概述
透過todo-mvp來說明MVP中的Model
todo-mvp 是 Android 官方用來說明 MVP Pattern的範例，參考 https://github.com/googlesamples/android-architecture
todo-mvp 裡的 Model 為TaskRepository，TaskRepository繼承TasksDataSource。
TaskDataSource實際上是一個interface，其中2個內部介面LoadTasksCallback和GetTasksCallback用來作callback使用，在內部介面的onTasksLoaded方法用來當取得task成功之後把task傳回呼叫點的用途，而onDataNotAvailable方法用來當取得task失敗後的後續處理。
其餘在TasksDataSource介面的方法都是存取資料的共用方法，只要是Model都要實作這些方法。
TasksDataSource.java
public interface TasksDataSource {
interface LoadTasksCallback {
void onTasksLoaded(List<Task> tasks);
void onDataNotAvailable();
}
interface GetTaskCallback {
void onTaskLoaded(Task task);
void onDataNotAvailable();
}
void getTasks(@NonNull LoadTasksCallback callback);
void getTask(@NonNull String taskId, @NonNull GetTaskCallback callback);
void saveTask(@NonNull Task task);
void completeTask(@NonNull Task task);
void completeTask(@NonNull String taskId);
void activateTask(@NonNull Task task);
void activateTask(@NonNull String taskId);
void clearCompletedTasks();
void refreshTasks();
void deleteAllTasks();
void deleteTask(@NonNull String taskId);
}
接著看Presenter如何關聯 Model以及使用Model。把焦點放在AddEditTaskPresenter。
1.AddEditTaskPresenter(Presenter)本身不會持有任何資料，資料放在Model中。
2.Presenter會通知Model去改變資料。
3.Presenter會持有Model和View的變數並在建構式初始化他們。
4.Presenter會在建構式初始化Model，接著在需要改變資料的位置去操縱Model改變資料，Model改變資料後Presenter再通知View重新載入資料。
AddEditTaskPresenter.java
public class AddEditTaskPresenter implements AddEditTaskContract.Presenter,
TasksDataSource.GetTaskCallback {
@NonNull
private final TasksDataSource mTasksRepository;
@NonNull
private final AddEditTaskContract.View mAddTaskView;
@Nullable
private String mTaskId;
private boolean mIsDataMissing;
/**
* Creates a presenter for the add/edit view.
*
* @param taskId ID of the task to edit or null for a new task
* @param tasksRepository a repository of data for tasks
* @param addTaskView the add/edit view
* @param shouldLoadDataFromRepo whether data needs to be loaded or not (for config changes)
*/
public AddEditTaskPresenter(@Nullable String taskId, @NonNull TasksDataSource tasksRepository,
@NonNull AddEditTaskContract.View addTaskView, boolean shouldLoadDataFromRepo) {
mTaskId = taskId;
mTasksRepository = checkNotNull(tasksRepository);
mAddTaskView = checkNotNull(addTaskView);
mIsDataMissing = shouldLoadDataFromRepo;
mAddTaskView.setPresenter(this);
}
@Override
public void start() {
if (!isNewTask() && mIsDataMissing) {
populateTask();
}
}
@Override
public void saveTask(String title, String description) {
if (isNewTask()) {
createTask(title, description);
} else {
updateTask(title, description);
}
}
@Override
public void populateTask() {
if (isNewTask()) {
throw new RuntimeException("populateTask() was called but task is new.");
}
mTasksRepository.getTask(mTaskId, this);
}
@Override
public void onTaskLoaded(Task task) {
// The view may not be able to handle UI updates anymore
if (mAddTaskView.isActive()) {
mAddTaskView.setTitle(task.getTitle());
mAddTaskView.setDescription(task.getDescription());
}
mIsDataMissing = false;
}
@Override
public void onDataNotAvailable() {
// The view may not be able to handle UI updates anymore
if (mAddTaskView.isActive()) {
mAddTaskView.showEmptyTaskError();
}
}
@Override
public boolean isDataMissing() {
return mIsDataMissing;
}
private boolean isNewTask() {
return mTaskId == null;
}
private void createTask(String title, String description) {
Task newTask = new Task(title, description);
if (newTask.isEmpty()) {
mAddTaskView.showEmptyTaskError();
} else {
mTasksRepository.saveTask(newTask);
mAddTaskView.showTasksList();
}
}
private void updateTask(String title, String description) {
if (isNewTask()) {
throw new RuntimeException("updateTask() was called but task is new.");
}
mTasksRepository.saveTask(new Task(title, description, mTaskId));
mAddTaskView.showTasksList(); // After an edit, go back to the list.
