2020-04-21-Android四大组件-ContentPr

应用层

ContentProvider平时使用比较少，今天大概看一下它的使用和简单原理。
通常ContentProvider会结合SQLiteDatabase数据库一起使用。所以我们再构造函数中初始化一个SQLiteDatabase对象。

    private SQLiteDatabase sqLiteDatabase;

    @Override
    public boolean onCreate() {
        sqLiteDatabase = new DatabaseHelper(getContext()).getWritableDatabase();
        return true;
    }

同时，一个数据库可能有多张表，每张表有多行多列，需要使用UriMatcher对Uri进行匹配处理。所以需要一个UriMatcher静态变量。

    public static final String AUTHORITIES = "com.one.provider";

    private static final UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);

    static {
        sUriMatcher.addURI(AUTHORITIES, DatabaseHelper.TABLE_NAME, DatabaseHelper.TABLE_CODE);
    }

然后，我们的增删查改操作都是依赖SQLiteDatabase实现的。

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        Log.d(TAG, "query");
        if (sUriMatcher.match(uri) == DatabaseHelper.TABLE_CODE) {
            String tableName = DatabaseHelper.TABLE_NAME;
            return sqLiteDatabase.query(tableName, projection, selection, selectionArgs, null, null, sortOrder);
        } else {
            return null;
        }
    }

    @Override
    public Uri insert(Uri uri, ContentValues contentValues) {
        if (sUriMatcher.match(uri) == DatabaseHelper.TABLE_CODE) {
            String tableName = DatabaseHelper.TABLE_NAME;
            sqLiteDatabase.insert(tableName, null, contentValues);
            return uri;
        } else {
            return null;
        }
    }

    @Override
    public int delete(Uri uri, String selection, String[] args) {
        if (sUriMatcher.match(uri) == DatabaseHelper.TABLE_CODE) {
            String tableName = DatabaseHelper.TABLE_NAME;
            return sqLiteDatabase.delete(tableName, selection, args);
        } else {
            return -1;
        }
    }

    @Override
    public int update(Uri uri, ContentValues contentValues, String selection, String[] args) {
        if (sUriMatcher.match(uri) == DatabaseHelper.TABLE_CODE) {
            String tableName = DatabaseHelper.TABLE_NAME;
            return sqLiteDatabase.update(tableName, contentValues, selection, args);
        } else {
            return -1;
        }
    }

而DatabaseHelper主要是帮助我们建表和进行数据库版本控制。

    public static final int VERSION = 100;

    public static final int TABLE_CODE = 1;
    public static final String TABLE_NAME = "firstTable";
    public static final String COLUMN_ID = "_id";
    public static final String PKG_NAME = "pkgName";
    public static final String LAST_TIME = "lastTime";

    private static final String CREATE_TABLE = "CREATE TABLE IF NOT EXISTS "
            + TABLE_NAME
            + " ("
            + COLUMN_ID
            + " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + PKG_NAME
            + " TEXT NOT NULL,"
            + LAST_TIME
            + " LONG"
            + ");";

    public DatabaseHelper(Context context) {
        super(context, MyProvider.class.getName(), null, VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL(CREATE_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

    }

最后，需要在AndroidManifest进行声明。

<provider
    android:authorities="com.one.provider"
    android:name="com.one.provider.MyProvider"/>

这里authorities的作用是用于标志唯一的Uri，前面的UriMatcher 就需要根据这个值进行匹配。
完成注册后，我们就可以通过ContentResolver对ContentProvider进行读写了。
首先根据authorities获得Uri对象。

    private Uri uri = Uri.parse("content://" + MyProvider.AUTHORITIES + "/" + DatabaseHelper.TABLE_NAME);

首先看下简单的查询操作：

    public void query() {
        Cursor cursor = mResolver.query(uri, null, null, null);
        while (cursor.moveToNext()) {
            Log.d(TAG, "query " + cursor.getColumnName(0) + "=" + cursor.getInt(0)
                    +" " + cursor.getColumnName(1) + "=" + cursor.getString(1)
                    + " " + cursor.getColumnName(2) + "=" + cursor.getLong(2));
        }
        cursor.close();
    }

插入操作则需要考虑Primary Key冲突的问题。

    public void insert() {
        ContentValues values = new ContentValues();
        values.put(DatabaseHelper.COLUMN_ID, 101);
        values.put(DatabaseHelper.PKG_NAME, mContext.getPackageName());
        long lastTime = System.currentTimeMillis();
        Log.d(TAG, "lastTime=" + lastTime);
        values.put(DatabaseHelper.LAST_TIME, lastTime);
        String[] projections = new String[]{DatabaseHelper.COLUMN_ID};
        String[] selectionArgs = new String[]{101 + ""};
        String selection = DatabaseHelper.COLUMN_ID + " =?";
        Cursor cursor = mResolver.query(uri, projections, selection, selectionArgs, null);
        if (cursor != null && cursor.moveToFirst()) {
            Log.d(TAG, "update id=" + cursor.getInt(cursor.getColumnIndex(DatabaseHelper.COLUMN_ID)));
            String where = DatabaseHelper.COLUMN_ID + "=?";
            String[] conditions = new String[]{101 + ""};
            int code = mResolver.update(uri, values, where, conditions);
            Log.d(TAG, "update code=" + code);
        } else {
            mResolver.insert(uri, values);
        }
        cursor.close();
    }

最后看一下ContentProvider的启动过程，跟其他组件一样，它也是通过ActivityThread启动的。

启动

ContentProvider.jpg

查询

ContentProvider (2).jpg

参考

理解ContentProvider原理
ContentProvider使用详解
Android：关于ContentProvider的知识都在这里了！