什么是合同类以及如何使用它

Question

在最近更新的Android开发指南中,内容提供商的文档包含标题为合同类的部分.虽然有一个指向联系人示例的链接,但目前还不清楚什么是合同类,以及如何为我的自定义内容提供商创建一个

希望得到一些帮助.

谢谢!

Answer 1

什么是合同类？

契约类是一个public final包含URI,列名,MIME类型和其他元数据的常量定义的类ContentProvider.它还可以包含static帮助方法来操作URI.

为什么用它？

1. 合同类在内容提供商和其他应用程序之间建立合同.它确保即使URI,列名等的实际值发生更改,也可以正确访问您的内容提供程序.
2. 由于它为其常量提供助记符名称,因此开发人员不太可能对列名或URI使用不正确的值.
3. 将Javadoc文档提供给想要使用内容提供程序的客户端很容易.

怎么用？

以下是为包含两个表的天气应用程序设计的示例Contract类代码段:weather table和location table.跳过注释和一些方法以保持小.一般来说,应该评论很好.

public class WeatherContract {

    public static final String CONTENT_AUTHORITY = 
            "com.example.android.sunshine.app";
    public static final Uri BASE_CONTENT_URI = 
            Uri.parse("content://" + CONTENT_AUTHORITY);
    public static final String PATH_WEATHER = "weather";
    public static final String PATH_LOCATION = "location";

    /**Inner class that defines the table contents of the location table. */
    public static final class LocationEntry implements BaseColumns {
        public static final String TABLE_NAME = "location";
        public static final String COLUMN_LOCATION_SETTING = "location_setting";
        public static final String COLUMN_CITY_NAME = "city_name";
        public static final String COLUMN_COORD_LAT = "coord_lat";
        public static final String COLUMN_COORD_LONG = "coord_long";

        public static final Uri CONTENT_URI =
                BASE_CONTENT_URI.buildUpon().appendPath(PATH_LOCATION).build();

        // Custom MIME types
        public static final String CONTENT_TYPE =
                ContentResolver.CURSOR_DIR_BASE_TYPE +
                        "/" + CONTENT_AUTHORITY + "/" + PATH_LOCATION;

        public static final String CONTENT_ITEM_TYPE =
                ContentResolver.CURSOR_ITEM_BASE_TYPE +
                        "/" + CONTENT_AUTHORITY + "/" + PATH_LOCATION;

        // Helper method
        public static Uri buildLocationUri(long id) {
            return ContentUris.withAppendedId(CONTENT_URI, id);
        }
    }


    /** Inner class that defines the table contents of the weather table. */
    public static final class WeatherEntry implements BaseColumns {

        public static final String TABLE_NAME = "weather";

        public static final String COLUMN_LOC_KEY = "location_id";
        public static final String COLUMN_DATE = "date";
        public static final String COLUMN_WEATHER_ID = "weather_id";
        public static final String COLUMN_SHORT_DESC = "short_desc";
        public static final String COLUMN_MIN_TEMP = "min";
        public static final String COLUMN_MAX_TEMP = "max";
        public static final String COLUMN_HUMIDITY = "humidity";
        public static final String COLUMN_PRESSURE = "pressure";
        public static final String COLUMN_WIND_SPEED = "wind";
        public static final String COLUMN_DEGREES = "degrees";

        public static final Uri CONTENT_URI =
                BASE_CONTENT_URI.buildUpon().appendPath(PATH_WEATHER).build();

        public static final String CONTENT_TYPE =
                ContentResolver.CURSOR_DIR_BASE_TYPE + "/" + CONTENT_AUTHORITY + 
                        "/" + PATH_WEATHER;

        public static final String CONTENT_ITEM_TYPE =
                ContentResolver.CURSOR_ITEM_BASE_TYPE + "/" + CONTENT_AUTHORITY + 
                        "/" + PATH_WEATHER;

        // Helper method.
        public static Uri buildWeatherUri(long id) {
            return ContentUris.withAppendedId(CONTENT_URI, id);
        }

        .
        .
        .
    }
}

Answer 2

契约类定义了有助于应用程序使用内容URI,列名,意图操作和内容提供程序的其他功能的常量.合同类不会自动包含在提供者中; 提供者的开发人员必须定义它们,然后将其提供给其他开发人员.

您可以创建自己的Contract类并在那里定义一些常量.例如,以后可以在对数据库进行查询的代码中调用的列名等.

如何使用Contract类的好例子请参阅此线程Android - 如何加载联系人照片？