}
}
注意雖然Model的變數型態為TasksDataSource(interface)，但在Presenter建構式傳入的其實是TaskRepository(繼承自TasksDataSource)。
AddEditTaskActivity.java
...
mPresenter = new AddEditTaskPresenter(
taskId,
Injection.provideTasksRepository(getApplicationContext()),
addEditTaskView);
...
Injection.java
public class Injection {
public static TasksRepository provideTasksRepository(@NonNull Context context) {
checkNotNull(context);
return TasksRepository.getInstance(FakeTasksRemoteDataSource.getInstance(),
TasksLocalDataSource.getInstance(context));
}
}
因此我們需要看的是TasksRepository的內容。
TasksRepository.java
public class TasksRepository implements TasksDataSource {
private static TasksRepository INSTANCE = null;
private final TasksDataSource mTasksRemoteDataSource;
private final TasksDataSource mTasksLocalDataSource;
/**
* This variable has package local visibility so it can be accessed from tests.
*/
Map<String, Task> mCachedTasks;
/**
* Marks the cache as invalid, to force an update the next time data is requested. This variable
* has package local visibility so it can be accessed from tests.
*/
boolean mCacheIsDirty = false;
// Prevent direct instantiation.
private TasksRepository(@NonNull TasksDataSource tasksRemoteDataSource,
@NonNull TasksDataSource tasksLocalDataSource) {
mTasksRemoteDataSource = checkNotNull(tasksRemoteDataSource);
mTasksLocalDataSource = checkNotNull(tasksLocalDataSource);
}
/**
* Returns the single instance of this class, creating it if necessary.
*
* @param tasksRemoteDataSource the backend data source
* @param tasksLocalDataSource the device storage data source
* @return the {@link TasksRepository} instance
*/
public static TasksRepository getInstance(TasksDataSource tasksRemoteDataSource,
TasksDataSource tasksLocalDataSource) {
if (INSTANCE == null) {
INSTANCE = new TasksRepository(tasksRemoteDataSource, tasksLocalDataSource);
}
return INSTANCE;
}
/**
* Used to force {@link #getInstance(TasksDataSource, TasksDataSource)} to create a new instance
* next time it's called.
*/
public static void destroyInstance() {
INSTANCE = null;
}
/**
* Gets tasks from cache, local data source (SQLite) or remote data source, whichever is
* available first.
* <p>
* Note: {@link LoadTasksCallback#onDataNotAvailable()} is fired if all data sources fail to
* get the data.
*/
@Override
public void getTasks(@NonNull final LoadTasksCallback callback) {
checkNotNull(callback);
// Respond immediately with cache if available and not dirty
if (mCachedTasks != null && !mCacheIsDirty) {
callback.onTasksLoaded(new ArrayList<>(mCachedTasks.values()));
return;
}
if (mCacheIsDirty) {
// If the cache is dirty we need to fetch new data from the network.
getTasksFromRemoteDataSource(callback);
} else {
// Query the local storage if available. If not, query the network.
