如何使用Android MVVM模式与片段？

Question

首先,我要求对我的英语不好道歉.

我开发了很多年的Java SE软件,而且我曾经使用过MVC设计模式.现在我开发了Android应用程序,我不满意那个说已经使用了MVC模式,xml文件充当视图的论点.

我在网上做了很多研究,但似乎对这个话题没有一致意见.有些人使用MVC模式,有些则使用MVP模式,但我认为,没有一致意见.

最近我买了一本书(Android Best Practices,来自Godfrey Nolan,Onur Cinar和David Truxall),在第二章中,您可以找到MVC,MVVM和依赖注入模式.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,最好的是MVVM模式.

我发现这种模式在使用活动进行编程时非常容易使用,但是我对使用片段编程时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,该模板是从"Android Best Practices"一书的网站下载的.

观点(活动)

   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);
    }
   }

该模型

 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);
    }
   }

ViewModel

 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片段时重现这种模式时,我不知道如何继续.我可以为每个片段设置一个视图模型和一个模型,还是仅针对包含这些片段的活动？

使用经典的片段方法(片段是活动内部的一个内部类),它很容易与活动交互,或者访问片段管理器进行更改,但如果我解耦代码,并把我的逻辑在活动之外的程序中,我已经看到我经常需要引用我的ViewModel中的活动(不是对活动视图的引用,而是对活动本身的引用).

或者,例如,假设带有片段的活动正在处理从意图而不是从模型(数据库或休息服务)接收的数据.然后,我觉得我不需要模特.也许我可以在活动中收到意图时创建模型,但我觉得这不正确(视图不应该与模型有关,只有视图模型...).

可以有人向我提供有关如何在使用片段时使用MVVM模式的解释吗？

提前致谢.

Answer 1

注意:以下内容已过时,我不再推荐它了.主要是因为在此设置中测试Viewsmodel很困难.看看Google Architecture Blueprints.

旧答案:

就个人而言,我更喜欢备用设置:

该模型

你的模特.不需要改变(使用MVVM的美丽:))

视图(片段)

稍微不一样.View(Fragment)在我的设置中引用了ViewModel(Activity).而不是初始化您的委托,如:

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

我建议你使用一个着名的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,View(Fragment)强制它所附加的Activity实现ITaskListManager接口.当Fragment与Activity分离时,一些默认实现被设置为委托.当你有一个没有附加到Activity的片段实例时,这可以防止出错(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(活动)

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)添加到活动中......

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

应在活动中处理添加新视图或不同视图.这很好,因为您现在可以侦听配置更改,并将特殊片段换成不同的方向...

Answer 2

我是RoboBinding的贡献者 - 一个Android平台的数据绑定表示模型(MVVM)框架.我会在这里提出我的理解.MVVM通常用于Microsoft社区,实际上源自Martin Fowler的Presentation Model.MVVM模式的简化图片是视图 - 同步机制(或数据绑定) - >视图模型 - >模型.使用MVVM的主要动机和好处是ViewModel变为纯POJO,可以进行单元测试(非Android单元测试,需要很长时间).在Android中,应用MVVM的一种可能方式是:View(Layout + Activity)---->同步机制(或数据绑定) - > ViewModel(纯POJO) - > Model(业务模型).箭头方向也表示依赖关系.您可以在View Layer中实例化您的业务模型,然后传递到ViewModel,但访问流始终是View to ViewModel,ViewModel是Business Model.在RoboBinding下有一个简单的Android MVVM示例应用程序.我建议你阅读Martin Fowler关于Presentation Model的原创文章.

要应用MVVM,需要实现一个同步机制模块,当没有第三方库时,这可能很复杂.如果您不想依赖第三方库,则可以尝试应用MVP(被动视图).但请注意,使用Test Double进行查看.两种模式的动机都试图使ViewModel或Presenter不依赖(或不直接依赖)View,以便它们可以进行正常单元测试(不是Android单元测试).