我正在 android studio 上制作一个考勤应用程序，但是每当我尝试使用正确的凭据登录时，我都需要在登录按钮上单击两次才能前进到下一个活动。我尝试了 Asynctasks 因为它的后台线程，在相同的结果之后我只是恢复了
所以，一开始点击什么都没有发生，但只要等待对话框消失，如果我在一两秒钟内点击，然后它就会移动到下一个活动。
在服务器验证登录存在且正确后，单击登录会从服务器获取一些数据。当接收到数据时，新活动应该开始，因为接收到的数据将显示在下一个活动中。 （我有一个列表，检查它是否有数据然后向前移动）
登录活动代码：
import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;
import android.content.Intent;
import android.content.SharedPreferences;
import android.graphics.Color;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.math.BigInteger;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
public class MainActivity extends AppCompatActivity {
//sign up and login button
private Button btnSignUp;
private Button btnLogin;
private ProgressDialog progressDialog;
private RequestQueue Queue;
//data URL
private String extractStudentDataURL = "https://asuiot12.000webhostapp.com/checkLoginCredentials.php";
private String extractEnrolmentsDataURL = "https://asuiot12.000webhostapp.com/retrieveEnrolledCoursesData.php";
//editText
private EditText editTextEmail, editTextPassword;
private ConstraintLayout login;
//string variables
public static String inputEmail;
private String inputPassword;
//array list to store enrolled courses data
public static LinkedList<Courses> enrolledCoursesData = new LinkedList();
//static variable
public static StudentData studentData;
//shared preferences to maintain the login status of the user...
public static SharedPreferences sp;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
login = findViewById(R.id.login_activity);
login.setBackgroundColor(Color.WHITE);
btnSignUp = findViewById(R.id.button_sign_up);
btnLogin = findViewById(R.id.button_login);
// sp = getSharedPreferences("login", MODE_PRIVATE);
editTextEmail = findViewById(R.id.editText_RA_emailAddress);
editTextPassword = findViewById(R.id.editText_RA_password);
//login button listener
btnLogin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
inputEmail = editTextEmail.getText().toString();
inputPassword = editTextPassword.getText().toString();
if (inputEmail.isEmpty()) {
editTextEmail.setError("Email Required");
editTextEmail.requestFocus();
} else if (inputPassword.isEmpty()) {
editTextPassword.setError("Password Required");
editTextPassword.requestFocus();
} else {
enrolledCoursesData.clear();
ExtractData();
}
}
});
//signUp button listener
btnSignUp.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, RegistrationActivity.class);
startActivityForResult(intent, 1);
}
});
}
//retrieving data from the server and checking the login credentials
private void ExtractData() {
StringRequest stringRequest = new StringRequest(Request.Method.POST, extractStudentDataURL, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
try {
JSONArray array = new JSONArray(response);
for (int i = 0; i < array.length(); i++) {
JSONObject data = array.getJSONObject(i);
int success = data.getInt("success");
if (success == 1) {
Long CNIC = data.getLong("CNIC");
String name = data.getString("Full_Name");
String fss = data.getString("FSS");
String regNo = data.getString("Reg_No");
String batch = data.getString("Batch");
String department = data.getString("Department");
int semester = data.getInt("Semester");
Long mobileNo = data.getLong("Mobile_No");
String nationality = data.getString("Nationality");
String MACAddress = data.getString("MAC_Address");
String homeAddress = data.getString("Home_Address");
String email = data.getString("Email");
String loginPassword = data.getString("Login_Password");
String photo = data.getString("Photo");
studentData = new StudentData(new BigInteger(String.valueOf(CNIC)), new BigInteger(String.valueOf(mobileNo))
, semester, name, fss, regNo, batch, department, nationality,
MACAddress, homeAddress, email, loginPassword, photo);
Toast.makeText(MainActivity.this, "Plz wait....", Toast.LENGTH_LONG).show();
// to get data of courses in which student is enrolled
extractDataOfEnrolments(email);
if (!enrolledCoursesData.isEmpty()) {
Intent intent = new Intent(MainActivity.this, CoursesActivity.class);
Bundle bundle = new Bundle();
bundle.putSerializable("list", enrolledCoursesData);
intent.putExtras(bundle);
startActivity(intent);
}
} else if (success == 0) {
Toast.makeText(MainActivity.this, "Incorrect username or password",
Toast.LENGTH_SHORT).show();
}
}
} catch (Exception e) {
}
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(MainActivity.this, "Error " + error, Toast.LENGTH_SHORT).show();
}
}) {
public Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
params.put("email", editTextEmail.getText().toString().trim());
params.put("password", editTextPassword.getText().toString().trim());
return params;
}
};
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);
}
//retrieving data of enrolments from server
private void extractDataOfEnrolments(final String email) {
StringRequest stringRequest = new StringRequest(Request.Method.POST, extractEnrolmentsDataURL, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
try {
JSONArray array = new JSONArray(response);
for (int i = 0; i < array.length(); i++) {
JSONObject data = array.getJSONObject(i);
String courseCode = data.getString("Course_Code");
String courseName = data.getString("Course_Name");
Courses courses = new Courses(courseCode, courseName);
enrolledCoursesData.add(courses);
}
} catch (Exception e) {
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Toast.makeText(MainActivity.this, "Error " + error, Toast.LENGTH_SHORT).show();
}
}) {
public Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
params.put("email", email);
return params;
}
};
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);
}
}
如果需要布局代码或服务器端 PHP 代码来更好地理解问题，请务必提问。
答案 0 :(得分：0)
您对线程的工作方式有误解。
在您的 ExtractData 方法及其 onResponse 处理程序中，您调用 extractDataOfEnrolments()，它本身是异步的：
// to get data of courses in which student is enrolled
extractDataOfEnrolments(email);
// the list hasn't been filled yet, will be empty!!!
if (!enrolledCoursesData.isEmpty()) {
}
然后您立即检查 enrolledCoursesData 列表，如果不为空则启动 CoursesActivity。 extractDataOfEnrolments() 有一个尚未完成的异步回调 (onResponse())，因此列表将为空。
您可以考虑将 startActivity 代码移至 extractDataOfEnrolments()，例如：
@Override
public void onResponse(String response) {
try {
JSONArray array = new JSONArray(response);
for (int i = 0; i < array.length(); i++) {
JSONObject data = array.getJSONObject(i);
String courseCode = data.getString("Course_Code");
String courseName = data.getString("Course_Name");
Courses courses = new Courses(courseCode, courseName);
enrolledCoursesData.add(courses);
}
// now your list is populated, so now Ok to startActivity
if (!enrolledCoursesData.isEmpty()) {
Intent intent = new Intent(MainActivity.this, CoursesActivity.class);
Bundle bundle = new Bundle();
bundle.putSerializable("list", enrolledCoursesData);
intent.putExtras(bundle);
startActivity(intent);
}
} catch (Exception e) {
}