mTasksLocalDataSource.getTasks(new LoadTasksCallback() {
@Override
public void onTasksLoaded(List<Task> tasks) {
refreshCache(tasks);
callback.onTasksLoaded(new ArrayList<>(mCachedTasks.values()));
}
@Override
public void onDataNotAvailable() {
getTasksFromRemoteDataSource(callback);
}
});
}
}
@Override
public void saveTask(@NonNull Task task) {
checkNotNull(task);
mTasksRemoteDataSource.saveTask(task);
mTasksLocalDataSource.saveTask(task);
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
}
@Override
public void completeTask(@NonNull Task task) {
checkNotNull(task);
mTasksRemoteDataSource.completeTask(task);
mTasksLocalDataSource.completeTask(task);
Task completedTask = new Task(task.getTitle(), task.getDescription(), task.getId(), true);
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), completedTask);
}
@Override
public void completeTask(@NonNull String taskId) {
checkNotNull(taskId);
completeTask(getTaskWithId(taskId));
}
@Override
public void activateTask(@NonNull Task task) {
checkNotNull(task);
mTasksRemoteDataSource.activateTask(task);
mTasksLocalDataSource.activateTask(task);
Task activeTask = new Task(task.getTitle(), task.getDescription(), task.getId());
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), activeTask);
}
@Override
public void activateTask(@NonNull String taskId) {
checkNotNull(taskId);
activateTask(getTaskWithId(taskId));
}
@Override
public void clearCompletedTasks() {
mTasksRemoteDataSource.clearCompletedTasks();
mTasksLocalDataSource.clearCompletedTasks();
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
Iterator<Map.Entry<String, Task>> it = mCachedTasks.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, Task> entry = it.next();
if (entry.getValue().isCompleted()) {
it.remove();
}
}
}
/**
* Gets tasks from local data source (sqlite) unless the table is new or empty. In that case it
* uses the network data source. This is done to simplify the sample.
* <p>
* Note: {@link GetTaskCallback#onDataNotAvailable()} is fired if both data sources fail to
* get the data.
*/
@Override
public void getTask(@NonNull final String taskId, @NonNull final GetTaskCallback callback) {
checkNotNull(taskId);
checkNotNull(callback);
Task cachedTask = getTaskWithId(taskId);
// Respond immediately with cache if available
if (cachedTask != null) {
callback.onTaskLoaded(cachedTask);
return;
}
// Load from server/persisted if needed.
// Is the task in the local data source? If not, query the network.
mTasksLocalDataSource.getTask(taskId, new GetTaskCallback() {
@Override
public void onTaskLoaded(Task task) {
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
callback.onTaskLoaded(task);
}
@Override
public void onDataNotAvailable() {
mTasksRemoteDataSource.getTask(taskId, new GetTaskCallback() {
@Override
public void onTaskLoaded(Task task) {
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
callback.onTaskLoaded(task);
}
@Override
public void onDataNotAvailable() {
callback.onDataNotAvailable();
}
});
}
});
}
@Override
public void refreshTasks() {
mCacheIsDirty = true;
}
@Override
public void deleteAllTasks() {
mTasksRemoteDataSource.deleteAllTasks();
mTasksLocalDataSource.deleteAllTasks();
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.clear();
