如何在Android中使用合同类？ [英] How to use a contract class in android?
问题描述
我有点困惑，因为我不知道我应该怎么跨preT这里的教程：<一href="http://developer.android.com/training/basics/data-storage/databases.html#DbHelper">http://developer.android.com/training/basics/data-storage/databases.html#DbHelper
I'm a little bit confused because I don't know how I should interpret the tutorial here: http://developer.android.com/training/basics/data-storage/databases.html#DbHelper
我的code到现在为止是这样的：
My code until now looks like this:
public final class DatabaseContract {
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
public DatabaseContract() {}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME = "nameOfTable";
public static final String COLUMN_NAME_COL1 = "column1";
public static final String COLUMN_NAME_COL2 = "column2";
public static final String COLUMN_NAME_COL3 = "column3";
}
public class DatabaseHelper extends SQLiteOpenHelper {
// If you change the database schema, you must increment the database version.
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "database.db";
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES = "CREATE TABLE " +
Table1.TABLE_NAME + " (" +
Table1._ID + " INTEGER PRIMARY KEY," +
Table1.COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
Table1.COLUMN_NAME_COL3 + TEXT_TYPE + COMMA_SEP + " )";
private static final String SQL_DELETE_ALL_ENTRIES = "DROP TABLE IF EXISTS " + Table1.TABLE_NAME;
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// Method is called during creation of the database
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(SQL_CREATE_ENTRIES);
}
// Method is called during an upgrade of the database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(DatabaseHelper.class.getName(), "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data");
db.execSQL(SQL_DELETE_ALL_ENTRIES);
onCreate(db);
}
}
}
难道我除preTED是正确的或有第6个变量在Helper类是在外面的合同类？ 还是应该帮助类不是一个内部类合同类的？
Did I interpreted it right or have the first 6 variables in the Helper class to be outside in the Contract class? Or should the Helper Class not be a inner class of the contract class?
希望你能帮助我。
问候 西蒙
推荐答案
您的合同基本上定义您的数据库和人应该如何通过内容提供商与它进行交互。
Your contract basically defines your database and how people should interact with it through the Content Provider.
一个合同类定义的常量，帮助应用程序一起工作 内容的URI，列名，故意行为，以及其他功能 内容提供商。合同类不会自动包含 与提供者;供应商的开发者来定义他们，然后 让他们给其他开发者使用。
A contract class defines constants that help applications work with the content URIs, column names, intent actions, and other features of a content provider. Contract classes are not included automatically with a provider; the provider's developer has to define them and then make them available to other developers.
说了这么多，你不一定需要一个内容提供商使用合同类。我的例子包含了由内容提供商（该MIME和URI部分）的常量。如果您使用的不是内容提供商，则不需要这些部分。
Having said that, you do not necessarily need a Content Provider to use a Contract class. My example contains constants that are used by the Content Provider (The MIME and URI parts). If you are not using a Content Provider, you do not need these sections.
我喜欢把它定义你的数据库是如何设置的合同类作为数据库模式，或者说什么的。您可能会注意到，合同类中都被声明为静态的。那是因为你将永远不会被实例化一个合同类，但只有参照它定义的常量。你可以在我的例子，我的合同类只是有一堆声明为static final变量看。本合同类可以是它自己的文件，例如我的文件名为TransitContract.java。
I like to think of the contract class as a database schema, or in other words something that defines how your database is set up. You might notice that everything within the contract class is declared as static. That is because you will never be instantiating a Contract class, but only referring to the constants defined in it. You can see in my example that my Contract class just has a bunch of static final variables declared. This Contract class can be its own file, e.g. my file is called TransitContract.java.
说，例如，你想改变你的其中一列的名称。而不是更改到多个文件，所有你需要做的是改变该值在合同类列。 你是不是在做任何类型的计算工作合同类里面。
Say for example you wanted to change the name of one of your columns. Rather than making changes to multiple files, all you have to do is change the value for the column in the contract class. You are not doing any kind of computation work inside of the contract class.
