MVP（Android）中的演示者对象获取空对象引用错误

Question

我正在使用 MVP。我的活动包含一个列表视图。我在我的 Activity 的 on Create 方法中初始化演示者，然后使用该演示者对象调用接口方法以获取数据并设置列表视图适配器。

我的问题是，当在任何 onClick 侦听器中使用 Presenter 对象时工作正常，但是当我直接使用它时，它会在 Presenter 对象上给出 NullPointerException。

这是我的代码 sn-p ：

这是我的活动：-

public class EmployeeListActivity extends AppCompatActivity implements EmployeeContract.View {

private static final String TAG = EmployeeListActivity.class.getSimpleName();
private ListView listView;
private EmployeeContract.GetPresenter presenter;

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

    presenter = new EmployeePresenter(this);    //--> Presenter inialization
}

@Override
public void initView() {
    try {
        listView = findViewById(R.id.listview);
        presenter.getEmployees(listView);     //--> interface method calling (Here I got exception.)

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                presenter.listViewClick(position);    //--> Same thing working perfectly here
            }
        });

    } catch (Exception e) {
        e.printStackTrace();
        String methodName = Thread.currentThread().getStackTrace()[2].getMethodName();
        ErrorLog.saveLog(getBaseContext(), TAG, methodName, e.getMessage(), String.valueOf(e.getCause()));
    }
}

@Override
protected void onResume() {
    super.onResume();
    presenter = new EmployeePresenter(this);
}

}

我们将提供帮助。提前谢谢你。

更新 这是我的错误日志 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.myfirstexamplemvp.contract.EmployeeContract$GetPresenter.getEmployees(android.widget.ListView)' on a null object reference 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.example.myfirstexamplemvp.view.EmployeeListActivity.initView(EmployeeListActivity.java:35) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.example.myfirstexamplemvp.presenter.EmployeePresenter.initPresenter(EmployeePresenter.java:40) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.example.myfirstexamplemvp.presenter.EmployeePresenter.<init>(EmployeePresenter.java:34) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.example.myfirstexamplemvp.view.EmployeeListActivity.onCreate(EmployeeListActivity.java:28) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.Activity.performCreate(Activity.java:6582) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1113) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2532) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2667) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.ActivityThread.-wrap11(ActivityThread.java) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1494) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.os.Handler.dispatchMessage(Handler.java:111) 01-29 16:11:54.709 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.os.Looper.loop(Looper.java:207) 01-29 16:11:54.710 28540-28540/com.example.myfirstexamplemvp W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5776) 01-29 16:11:54.710 28540-28540/com.example.myfirstexamplemvp W/System.err: at java.lang.reflect.Method.invoke(Native Method) 01-29 16:11:54.710 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 01-29 16:11:54.710 28540-28540/com.example.myfirstexamplemvp W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

这是我的演示者：-

public class EmployeePresenter implements EmployeeContract.AddPresenter, EmployeeContract.GetPresenter, EmployeeContract.EditPresenter {

private EmployeeContract.View view;
AddEmployeeModel model;
SqliteController controller;

public EmployeePresenter(EmployeeContract.View view) {
    this.view = view;
    initPresenter();
}

private void initPresenter() {
    model = new AddEmployeeModel();
    controller = new SqliteController((Context) view);
    view.initView();
}

@Override
public void getEmployees(final ListView listView) {

    List<AddEmployeeModel> list = controller.getEmployeeList();

    ArrayList<String> empname = new ArrayList<String>();

    if (list.size() > 0) {
        for (int i = 0; i < list.size(); i++) {
            empname.add(list.get(i).getFirstName() + " " + list.get(i).getLastName());
        }
        listView.setAdapter(new ArrayAdapter<String>((Context) view, android.R.layout.simple_expandable_list_item_1, empname));
    }
}

}

Answer 1

试试这个：

public class EmployeeListActivity extends AppCompatActivity implements EmployeeContract.View {

private static final String TAG = EmployeeListActivity.class.getSimpleName();
private ListView listView;
private EmployeeContract.GetPresenter presenter;

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

    presenter = new EmployeePresenter(this);
}

@Override
public void initView(List<Employee> employees, Presenter.Callback callback) {
    listView = findViewById(R.id.listview);
    EmployeesAdapter adapter = new EmployeesAdapter(this, employees);
    listView.setAdapter(adapter);

    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            callback.onClick(position);
        }
    })
}

@Override
protected void onResume() {
    super.onResume();
    //presenter = new EmployeePresenter(this);
}

进入演示者

public EmployeePresenter(EmployeeView view) {

    List<Employee> employees = ...; // get the list of employees
    Callback callback = new Callback(){
        void onClick(int position){
            listViewClick(position);
        }
    }
    view.initView(employees, callback);
}


public interface Callback{
    void onClick(int position);
}

Answer 2

我认为您的代码违反了 MVP。您的 Presenter 不应包含任何 android 代码。这就是 MVP 的强大功能，便于测试。

1- 在 init() 方法中初始化您的 listView，包括设置适配器。

public void initView() {
listView = findViewById(R.id.listview);
adapter = new EmployeesAdapter(this, employeesList);
listView.setAdapter(adapter);

listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        presenter.listItemWasClicked(position).
    }
})
}

2- 创建一个名为EmployeeData的接口。

public interface EmployeeData {
void add(Employee employee);
Employee getEmployee(int position);
int getSize();
}

3- 在您的适配器内部实现 EmployeeData。

{

//.....adapter methods

@Overrride
public Employee getEmployee(int position){
return myEmployeeList.get(position);
   }
     //.. other methods.
}

4- 在您的演示者中

   private EmployeeData empData;
    public  EmployeePresenter  (View view,EmployeeData empData){
    this.empData=empData;
    //... other init
    }
 @Override
 public void listItemWasClicked(int position){
 Employee emp=empData.getEmployee(position)
  }

5- 在onCreate() 中的活动中，例如，初始化您的 Presenter。

employeePresenter=new EmployeePresenter(this,listAdapter);

注意：在创建演示者之前不要忘记致电initView()