android - 如何将 Android MVVM 模式与 fragment 一起使用？

Question

首先我请求为我的英语不好向你道歉。

我开发Java SE软件很多年了，我曾经使用过MVC设计模式。现在我开发 android 应用程序，我对说 android 已经使用 MVC 模式的说法不满意，xml 文件充当 View 。

我在网上做了很多研究，但似乎对这个话题并没有一致意见。有些使用 MVC 模式，有些使用 MVP 模式，但我个人认为，没有一致意见。

最近我买了一本书(Android Best Practices, from Godfrey Nolan, Onur Cinar and David Truxall)，在第二章，你可以找到MVC、MVVM和依赖注入(inject)模式的解释。在尝试了所有这些之后，我认为对于我的应用程序和我的工作模式来说，最好的是 MVVM 模式。

我发现这种模式在使用 Activity 编程时非常容易使用，但是在使用 Fragment 编程时我对如何使用它感到困惑。我将重现应用于简单“todo app”的 MVVM 模式示例，该示例是从“Android 最佳实践”一书的网站下载的。

View ( Activity )

   package com.example.mvvm;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;

public class TodoActivity extends Activity
{
    public static final String APP_TAG = "com.logicdrop.todos";

    private ListView taskView;
    private Button btNewTask;
    private EditText etNewTask;
    private TaskListManager delegate;

    /*The View handles UI setup only. All event logic and delegation
     *is handled by the ViewModel.
     */

    public static interface TaskListManager
    {
        //Through this interface the event logic is
        //passed off to the ViewModel.
        void registerTaskList(ListView list);
        void registerTaskAdder(View button, EditText input);
    }

    @Override
    protected void onStop()
    {
        super.onStop();
    }

    @Override
    protected void onStart()
    {
        super.onStart();
    }

    @Override
    public void onCreate(final Bundle bundle)
    {
        super.onCreate(bundle);

        this.setContentView(R.layout.main);

        this.delegate = new TodoViewModel(this);
        this.taskView = (ListView) this.findViewById(R.id.tasklist);
        this.btNewTask = (Button) this.findViewById(R.id.btNewTask);
        this.etNewTask = (EditText) this.findViewById(R.id.etNewTask);
        this.delegate.registerTaskList(taskView);
        this.delegate.registerTaskAdder(btNewTask, etNewTask);
    }
   }

模型

 package com.example.mvvm;

import java.util.ArrayList;
import java.util.List;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

final class TodoModel
{
    //The Model should contain no logic specific to the view - only
    //logic necessary to provide a minimal API to the ViewModel.
    private static final String DB_NAME = "tasks";
    private static final String TABLE_NAME = "tasks";
    private static final int DB_VERSION = 1;
    private static final String DB_CREATE_QUERY = "CREATE TABLE " + TodoModel.TABLE_NAME + " (id integer primary key autoincrement, title text not null);";

    private final SQLiteDatabase storage;
    private final SQLiteOpenHelper helper;

    public TodoModel(final Context ctx)
    {
        this.helper = new SQLiteOpenHelper(ctx, TodoModel.DB_NAME, null, TodoModel.DB_VERSION)
        {
            @Override
            public void onCreate(final SQLiteDatabase db)
            {
                db.execSQL(TodoModel.DB_CREATE_QUERY);
            }

            @Override
            public void onUpgrade(final SQLiteDatabase db, final int oldVersion,
                    final int newVersion)
            {
                db.execSQL("DROP TABLE IF EXISTS " + TodoModel.TABLE_NAME);
                this.onCreate(db);
            }
        };

        this.storage = this.helper.getWritableDatabase();
    }

    /*Overrides are now done in the ViewModel. The Model only needs
     *to add/delete, and the ViewModel can handle the specific needs of the View.
     */
    public void addEntry(ContentValues data)
    {
        this.storage.insert(TodoModel.TABLE_NAME, null, data);
    }

    public void deleteEntry(final String field_params)
    {
        this.storage.delete(TodoModel.TABLE_NAME, field_params, null);
    }

    public Cursor findAll()
    {
        //Model only needs to return an accessor. The ViewModel will handle
         //any logic accordingly.
        return this.storage.query(TodoModel.TABLE_NAME, new String[]
        { "title" }, null, null, null, null, null);
    }
   }

