");
pieceName = line.substring(0, commaPoint);
isDrama = line.substring(commaPoint + 1, commaPoint + 2);
ContentValues pv = new ContentValues();
pv.put(pieceField, pieceName);
pv.put(typeField, (Integer.parseInt(isDrama)));
dataBase.insert(pieceTable, null, pv);
System.out.println(line
+ ": INSERTION SHOULD HAVE JUST FINISHED");
}
while ((line = specificScan.readLine()) != null) {
System.out.println("SpecificInsertion: ");
commaPoint = line.indexOf(",");
point = line.substring(0, commaPoint);
commaPoint2 = (line.substring(commaPoint + 1,
line.length() - 1).indexOf(",")) + point.length();
// pieceName = line.substring(commaPoint + 1,
// commaPoint2 +
// 1);
element = line.substring(commaPoint2 + 2, line.length());
ContentValues sv = new ContentValues();
sv.put(pointField, point);
// sv.put(pieceField, pieceName);
sv.put(elementField, element);
dataBase.insert(specificTable, null, sv);
System.out.println(line
+ ": INSERTION SHOULD HAVE JUST FINISHED");
}
while ((line = elementScan.readLine()) != null) {
commaPoint = line.indexOf(",");
element = line.substring(0, commaPoint);
commaPoint2 = (line.substring(commaPoint + 1,
line.length() - 1).indexOf(",")) + element.length();
pieceName = line.substring(commaPoint + 1, commaPoint2 + 1);
isCharacter = line.substring(commaPoint2 + 2,
commaPoint2 + 3);
ContentValues ev = new ContentValues();
ev.put(elementField, element);
ev.put(pieceField, pieceName);
ev.put(elTypeField, (Integer.parseInt(isCharacter)));
dataBase.insert(elementTable, null, ev);
System.out.println(line
+ ": INSERTION SHOULD HAVE JUST FINISHED");
}
while ((line = quoteScan.readLine()) != null) {
commaPoint = line.indexOf("^");
quote = line.substring(0, commaPoint);
System.out.println("Quote = " + quote);
point = line.substring(commaPoint + 1, line.length());
System.out.println("Point = " + point);
ContentValues qv = new ContentValues();
qv.put(quoteField, quote);
qv.put(pointField, point);
dataBase.insert(quoteTable, null, qv);
System.out.println(line
+ ": INSERTION SHOULD HAVE JUST FINISHED");
}
success = true;
} catch (UnsupportedEncodingException e) {
System.out.println("Unsupported Encoding Exception!");
e.printStackTrace();
success = false;
} catch (IOException e) {
System.out.println("IO Exception!");
e.printStackTrace();
success = false;
}
return success;
}
protected void onPostExecute(Boolean result) {
super.onPostExecute(result);
System.out.println("onPostExecute");
progressDialog.cancel();
String toastMessage = (result) ? "Imported" : "Import Failed";
Toast resultOut = Toast.makeText(context.getApplicationContext(),
toastMessage, Toast.LENGTH_LONG);
resultOut.show();
}
/**
* Method called before doInBackground is executed
*
* @override
*/
protected void onPreExecute() {
super.onPreExecute();
System.out.println("onPreExecute");
/* Dialog shown */
progressDialog.show();
}
}
修改 * 我现在已将以下方法添加到GenList类（负责ListView的类） *
public void refreshList() {
runOnUiThread(new Runnable() {
public void run() {
adapter.notifyDataSetChanged();
}
});
}
现在我在onPostExecute中调用该方法
protected void onPostExecute(Boolean result) {
super.onPostExecute(result);
System.out.println("onPostExecute");
progressDialog.cancel();
String toastMessage = (result) ? "Imported" : "Import Failed";
Toast resultOut = Toast.makeText(context.getApplicationContext(),
toastMessage, Toast.LENGTH_LONG);
resultOut.show();
genericList.refreshList();
}
* 但我遇到此错误，显示在LogCat输出*
02-06 20:10:54.723: E/AndroidRuntime(607): FATAL EXCEPTION: main
02-06 20:10:54.723: E/AndroidRuntime(607): java.lang.NullPointerException
02-06 20:10:54.723: E/AndroidRuntime(607): at com.lawson.englishlitrevision.GenList$1.run(GenList.java:312)
02-06 20:10:54.723: E/AndroidRuntime(607): at android.os.Handler.handleCallback(Handler.java:605)
02-06 20:10:54.723: E/AndroidRuntime(607): at android.os.Handler.dispatchMessage(Handler.java:92)
02-06 20:10:54.723: E/AndroidRuntime(607): at android.os.Looper.loop(Looper.java:137)
02-06 20:10:54.723: E/AndroidRuntime(607): at android.app.ActivityThread.main(ActivityThread.java:4424)
02-06 20:10:54.723: E/AndroidRuntime(607): at java.lang.reflect.Method.invokeNative(Native Method)
02-06 20:10:54.723: E/AndroidRuntime(607): at java.lang.reflect.Method.invoke(Method.java:511)
02-06 20:10:54.723: E/AndroidRuntime(607): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
02-06 20:10:54.723: E/AndroidRuntime(607): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
02-06 20:10:54.723: E/AndroidRuntime(607): at dalvik.system.NativeStart.main(Native Method)
任何建议为什么会受到高度赞赏。
答案 0 :(得分：0)
因此，您需要对列表视图进行notifyDataSetChanged调用。这样做有几种选择。您可以将listview传递给AsyncTask的构造函数，然后直接在onPostExecute中调用它，或者您可以在您的activity中创建一个“refreshList”方法（在listview上调用notifyDataSetChanged），并在onPostExecute中调用该方法。我更喜欢后者，因为它允许你做出你想做的其他改变。
请注意，如果您在活动中创建了一个方法，那么您需要更改构造函数以专门接受该活动以便能够对其进行调用，常规Activity不会知道该方法。