改造仅在执行整个代码后给出响应

Question

我正在使用Retrofit2将Multipart数据发送到我的PHP文件。

Call<ServerResponse> call = getResponse.uploadFile(files,size);
            call.enqueue(new Callback<ServerResponse>() {

                @Override
                public void onResponse(Call call1, Response response) {

                       ServerResponse serverResponse = (ServerResponse) response.body();
                        if (serverResponse != null) {
                             if(serverResponse.getMessage().equals("started")) {
                            Intent intent = new Intent(FileUpload.this, MainActivity.class);
                            startActivity(intent);
                            finishAffinity();
                        }
                        } else {
                            Toast.makeText(context,"ERROR",Toast.LENGTH_SHORT).show();
                        }
                       loadingDialog.dismissDialog();
                }
                 @Override
                public void onFailure(Call call, Throwable t) {
                    Toast.makeText(context,"ERROR",Toast.LENGTH_SHORT).show();
                    loadingDialog.dismissDialog();
                }
            }); 

import com.google.gson.annotations.SerializedName;

public class ServerResponse {
    @SerializedName("success")
    boolean success;
    @SerializedName("message")
    String message;
    public String getMessage() {
        return message;
    }
    public boolean getSuccess() {
        return success;
    }
}

当PHP提供一个包含消息“ started”的json数组时，我需要启动MainActivity类。这是我的php代码

<?php
  if(isset($_POST['size'])&&!empty($_FILES)) {
   echo json_encode(array('success'=> 0, 'message'=>'started'));
      for($x=0; $x < $_POST['size']; $x++) {
      //CODE FOR VALIDATING THE FILES
      }
}
?>

但这不会立即被调用，只有在执行了整个代码之后才被调用。我究竟做错了什么？请帮忙。问候。