}
@Override
public void deleteTask(@NonNull String taskId) {
mTasksRemoteDataSource.deleteTask(checkNotNull(taskId));
mTasksLocalDataSource.deleteTask(checkNotNull(taskId));
mCachedTasks.remove(taskId);
}
private void getTasksFromRemoteDataSource(@NonNull final LoadTasksCallback callback) {
mTasksRemoteDataSource.getTasks(new LoadTasksCallback() {
@Override
public void onTasksLoaded(List<Task> tasks) {
refreshCache(tasks);
refreshLocalDataSource(tasks);
callback.onTasksLoaded(new ArrayList<>(mCachedTasks.values()));
}
@Override
public void onDataNotAvailable() {
callback.onDataNotAvailable();
}
});
}
private void refreshCache(List<Task> tasks) {
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.clear();
for (Task task : tasks) {
mCachedTasks.put(task.getId(), task);
}
mCacheIsDirty = false;
}
private void refreshLocalDataSource(List<Task> tasks) {
mTasksLocalDataSource.deleteAllTasks();
for (Task task : tasks) {
mTasksLocalDataSource.saveTask(task);
}
}
@Nullable
private Task getTaskWithId(@NonNull String id) {
checkNotNull(id);
if (mCachedTasks == null || mCachedTasks.isEmpty()) {
return null;
} else {
return mCachedTasks.get(id);
}
}
}
TasksRepository實現3層緩存，首先第1層緩存為記憶體也就是Map<String, Task> mCachedTasks;
第2層緩存為本地端資料來源，也就是private final TasksDataSource mTasksLocalDataSource;
因為該變數的型態也是TasksDataSource為interface，因此不會被資料來源的實現綁住，也就是說若想更換不同的資料庫，也只要新增TasksDataSource的子類別繼承TasksDataSource即可(OCP)。
第3層緩存為遠端資料來源，為 private final TasksDataSource mTasksRemoteDataSource;
變數型態也是TasksDataSource，也可以簡單替換遠端來源的實現(OCP)，如volley, okhttp, retrofit等等。
若以儲存資料來說，在順序性來說沒有分別，這3層都會儲存資料，如下面的TasksRepository.saveTask方法的實作內容
@Override
public void saveTask(@NonNull Task task) {
checkNotNull(task);
mTasksRemoteDataSource.saveTask(task);
mTasksLocalDataSource.saveTask(task);
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
}
若是讀取資料，則會先從第1層緩存記憶體(mCachedTasks)去讀取資料，若資料存在就直接回傳，若資料不存在，則從第2層緩存本地端資料庫(mTasksLocalDataSource)去讀取資料，若有資料則把該資料加到記憶體(mCachedTasks)後再回傳資料。
若還是沒有資料則從第3層緩存遠端網路(mTasksRemoteDataSource)去讀取資料，若有資料則把該資料加到記憶體(mCachedTasks)後再回傳資料，若資料不存在則顯示該資料不存在訊息。
如下方的TasksRepository.getTask方法內容
@Override
public void getTask(@NonNull final String taskId, @NonNull final GetTaskCallback callback) {
checkNotNull(taskId);
checkNotNull(callback);
Task cachedTask = getTaskWithId(taskId);
// Respond immediately with cache if available
if (cachedTask != null) {
callback.onTaskLoaded(cachedTask);
return;
}
// Load from server/persisted if needed.
// Is the task in the local data source? If not, query the network.
mTasksLocalDataSource.getTask(taskId, new GetTaskCallback() {
@Override
public void onTaskLoaded(Task task) {
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
callback.onTaskLoaded(task);
}
@Override
public void onDataNotAvailable() {
mTasksRemoteDataSource.getTask(taskId, new GetTaskCallback() {
@Override
public void onTaskLoaded(Task task) {
// Do in memory cache update to keep the app UI up to date
if (mCachedTasks == null) {
mCachedTasks = new LinkedHashMap<>();
}
mCachedTasks.put(task.getId(), task);
callback.onTaskLoaded(task);
}
@Override
public void onDataNotAvailable() {
callback.onDataNotAvailable();
}
});
}
});
}
接著來看看在TasksRepository建構式，存取權限為私有，代表只能透過該纇別內部呼叫。
// Prevent direct instantiation.