在另一方面，SQLLiteOpenhelper类是提供了由谷歌，为了使与数据库工作更容易一些。这是你实现了创建方法和设置初始数据库。请参阅<一href="http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html">http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html.当你实现这些方法，你所要做的就是实例化助手类的一个实例，然后调用helperClassInstance.getWriteableDatabase（）（或getReadableDataBase（）），然后助手类会自动在必要时创建新数据库的照顾，或返回已经存在一个，等等。
The SQLLiteOpenhelper class on the other hand is something that was provided by Google to make working with databases easier. This is where you implement methods that create and set up the initial database. See http://developer.android.com/reference/android/database/sqlite/SQLiteOpenHelper.html. After you implement these methods, all you have to do is instantiate an instance of your helper class, and then call helperClassInstance.getWriteableDatabase() (or getReadableDataBase()) and then your helper class automatically takes care of creating a new database if necessary, or returning the one that already exists, etc.
本助手通常是实现为一个内部类，但可能是它自己的独立的类。这不过想要实现它。
This helper is generally implemented as an inner class but could be its own standalone class. It's however you want to implement it.
我强烈建议看的记事本的例子，谷歌提供了，因为它有一个pretty的如何你可能订下契约等级达到很好的例子。请注意，他们还利用一个内容提供者。如果您有兴趣了解的内容提供商，我建议你阅读了一些在<一href="http://developer.android.com/guide/topics/providers/content-provider-basics.html">http://developer.android.com/guide/topics/providers/content-provider-basics.html.它进入有关内容供应商和合同类的更深入。
I highly recommend looking at the Notepad example that Google provides as it has a pretty good example of how you could possibly set a contract class up. Please note that they also use a Content Provider. If you are interested in learning about Content Providers I recommend you read up some more at http://developer.android.com/guide/topics/providers/content-provider-basics.html. It goes into much more depth about Content Providers and Contract classes.
下面是使用code的例子。我没有实际测试此code所以它可能存在错误。正如你所看到的，你可以实例化你的数据库帮助任何地方，你认为有必要的。在这个例子中我做的主要活动的onCreate，但在现实中，这是不好的做法。
DatabaseContract.java
DatabaseContract.java
public final class DatabaseContract {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = "database.db";
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
private DatabaseContract() {}
public static abstract class Table1 implements BaseColumns {
public static final String TABLE_NAME = "nameOfTable";
public static final String COLUMN_NAME_COL1 = "column1";
public static final String COLUMN_NAME_COL2 = "column2";
public static final String COLUMN_NAME_COL3 = "column3";
public static final String CREATE_TABLE = "CREATE TABLE " +
TABLE_NAME + " (" +
_ID + " INTEGER PRIMARY KEY," +
COLUMN_NAME_COL1 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL2 + TEXT_TYPE + COMMA_SEP +
COLUMN_NAME_COL3 + TEXT_TYPE + COMMA_SEP + " )";
public static final String DELETE_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;
}
}
DatabaseHelper.java
DatabaseHelper.java
public class DatabaseHelper extends SQLiteOpenHelper {
public DatabaseHelper(Context context) {
super(context, DatabaseContract.DATABASE_NAME, null, DatabaseContract.DATABASE_VERSION);
}
// Method is called during creation of the database
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DatabaseContract.Table1.CREATE_TABLE);
}
// Method is called during an upgrade of the database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL(DatabaseContract.Table1.DELETE_TABLE);
onCreate(db);
}
}
MainActivity.java
MainActivity.java
public class MainActivity extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Create new helper
DatabaseHelper dbHelper = new DatabaseHelper(getContext());
// Get the database. If it does not exist, this is where it will
// also be created.