View 模型

 package com.example.mvvm;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;

public class TodoViewModel implements TodoActivity.TaskListManager
{
    /*The ViewModel acts as a delegate between the ToDoActivity (View)
     *and the ToDoProvider (Model).
     * The ViewModel receives references from the View and uses them
     * to update the UI.
     */

    private TodoModel db_model;
    private List<String> tasks;
    private Context main_activity;
    private ListView taskView;
    private EditText newTask;

    public TodoViewModel(Context app_context)
    {
        tasks = new ArrayList<String>();
        main_activity = app_context;
        db_model = new TodoModel(app_context);
    }

    //Overrides to handle View specifics and keep Model straightforward.

    private void deleteTask(View view)
    {
        db_model.deleteEntry("title='" + ((TextView)view).getText().toString() + "'");
    }

    private void addTask(View view)
    {
        final ContentValues data = new ContentValues();

        data.put("title", ((TextView)view).getText().toString());
        db_model.addEntry(data);
    }

    private void deleteAll()
    {
        db_model.deleteEntry(null);
    }

    private List<String> getTasks()
    {
        final Cursor c = db_model.findAll();
        tasks.clear();

        if (c != null)
        {
            c.moveToFirst();

            while (c.isAfterLast() == false)
            {
                tasks.add(c.getString(0));
                c.moveToNext();
            }

            c.close();
        }

        return tasks;
    }

    private void renderTodos()
    {
        //The ViewModel handles rendering and changes to the view's
        //data. The View simply provides a reference to its
        //elements.
        taskView.setAdapter(new ArrayAdapter<String>(main_activity,
                android.R.layout.simple_list_item_1,
                getTasks().toArray(new String[]
                        {})));
    }

    public void registerTaskList(ListView list)
    {
        this.taskView = list; //Keep reference for rendering later
        if (list.getAdapter() == null) //Show items at startup
        {
            renderTodos();
        }

        list.setOnItemClickListener(new AdapterView.OnItemClickListener()
        {
            @Override
            public void onItemClick(final AdapterView<?> parent, final View view, final int position, final long id)
            { //Tapping on any item in the list will delete that item from the database and re-render the list
                deleteTask(view);
                renderTodos();
            }
        });
    }

    public void registerTaskAdder(View button, EditText input)
    {
        this.newTask = input;
        button.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(final View view)
            { //Add task to database, re-render list, and clear the input
                addTask(newTask);
                renderTodos();
                newTask.setText("");
            }
        });
    }
   }

问题是，当我尝试在使用 fragment 时重现此模式时，我不知道如何继续。我可以为每个 fragment 或仅为包含这些 fragment 的 Activity 提供 View 模型和模型吗？

使用经典的fragment方式(fragment是activity内部的一个内部类)，很容易和activity交互，或者访问fragment manager做修改，但是如果我解耦代码，把我的程序在 Activity 之外的逻辑，我发现我经常需要在我的 ViewModel 中引用 Activity (不是对 Activity View 的引用，而是对 Activity 本身的引用)。

或者，例如，假设带有 fragment 的 Activity 正在处理从 Intent 接收的数据，而不是来自模型(数据库或休息服务)的数据。然后，我觉得我不需要模型。也许我可以在activity中收到intent时创建模型，但我觉得这样不正确( View 不应该和模型有关系，只有viewmodel...)。

谁能给我解释一下在使用 fragment 时如何在android中使用MVVM模式？

提前致谢。

Answer 1

注意:以下内容已过时，我不再推荐。主要是因为在此设置中很难测试 Viewsmodel。查看 Google 架构蓝图。

旧答案:

就个人而言，我更喜欢另一种设置:

模型

你的模特。不需要更改(使用 MVVM 的美妙之处:))

View ( fragment )

略有不同。 View (Fragment) 在我的设置中有对 ViewModel ( Activity ) 的引用。而不是像这样初始化您的委托(delegate):

// Old way -> I don't like it
this.delegate = new TodoViewModel(this);

我建议你使用众所周知的 Android 模式:

@Override
public void onAttach(final Activity activity) {
    super.onAttach(activity);
    try {
        delegate = (ITaskListManager) activity;
    } catch (ClassCastException ignore) {
        throw new IllegalStateException("Activity " + activity + " must implement ITaskListManager");
    }
}

@Override
public void onDetach() {
    delegate = sDummyDelegate;
    super.onDetach();
}

