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BasicSample項目說明

2023-07-30 11:31 ttylinux 閱讀(50) 評論(0) 收藏舉報

整個示例項目，兩個Fragment，ProductListFragment和ProductFragment，一個MainActivity。在MainActivity里面展示的是ProductListFragment，點擊每個Item，

會進入相應的ProductFragment。

相關技術點說明：

LiveData應用：

LiveData的應用，當LiveData包裝的數據發生變化的時候，更新List

private void subscribeUi(LiveData<List<ProductEntity>> liveData) {
    // Update the list when the data changes
    liveData.observe(getViewLifecycleOwner(), myProducts -> {
        if (myProducts != null) {
            mBinding.setIsLoading(false);
            mProductAdapter.setProductList(myProducts);
        } else {
            mBinding.setIsLoading(true);
        }
        // espresso does not know how to wait for data binding's loop so we execute changes
        // sync.
        mBinding.executePendingBindings();
    });
}

保存LiveData的狀態數據，并且根據狀態數據的變化，更新LiveData：

Transformations.switchMap和SavedStateHandle的應用

public ProductListViewModel(@NonNull Application application,
        @NonNull SavedStateHandle savedStateHandle) {
    super(application);
    mSavedStateHandler = savedStateHandle;


    mRepository = ((BasicApp) application).getRepository();


    // Use the savedStateHandle.getLiveData() as the input to switchMap,
    // allowing us to recalculate what LiveData to get from the DataRepository
    // based on what query the user has entered
    //LiveData<String> query，數據發生變化，就會觸發執行向數據庫查詢的動作，將查詢結果添加到已有的LiveData中
    mProducts = Transformations.switchMap(
            savedStateHandle.getLiveData("QUERY", null),
            (Function<CharSequence, LiveData<List<ProductEntity>>>) query -> {
                if (TextUtils.isEmpty(query)) {
                    return mRepository.getProducts();
                }
                return mRepository.searchProducts("*" + query + "*");
            });
}


public void setQuery(CharSequence query) {
    // Save the user's query into the SavedStateHandle.
    // This ensures that we retain the value across process death
    // and is used as the input into the Transformations.switchMap above
    //保存在SavedStateHandler的數據，即使進程銷毀重建都會存在
    mSavedStateHandler.set(QUERY_KEY, query);
}

　實際的數據獲取實現，封裝在DataRepository mRepository;中。

RecyclerView - DiffUtil 局部刷新

輸入兩個數據集合，計算兩個數據集合的差異化，然后根據差異化結果更新列表。差異化的計算過程是在UI線程中進行的。

如果數據量巨大，可以在異步線程更新，使用AsyncListDiffer和ListAdapter。

public void setProductList(final List<? extends Product> productList) {
    if (mProductList == null) {
        mProductList = productList;
        notifyItemRangeInserted(0, productList.size());
    } else {
        DiffUtil.DiffResult result = DiffUtil.calculateDiff(new DiffUtil.Callback() {
            @Override
            public int getOldListSize() {
                return mProductList.size();
            }


            @Override
            public int getNewListSize() {
                return productList.size();
            }


            @Override
            public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
                return mProductList.get(oldItemPosition).getId() ==
                        productList.get(newItemPosition).getId();
            }


            @Override
            public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
                Product newProduct = productList.get(newItemPosition);
                Product oldProduct = mProductList.get(oldItemPosition);
                return newProduct.getId() == oldProduct.getId()
                        && TextUtils.equals(newProduct.getDescription(), oldProduct.getDescription())
                        && TextUtils.equals(newProduct.getName(), oldProduct.getName())
                        && newProduct.getPrice() == oldProduct.getPrice();
            }
        });
        mProductList = productList;
        result.dispatchUpdatesTo(this);
    }
}

回調函數方法的含義

public abstract static class Callback {
    /**
    * 舊數據 size
    */
    public abstract int getOldListSize();
    /**
    * 新數據 size
    */
    public abstract int getNewListSize();
    /**
    * DiffUtil 調用判斷兩個 itemview 對應的數據對象是否一樣. 由于 DiffUtil 是對兩個不同數據集合的對比, 所以比較對象引用肯定是不行的, 一般會使用 id 等具有唯一性的字段進行比較.
   
    * @param oldItemPosition 舊數據集合中的下標
    * @param newItemPosition 新數據集合中的下標
    * @return True 返回 true 即判斷兩個對象相等, 反之則是不同的兩個對象.
    */
    public abstract boolean areItemsTheSame(int oldItemPosition, int newItemPosition);
    /**
    * Diffutil 調用判斷兩個相同對象之間的數據是否不同. 此方法僅會在 areItemsTheSame() 返回 true 的情況下被調用.
    *
    * @param oldItemPosition 舊數據集合中的下標
    * @param newItemPosition 新數據集合中用以替換舊數據集合數據項的下標
    * @return True 返回 true 代表 兩個對象的數據相同, 反之則有差別.
    */
    public abstract boolean areContentsTheSame(int oldItemPosition, int newItemPosition);
    /**
    * 當 areItemsTheSame() 返回true areContentsTheSame() 返回 false 時, 此方法將被調用, 來完成局部刷新功能.
    */
    @Nullable
    public Object getChangePayload(int oldItemPosition, int newItemPosition);
}

　DataBinding，數據綁定

<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">
    <data>
        <variable name="product"
                  type="com.example.android.persistence.model.Product"/>
        <variable name="callback"
                  type="com.example.android.persistence.ui.ProductClickCallback"/>
    </data>


