我如何在 Android 中使用 onResponse 之外的变量？

Question

I have created an activity in which i insert some records into a mysql database. I declared a global variable named lastInsertId . When i try to println the variable inside the onResponse method, works fine but when i try to println outside the method returns null . I need to use this variable also outside the method. What can be done? Here is my code:

String insertUrl = "http://localhost/file.php";
String lastInsertId;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());

    StringRequest request = new StringRequest(Request.Method.POST, insertUrl, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {
            lastInsertId = response.toString();
            System.out.println(lastInsertId); // returns the lastInsertId
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    }) {

        @Override
        protected Map<String, String> getParams() throws AuthFailureError {
            Map<String, String> parameters = new HashMap<String, String>();

            // parameters

            return parameters;
        }
    };
    requestQueue.add(request);
    System.out.println(lastInsertId); // get's null
}

Thanks!

Answer 1

They are different results because although you are firing your request to your local HTTP server, your last println fires BEFORE you set the lastInsertId.

You need to think multi-threaded. Your HTTP request is running in the background and the UI thread is continuing. So the order of execution is not in the order the code appears in your sample.

1. Request queue created
2. Request created
3. Request added to queue (presumably starting HTTP call also)
4. Prints out lastInsertId (null)
5. lastInsertId is set from your response
6. Prints out lastInsertId

Answer 2

在 onResponse 中使用您的变量的 setter 方法

Answer 3

This is happening because the lastInsertId is getting declared, but never initialized, then the StringRequest request is using a callback through anonym interfaces to print the value, that is happening asynchronous way, but your code is going forward and tryinh to print the value before that happens.

you don net extra setters o getters, you need to validate / that the string is not empty, OR only print its value inside that callack.

Answer 4

I figure it out. I answered this question after about a year, beacuse i saw that this post had a few hundred visitor. Hope my answer will help other feature visitors to get data out from onResponse method. Here is the code:

String insertUrl = "http://localhost/file.php";
String lastInsertId;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    RequestQueue requestQueue = Volley.newRequestQueue(getApplicationContext());

    StringRequest request = new StringRequest(Request.Method.POST, insertUrl, new Response.Listener<String>() {
        @Override
        public void onResponse(String response) {
            lastInsertId = response.toString();
            System.out.println(lastInsertId); // returns the lastInsertId
            callback.onSuccess(lastInsertId);
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    }) {

        @Override
        protected Map<String, String> getParams() throws AuthFailureError {
            Map<String, String> parameters = new HashMap<String, String>();

            // parameters

            return parameters;
        }
    };
    requestQueue.add(request);
}

public interface VolleyCallback{
    void onSuccess(ArrayList<Data> dataArrayList);
}

And this is the code we need inside the Activity.

public void onResume(){
    super.onResume();
    getString(new VolleyCallback(){
        @Override
        public void onSuccess(String result){
            System.out.println(result); // returns the value of lastInsertId
        }
    });
}