这样，您的 View ( fragment )强制它所附加的 Activity 实现 ITaskListManager 接口(interface)。当 Fragment 与 Activity 分离时，一些默认实现被设置为委托(delegate)。这可以防止在您拥有未附加到 Activity 的 fragment 实例时出现错误(是的，这可能会发生)。

这是我的 ViewFragment 的完整代码:

public class ViewFragment extends Fragment {

    private ListView taskView;
    private Button btNewTask;
    private EditText etNewTask;
    private ITaskListManager delegate;

    /**
     * Dummy delegate to avoid nullpointers when
     * the fragment is not attached to an activity
     */
    private final ITaskListManager sDummyDelegate = new ITaskListManager() {

        @Override
        public void registerTaskList(final ListView list) {
        }

        @Override
        public void registerTaskAdder(final View button, final EditText input) {
        }
    };

    /*
     * The View handles UI setup only. All event logic and delegation
     * is handled by the ViewModel.
     */

    public static interface ITaskListManager {

        // Through this interface the event logic is
        // passed off to the ViewModel.
        void registerTaskList(ListView list);

        void registerTaskAdder(View button, EditText input);
    }

    @Override
    public void onAttach(final Activity activity) {
        super.onAttach(activity);
        try {
            delegate = (ITaskListManager) activity;
        } catch (ClassCastException ignore) {
            throw new IllegalStateException("Activity " + activity + " must implement ITaskListManager");
        }
    }

    @Override
    public void onDetach() {
        delegate = sDummyDelegate;
        super.onDetach();
    }

    @Override
    public View onCreateView(final LayoutInflater inflater, final ViewGroup container, final Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.activity_view_model, container, false);
        taskView = (ListView) view.findViewById(R.id.tasklist);
        btNewTask = (Button) view.findViewById(R.id.btNewTask);
        etNewTask = (EditText) view.findViewById(R.id.etNewTask);
        delegate.registerTaskList(taskView);
        delegate.registerTaskAdder(btNewTask, etNewTask);
        return view;
    }
}

ViewModel( Activity )

使用 Activity 作为 ViewModel 几乎是一样的。相反，您只需要确保在此处创建模型，并将 View ( fragment )添加到 Activity 中...

public class ViewModelActivity extends ActionBarActivity implements ITaskListManager {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().add(R.id.container, new ViewFragment()).commit();
        }

        initViewModel();
    }

    @Override
    public boolean onCreateOptionsMenu(final Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.view_model, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(final MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private Model db_model;
    private List<String> tasks;
    private ListView taskView;
    private EditText newTask;

    /**
     * Initialize the ViewModel
     */    
    private void initViewModel() {
        tasks = new ArrayList<String>();
        db_model = new Model(this);
    }

    private void deleteTask(final View view) {
        db_model.deleteEntry("title='" + ((TextView) view).getText().toString() + "'");
    }

    private void addTask(final View view) {
        final ContentValues data = new ContentValues();

        data.put("title", ((TextView) view).getText().toString());
        db_model.addEntry(data);
    }

    private void deleteAll() {
        db_model.deleteEntry(null);
    }

    private List<String> getTasks() {
        final Cursor c = db_model.findAll();
        tasks.clear();

        if (c != null) {
            c.moveToFirst();

            while (c.isAfterLast() == false) {
                tasks.add(c.getString(0));
                c.moveToNext();
            }

            c.close();
        }

        return tasks;
    }

    private void renderTodos() {
        // The ViewModel handles rendering and changes to the view's
        // data. The View simply provides a reference to its
        // elements.
        taskView.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, getTasks().toArray(new String[] {})));
    }

    @Override
    public void registerTaskList(final ListView list) {
        taskView = list; // Keep reference for rendering later
        if (list.getAdapter() == null) // Show items at startup
        {
            renderTodos();
        }    

        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(final AdapterView<?> parent, final View view, final int position, final long id) { // Tapping on any
                                                                                                                   // item in the list
                                                                                                                   // will delete that
                                                                                                                   // item from the
                                                                                                                   // database and
                                                                                                                   // re-render the list
                deleteTask(view);
                renderTodos();
            }
        });
    }

    @Override
    public void registerTaskAdder(final View button, final EditText input) {
        newTask = input;
        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(final View view) { // Add task to database, re-render list, and clear the input
                addTask(newTask);
                renderTodos();
                newTask.setText("");
            }
        });
    }
}

额外

应在 Activity 中处理添加新 View 或不同 View 。这很好，因为您现在可以监听配置更改，并为不同的方向交换一个特殊的 Fragment...