private TasksRepository(@NonNull TasksDataSource tasksRemoteDataSource,
@NonNull TasksDataSource tasksLocalDataSource) {
mTasksRemoteDataSource = checkNotNull(tasksRemoteDataSource);
mTasksLocalDataSource = checkNotNull(tasksLocalDataSource);
}
呼叫該建構式的位置為getInstance()方法，會透過其方法的參數設定tasksRemoteDataSource和tasksLocalDataSource。
public static TasksRepository getInstance(TasksDataSource tasksRemoteDataSource,
TasksDataSource tasksLocalDataSource) {
if (INSTANCE == null) {
INSTANCE = new TasksRepository(tasksRemoteDataSource, tasksLocalDataSource);
}
return INSTANCE;
}
而getInstance方法的呼叫者為Injection類別的ProvideTasksRepository方法。
public class Injection {
public static TasksRepository provideTasksRepository(@NonNull Context context) {
checkNotNull(context);
ToDoDatabase database = ToDoDatabase.getInstance(context);
return TasksRepository.getInstance(FakeTasksRemoteDataSource.getInstance(),
TasksLocalDataSource.getInstance(new AppExecutors(),
database.taskDao()));
}
}
可以看到provideTasksRepository方法內即為FakeTasksRemoteDataSource.getIntance()和TasksLocalDataSource.getInstance()分別代表遠端資料來源和本地端資料來源。
接著看看FakeTasksRemoteDataSource類別。其內容相當簡單，儲存資料的方式是透過一個Map<String, Task> TASKS_SERVICE_DATA 來儲存資料。
FakeTasksRemoteDataSource.java
public class FakeTasksRemoteDataSource implements TasksDataSource {
private static FakeTasksRemoteDataSource INSTANCE;
private static final Map<String, Task> TASKS_SERVICE_DATA = new LinkedHashMap<>();
// Prevent direct instantiation.
private FakeTasksRemoteDataSource() {}
public static FakeTasksRemoteDataSource getInstance() {
if (INSTANCE == null) {
INSTANCE = new FakeTasksRemoteDataSource();
}
return INSTANCE;
}
@Override
public void getTasks(@NonNull LoadTasksCallback callback) {
callback.onTasksLoaded(Lists.newArrayList(TASKS_SERVICE_DATA.values()));
}
@Override
public void getTask(@NonNull String taskId, @NonNull GetTaskCallback callback) {
Task task = TASKS_SERVICE_DATA.get(taskId);
callback.onTaskLoaded(task);
}
@Override
public void saveTask(@NonNull Task task) {
TASKS_SERVICE_DATA.put(task.getId(), task);
}
@Override
public void completeTask(@NonNull Task task) {
Task completedTask = new Task(task.getTitle(), task.getDescription(), task.getId(), true);
TASKS_SERVICE_DATA.put(task.getId(), completedTask);
}
@Override
public void completeTask(@NonNull String taskId) {
// Not required for the remote data source.
}
@Override
public void activateTask(@NonNull Task task) {
Task activeTask = new Task(task.getTitle(), task.getDescription(), task.getId());
TASKS_SERVICE_DATA.put(task.getId(), activeTask);
}
@Override
public void activateTask(@NonNull String taskId) {
// Not required for the remote data source.
}
@Override
public void clearCompletedTasks() {
Iterator<Map.Entry<String, Task>> it = TASKS_SERVICE_DATA.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, Task> entry = it.next();
if (entry.getValue().isCompleted()) {
it.remove();
}
}
}
public void refreshTasks() {
// Not required because the {@link TasksRepository} handles the logic of refreshing the
// tasks from all the available data sources.
}
@Override
public void deleteTask(@NonNull String taskId) {
TASKS_SERVICE_DATA.remove(taskId);
}
@Override
public void deleteAllTasks() {
TASKS_SERVICE_DATA.clear();
}
@VisibleForTesting
public void addTasks(Task... tasks) {
for (Task task : tasks) {
TASKS_SERVICE_DATA.put(task.getId(), task);
}
}
}
最後看看 TasksLocalDataSource 類別。
該類別有TaskDao以及AppExecutors 變數，其中TaskDao提供存取Task的介面，為使用Room的寫法。關於Room可以參考這篇。
而AppExecutors主要負責Executor的執行。
TasksLocalDataSource.java
public class TasksLocalDataSource implements TasksDataSource {
private static volatile TasksLocalDataSource INSTANCE;
private TasksDao mTasksDao;
private AppExecutors mAppExecutors;
// Prevent direct instantiation.