SQLiteDatabase db = dbHelper.getWriteableDatabase();
// Create insert entries
ContentValues values = new ContentValues();
values.put(DatabaseContract.Table1.COLUMN_NAME_COL1, "value1");
values.put(DatabaseContract.Table1.COLUMN_NAME_COL2, "value2");
values.put(DatabaseContract.Table1.COLUMN_NAME_COL3, "value3");
// Insert the new row, returning the primary key value of the new row
long newRowId;
newRowId = db.insert(
DatabaseContract.Table1.TABLE_NAME,
null,
values);
}
}
和我的例子
public final class TransitContract {
public static final String AUTHORITY = "com.example.TransitProvider";
public static final String SCHEME = "content://";
public static final String SLASH = "/";
public static final String DATABASE_NAME = "transit.db";
/* An array list of all the SQL create table statements */
public static final String[] SQL_CREATE_TABLE_ARRAY = {
Agency.CREATE_TABLE,
CalendarDates.CREATE_TABLE,
Calendar.CREATE_TABLE,
Routes.CREATE_TABLE,
Shapes.CREATE_TABLE,
Stops.CREATE_TABLE,
StopTimes.CREATE_TABLE,
Trips.CREATE_TABLE
};
/**
* Array of resource ids for each GTFS data file that will be loaded into
* database
*/
public static final int[] RAW_IDS = {
R.raw.agency,
R.raw.calendar_dates,
R.raw.calendar,
R.raw.routes,
R.raw.shapes,
R.raw.stops,
R.raw.stop_times,
R.raw.trips,
};
/* Do not allow this class to be instantiated */
private TransitContract() {}
public static final class Agency implements BaseColumns {
/* Do not allow this class to be instantiated */
private Agency() {}
public static final String TABLE_NAME = "Agency";
public static final String KEY_AGENCY_ID = "AgencyId";
public static final String KEY_NAME = "Name";
public static final String KEY_URL = "Url";
public static final String KEY_TIMEZONE = "Timezone";
public static final String KEY_LANG = "Language";
public static final String KEY_PHONE = "PhoneNumber";
public static final String KEY_FARE_URL = "FareUrl";
/*
* URI definitions
*/
/**
* The content style URI
*/
public static final Uri CONTENT_URI = Uri.parse(SCHEME + AUTHORITY + SLASH + TABLE_NAME);
/**
* The content URI base for a single row. An ID must be appended.
*/
public static final Uri CONTENT_ID_URI_BASE = Uri.parse(SCHEME + AUTHORITY + SLASH + TABLE_NAME + SLASH);
/**
* The default sort order for this table
*/
public static final String DEFAULT_SORT_ORDER = KEY_AGENCY_ID + " ASC";
/*
* MIME type definitions
*/
/**
* The MIME type of {@link #CONTENT_URI} providing rows
*/
public static final String CONTENT_TYPE = ContentResolver.CURSOR_DIR_BASE_TYPE +
"/vnd.com.marylandtransitcommuters.agency";
/**
* The MIME type of a {@link #CONTENT_URI} single row
*/
public static final String CONTENT_ITEM_TYPE = ContentResolver.CURSOR_ITEM_BASE_TYPE +
"/vnd.com.marylandtransitcommuters.agency";
/**
* SQL Statement to create the routes table
*/
public static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " ("
+ _ID + " INTEGER PRIMARY KEY,"
+ KEY_AGENCY_ID + " TEXT,"
+ KEY_NAME + " TEXT,"
+ KEY_URL + " TEXT,"
+ KEY_TIMEZONE + " TEXT,"
+ KEY_LANG + " TEXT,"
+ KEY_PHONE + " TEXT,"
+ KEY_FARE_URL + " TEXT"
+ ");";
/**
* SQL statement to delete the table
*/
public static final String DELETE_TABLE = "DROP TABLE IF EXISTS " + TABLE_NAME;
/**
* Array of all the columns. Makes for cleaner code
*/
public static final String[] KEY_ARRAY = {
KEY_AGENCY_ID,
KEY_NAME,
KEY_URL,
KEY_TIMEZONE,
KEY_LANG,
KEY_PHONE,
KEY_FARE_URL
};
}
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