Android在远程MySQL中验证登录

Question

Ok. I have a remote mySQL set up and the table have two pair values. Email: aa, password: aa and Email: bb, password: bb.

In my main activity, in the loginOnClickListener, I declare a backgroundworker class which extends the AsyncTask class to do the remote connection and verify the login. If email and password are correct, the value "Success" would return to String result in the onPostExecute(String result). I declare a public static String result in the MainActivity. Inside the onPostExecute(String result) of the backgoundworker class

@Override   
protected void onPostExecute(String result) {
    LoginActivity.get=result;
    delegate.processFinish(result);
    alertDialog.setMessage(result);
    alertDialog.show(); // alertDiaglog can always show the correct message, no "delay"

}

In the MainActivity

public class MainActivity extends AppCompatActivity 

    public static String result;




 public void loginButtonListener(){
    button_login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String type = "login";
            String email = editText_email.getText().toString();
            String password = editText_password.getText().toString();

            BackgroundWorker worker = new BackgroundWorker(LoginActivity.this);
            worker.delegate=LoginActivity.this;
            worker.execute(type, email, password);
            Toast.makeText(LoginActivity.this,get,Toast.LENGTH_SHORT).show(); // result is null in the first time
        }
    });

}

So the value of result would return to MainActivity and I can verify if the string is "Success" and it will go to another activity once log in success. However, when I first sucessfully log in, the value of String result is null in Mainactivity, and then I entered a wrong password, and the result becomes "Success" and then becomes "Fails".

In general, the value of String result in MainActivity is always one step behind the results generated by the button click because of the first null string. Anybody has a idea how to verify login or how to solve this issue?

Answer 1

Since you are starting an Async task, which is asynchronous so naturally the value of result will be null initially. When postExecute() completes, it will assign result to this variable and then you can have its value.

Update

Declare an interface like which will have onLoginResult method

public interface LoginInterface {

  public void onLoginResult(String result);
}

Make your Activity implement this method

public class MainActivity extends AppCompatActivity implements LoginInterface {

  @Override
  public void onLoginResult(String result) {

  }



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

    MyAsyncTask task = new MyAsyncTask(this);
}

}

Assuming your AsyncTask is MyAsyncTask

class MyAsyncTask extends AsyncTask<String, Void, Boolean> {

    private LoginInterface loginInterface;

     //Change the number of arguments according to your requirements.
    AsyncTask(LoginInterface loginInterface) {
    this.loginInterface  = loginInterface;
    }

    @Override
    protected Boolean doInBackground(String... params) {

        // Some code

        return true;
    }

    @Override
    protected void onPostExecute(Boolean result) {
        super.onPostExecute(result);
        loginInterface.onLoginResult(result);
    }
}

Now once postExecute complete it will make a call to your MainActivity's onLoginResult with the actual result. Here you can do whatever stuff you want like starting an activity.

Answer 2

I striped down what you are trying to do to a Button and an inner AsyncTask class. Take note that the Toast and starting the new Activity are in the onPostExecute because the AsyncTask runs asynchronously, which means, it could return at any point in the future, not immediately after you click the button or call .execute() .

I don't have a SQL database to test against, so I just slept the running thread for a second to simulate a network operation.

It's also worth noting that by making the inner class, you can easily call startActivity and finish because you are still within scope of the current activity Context .

public class MainActivity extends Activity {

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

        Button button = (Button) findViewById(R.id.button);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String email = "test@test.com"; // blah, blah .getText()
                String password = "password"; // blah, blah .getText()

                new LoginTask().execute("type?", email, password);
            }
        });
    }

    class LoginTask extends AsyncTask<String, Void, Boolean> {

        private String email;

        @Override
        protected Boolean doInBackground(String... params) {
            String type = params[0];
            this.email = params[1]; // save the email, if you want
            String password = params[2];

            // Do your database stuff
            try {
                // Simulate some long running operation
                Thread.sleep(1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
                return false;
            }

            return true;
        }

        @Override
        protected void onPostExecute(Boolean result) {
            super.onPostExecute(result);

            Toast.makeText(getApplicationContext(), String.valueOf(result), Toast.LENGTH_SHORT).show();

            if (result) {
                Intent i = new Intent(MainActivity.this, DashboardActivity.class);
                i.putExtra("email", this.email); // send some arguments to the next activity
                startActivity(i); // You have logged-in, move to the next screen
                finish(); // Kill the login activity
            } 
        }
    }
}