private TasksLocalDataSource(@NonNull AppExecutors appExecutors,
@NonNull TasksDao tasksDao) {
mAppExecutors = appExecutors;
mTasksDao = tasksDao;
}
public static TasksLocalDataSource getInstance(@NonNull AppExecutors appExecutors,
@NonNull TasksDao tasksDao) {
if (INSTANCE == null) {
synchronized (TasksLocalDataSource.class) {
if (INSTANCE == null) {
INSTANCE = new TasksLocalDataSource(appExecutors, tasksDao);
}
}
}
return INSTANCE;
}
/**
* Note: {@link LoadTasksCallback#onDataNotAvailable()} is fired if the database doesn't exist
* or the table is empty.
*/
@Override
public void getTasks(@NonNull final LoadTasksCallback callback) {
Runnable runnable = new Runnable() {
@Override
public void run() {
final List<Task> tasks = mTasksDao.getTasks();
mAppExecutors.mainThread().execute(new Runnable() {
@Override
public void run() {
if (tasks.isEmpty()) {
// This will be called if the table is new or just empty.
callback.onDataNotAvailable();
} else {
callback.onTasksLoaded(tasks);
}
}
});
}
};
mAppExecutors.diskIO().execute(runnable);
}
/**
* Note: {@link GetTaskCallback#onDataNotAvailable()} is fired if the {@link Task} isn't
* found.
*/
@Override
public void getTask(@NonNull final String taskId, @NonNull final GetTaskCallback callback) {
Runnable runnable = new Runnable() {
@Override
public void run() {
final Task task = mTasksDao.getTaskById(taskId);
mAppExecutors.mainThread().execute(new Runnable() {
@Override
public void run() {
if (task != null) {
callback.onTaskLoaded(task);
} else {
callback.onDataNotAvailable();
}
}
});
}
};
mAppExecutors.diskIO().execute(runnable);
}
@Override
public void saveTask(@NonNull final Task task) {
checkNotNull(task);
Runnable saveRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.insertTask(task);
}
};
mAppExecutors.diskIO().execute(saveRunnable);
}
@Override
public void completeTask(@NonNull final Task task) {
Runnable completeRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.updateCompleted(task.getId(), true);
}
};
mAppExecutors.diskIO().execute(completeRunnable);
}
@Override
public void completeTask(@NonNull String taskId) {
// Not required for the local data source because the {@link TasksRepository} handles
// converting from a {@code taskId} to a {@link task} using its cached data.
}
@Override
public void activateTask(@NonNull final Task task) {
Runnable activateRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.updateCompleted(task.getId(), false);
}
};
mAppExecutors.diskIO().execute(activateRunnable);
}
@Override
public void activateTask(@NonNull String taskId) {
// Not required for the local data source because the {@link TasksRepository} handles
// converting from a {@code taskId} to a {@link task} using its cached data.
}
@Override
public void clearCompletedTasks() {
Runnable clearTasksRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.deleteCompletedTasks();
}
};
mAppExecutors.diskIO().execute(clearTasksRunnable);
}
@Override
public void refreshTasks() {
// Not required because the {@link TasksRepository} handles the logic of refreshing the
// tasks from all the available data sources.
}
@Override
public void deleteAllTasks() {
Runnable deleteRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.deleteTasks();
}
};
mAppExecutors.diskIO().execute(deleteRunnable);
}
@Override
public void deleteTask(@NonNull final String taskId) {
Runnable deleteRunnable = new Runnable() {
@Override
public void run() {
mTasksDao.deleteTaskById(taskId);
}
};
mAppExecutors.diskIO().execute(deleteRunnable);
}
@VisibleForTesting
static void clearInstance() {
INSTANCE = null;
}
}
整體來說因為Model提供了三層緩存，複雜度反而比Presenter和View還高。
從裡面的設計也可以看到設計原則(OCP)等等。
以上便是 todo-mvp 的 Model 。
以下為MVP相關內容
MVP Pattern in Android