    <androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:minHeight="@dimen/product_item_min_height"
        android:onClick="@{() ->  callback.onClick(product)}"
        android:orientation="horizontal"
        android:layout_marginStart="@dimen/item_horizontal_margin"
        android:layout_marginEnd="@dimen/item_horizontal_margin"
        app:cardUseCompatPadding="true">


        <RelativeLayout
            android:layout_marginStart="@dimen/item_horizontal_margin"
            android:layout_marginEnd="@dimen/item_horizontal_margin"
            android:layout_width="match_parent"
            android:layout_height="wrap_content">


            <TextView
                android:id="@+id/name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:contentDescription="@string/cd_product_name"
                android:text="@{product.name}"/>


            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_alignParentEnd="true"
                android:layout_marginEnd="5dp"
                android:text="@{@string/product_price(product.price)}"/>


            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_below="@id/name"
                android:text="@{product.description}"/>
        </RelativeLayout>


    </androidx.cardview.widget.CardView>
</layout>

實例化Binding，為Binding注入相應的對象

@Override
@NonNull
public ProductViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    ProductItemBinding binding = DataBindingUtil
            .inflate(LayoutInflater.from(parent.getContext()), R.layout.product_item,
                    parent, false);
    binding.setCallback(mProductClickCallback);
    return new ProductViewHolder(binding);
}


@Override
public void onBindViewHolder(@NonNull ProductViewHolder holder, int position) {
    holder.binding.setProduct(mProductList.get(position));
    //立即刷新數據
    holder.binding.executePendingBindings();
}

=================================================================================================================================

數據層，創建三個數據表，ProductFtsEntity，ProductEntity，CommentEntity。然后聲明Dao，通過Dao來獲取數據庫中的數據，與數據庫交互。

Dao：

@Dao
public interface CommentDao {
    @Query("SELECT * FROM comments where productId = :productId")
    LiveData<List<CommentEntity>> loadComments(int productId);


    @Query("SELECT * FROM comments where productId = :productId")
    List<CommentEntity> loadCommentsSync(int productId);


    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insertAll(List<CommentEntity> comments);
}

應用@TypeConverter，在聲明Entity的時候，使用屬性類型不是基本類型時，是復雜的數據類型時，比如是Date類型時，需要

定義TypeConverter，這樣數據庫才知道如何存儲該數據。

定義TypeConverter需要實現兩個方法，一個是從基礎數據獲取復雜數據的方法，一個是復雜數據轉換為基礎數據的方法，比如對于類型Date：

public class Converters {
  @TypeConverter
  public static Date fromTimestamp(Long value) {
    return value == null ? null : new Date(value);
  }


  @TypeConverter
  public static Long dateToTimestamp(Date date) {
    return date == null ? null : date.getTime();
  }
}

數據庫實例，設計為一個單例，使用單例模式：

單例模式，設計為線程安全的，首先檢測是否為null，二次檢測，加鎖，判斷是否為null，如果為null，

則在當前線程創建數據庫實例。

public static AppDatabase getInstance(final Context context, final AppExecutors executors) {
    if (sInstance == null) {
        synchronized (AppDatabase.class) {
            if (sInstance == null) {
                sInstance = buildDatabase(context.getApplicationContext(), executors);
                sInstance.updateDatabaseCreated(context.getApplicationContext());
            }
        }
    }
    return sInstance;
}

通過Room.databaseBuilder，builder模式構建數據庫實例。

在構建的時候，添加回調方法，首次創建的時候，數據庫創建成功，則在第三方線程為數據庫插入數據，也就是在onCreate方法里面執行。

添加數據庫升級策略。

/**
* Build the database. {@link Builder#build()} only sets up the database configuration and
* creates a new instance of the database.
* The SQLite database is only created when it's accessed for the first time.
*/
private static AppDatabase buildDatabase(final Context appContext,
        final AppExecutors executors) {
    return Room.databaseBuilder(appContext, AppDatabase.class, DATABASE_NAME)
            .addCallback(new Callback() {
                @Override
                public void onCreate(@NonNull SupportSQLiteDatabase db) {
                    super.onCreate(db);
                    executors.diskIO().execute(() -> {
                        // Add a delay to simulate a long-running operation
                        addDelay();
                        // Generate the data for pre-population
                        AppDatabase database = AppDatabase.getInstance(appContext, executors);
                        List<ProductEntity> products = DataGenerator.generateProducts();
                        List<CommentEntity> comments =
                                DataGenerator.generateCommentsForProducts(products);


                        insertData(database, products, comments);
                        // notify that the database was created and it's ready to be used
                        database.setDatabaseCreated();
                    });
                }
            })
        .addMigrations(MIGRATION_1_2)
        .build();
}

數據庫升級策略，添加Migrations，如下，從版本1升級到版本2，執行如下SQL語句：

如果不存在productFts數據庫表，則創建它；并且從products表查詢數據來填充productFts數據表。

private static final Migration MIGRATION_1_2 = new Migration(1, 2) {


    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("CREATE VIRTUAL TABLE IF NOT EXISTS `productsFts` USING FTS4("
            + "`name` TEXT, `description` TEXT, content=`products`)");
        database.execSQL("INSERT INTO productsFts (`rowid`, `name`, `description`) "
            + "SELECT `id`, `name`, `description` FROM products");


    }
};

https://medium.com/androiddevelopers/understanding-migrations-with-room-f01e04b07929

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BasicSample項目說明

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