下面是我的LoginActivity的代码，其中我基本上从URl获取JSON和图像 在AsyncTask中。奇怪的是，我的onClick（）的else部分总是执行（即toast shows）。只有再次单击该按钮才会跳转到下一个活动。
public class LoginActivity extends Activity
{
Button getBunks;
CheckBox detailscheck;
EditText regText, bdayText;
String storedReg, storedBDay;
public static final String PREFS_NAME = "MyPrefsFile";
String predataURL;
String prephotoURL;
public static URL dataUrl;
public static String dataUri = "http://ankit.im/websisAPI/parser.php";
ProgressDialog progressDialog;
JSONObject temp = null;
Bitmap tmpPhoto;
boolean doneFlag = false;
@Override
protected void onCreate(Bundle savedInstanceState)
{
this.requestWindowFeature(Window.FEATURE_NO_TITLE);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getBunks = (Button) findViewById(R.id.getBunkButton);
detailscheck = (CheckBox) findViewById(R.id.rememberOpts);
regText = (EditText) findViewById(R.id.regNo);
bdayText = (EditText) findViewById(R.id.bday);
detailscheck.setChecked(true); // To avoid Nag
try
{
dataUrl = new URL("http://ankit.im/websisAPI/parser.php");
} catch (MalformedURLException e)
{
e.printStackTrace();
}
// Check if login details are already stored
SharedPreferences prefs = this.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
storedReg = prefs.getString("regno", null);
storedBDay = prefs.getString("bday", null);
// The Following Listeners Check if the user has entered
// appropriate Data in the Fields
// Check is on enter key press and on Focus Change
SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
final SharedPreferences.Editor editor = settings.edit();
regText.setOnFocusChangeListener(new OnFocusChangeListener()
{
@Override
public void onFocusChange(View v, boolean hasFocus)
{
if (detailscheck.isChecked())
{
if (!hasFocus)
{
if (regText.getText().toString() != "" || regText.getText() != null)
{
editor.putString("regno", regText.getText().toString());
editor.commit();
// Toast.makeText(getApplicationContext(),
// "Reg No Stored", Toast.LENGTH_SHORT).show();
}
}
}
}
});
bdayText.setOnEditorActionListener(new OnEditorActionListener()
{
@Override
public boolean onEditorAction(TextView v, int actionId, KeyEvent event)
{
if (actionId == EditorInfo.IME_ACTION_SEARCH || actionId == EditorInfo.IME_ACTION_DONE || event.getAction() == KeyEvent.ACTION_DOWN
&& event.getKeyCode() == KeyEvent.KEYCODE_ENTER)
{
editor.putString("bday", bdayText.getText().toString());
editor.commit();
// Hide Virtual Keyboard
InputMethodManager imm = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
imm.hideSoftInputFromWindow(regText.getWindowToken(), 0);
return true;
}
return false;
}
});
bdayText.setOnFocusChangeListener(new OnFocusChangeListener()
{
@Override
public void onFocusChange(View v, boolean hasFocus)
{
if (detailscheck.isChecked())
{
if (!hasFocus)
{
if (bdayText.getText().toString() != "" || bdayText.getText().toString() != null)
{
editor.putString("bday", bdayText.getText().toString());
editor.commit();
// Toast.makeText(getApplicationContext(),
// "BDay Stored", Toast.LENGTH_SHORT).show();
}
}
}
}
});
// Handle the button click
getBunks.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View v)
{
new DownloadStudentData().execute();
if (doneFlag == true)
{
// GlobalVars.showAttendanceData();
Intent actChange = new Intent(v.getContext(), DetailsTabViewActivity.class);
startActivity(actChange);
finish();
} else if (doneFlag == false)
{
Toast.makeText(getApplicationContext(), "Error in Downloading, Please Check Details or Try Again", Toast.LENGTH_LONG).show();
}
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
protected void onStart()
{
super.onStart();
if (storedReg != null && storedBDay != null)
{
regText.setText(storedReg);
bdayText.setText(storedBDay);
}
}
private class DownloadStudentData extends AsyncTask<Void, Void, Void>
{
// ProgressDialog progressDialog;
final JSONParser jsp = new JSONParser();
@Override
protected void onPreExecute()
{
// super.onPreExecute();
progressDialog = new ProgressDialog(LoginActivity.this);
progressDialog.setCancelable(true);
progressDialog.setTitle(" Downloading Data ");
progressDialog.setMessage("Please Wait, Loading...");
progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
// progressDialog.setProgress(0);
progressDialog.show();
}
@Override
protected Void doInBackground(Void... voids)
{
temp = new JSONObject();
temp = jsp.getJSONFromUrl(dataUri); // get the data
prephotoURL ="http://218.248.47.9/websis/control/img/P12703.JPGimgId=P12703";
try
{
InputStream in = new java.net.URL(prephotoURL).openStream();
tmpPhoto = BitmapFactory.decodeStream(in);
} catch (Exception e)
{
Log.e("Error", e.getMessage());
e.printStackTrace();
}
return null;
}
protected void onPostExecute(Void v)
{
GlobalVars.setJSON(temp);
GlobalVars.setPhotoURL(prephotoURL);
GlobalVars.setBitmap(tmpPhoto);
doneFlag = true;
try
{
GlobalVars.setPhotoURL(temp.getString("photolink"));
} catch (JSONException e)
{
e.printStackTrace();
}
progressDialog.dismiss();
}
}
@Override
protected void onPause()
{
super.onPause();
if (progressDialog != null)
progressDialog.dismiss();
}
}
GlobalVars只是一个包含公共静态变量和方法的全局类。
此外，我不确定这是否是在后台从URL下载图像的正确方法，任何更好的选择都会受到赞赏（URLImageViewHelper不适合我的需要）。
答案 0 :(得分：4)
AsyncTask在不同的线程上运行，无法及时完成更改doneFlag的值。 （这就是异步的原因。）
new DownloadStudentData().execute(); // Takes time to complete
if (doneFlag == true) { ... } // Happens milliseconds later, AsyncTask still running
else if (doneFlag == false) { ... }
您需要在onPostExecute()中运行任何“完成”代码。
一些快速提示：
if (doneFlag)是if (doneFlag == true)。else if()，只需使用else {...}