java - contentResolver 返回 null

Question

我使用 contentResolver 而不是直接查询数据库，但它返回 null。我不知道我是否需要添加一些代码或者我的代码有错误。

private void displayDatabaseInfo() {
    // Define a projection that specifies which columns from the database
    // you will actually use after this query.
    String[] projection = {
            PetEntry._ID,
            PetEntry.COLUMN_PET_NAME,
            PetEntry.COLUMN_PET_BREED,
            PetEntry.COLUMN_PET_GENDER,
            PetEntry.COLUMN_PET_WEIGHT };

    // Perform a query on the provider using the ContentResolver.
    // Use the {@link PetEntry#CONTENT_URI} to access the pet data.
    Cursor cursor = getContentResolver().query(
            PetEntry.CONTENT_URI,   // The content URI of the words table
            projection,             // The columns to return for each row
            null,                   // Selection criteria
            null,                   // Selection criteria
            null);                  // The sort order for the returned rows

    Log.v(LOG_TAG, "Проверка курсора " + cursor);



    TextView displayView = (TextView) findViewById(R.id.text_view_pet);

    if (cursor != null){
    try {


            // Create a header in the Text View that looks like this:
            //
            // The pets table contains <number of rows in Cursor> pets.
            // _id - name - breed - gender - weight
            //
            // In the while loop below, iterate through the rows of the cursor and display
            // the information from each column in this order.
            displayView.setText("The pets table contains " + cursor.getCount() + " pets.\n\n");
            displayView.append(PetEntry._ID + " - " +
                    PetEntry.COLUMN_PET_NAME + " - " +
                    PetEntry.COLUMN_PET_BREED + " - " +
                    PetEntry.COLUMN_PET_GENDER + " - " +
                    PetEntry.COLUMN_PET_WEIGHT + "\n");

            // Figure out the index of each column
            int idColumnIndex = cursor.getColumnIndex(PetEntry._ID);
            int nameColumnIndex = cursor.getColumnIndex(PetEntry.COLUMN_PET_NAME);
            int breedColumnIndex = cursor.getColumnIndex(PetEntry.COLUMN_PET_BREED);
            int genderColumnIndex = cursor.getColumnIndex(PetEntry.COLUMN_PET_GENDER);
            int weightColumnIndex = cursor.getColumnIndex(PetEntry.COLUMN_PET_WEIGHT);

            // Iterate through all the returned rows in the cursor
            while (cursor.moveToNext()) {
                // Use that index to extract the String or Int value of the word
                // at the current row the cursor is on.
                int currentID = cursor.getInt(idColumnIndex);
                String currentName = cursor.getString(nameColumnIndex);
                String currentBreed = cursor.getString(breedColumnIndex);
                int currentGender = cursor.getInt(genderColumnIndex);
                int currentWeight = cursor.getInt(weightColumnIndex);
                // Display the values from each column of the current row in the cursor in the TextView
                displayView.append(("\n" + currentID + " - " +
                        currentName + " - " +
                        currentBreed + " - " +
                        currentGender + " - " +
                        currentWeight));
            }

    } finally {
        // Always close the cursor when you're done reading from it. This releases all its
        // resources and makes it invalid.
        cursor.close();
    }
    }
}

在此之前我遇到了错误 null 异常。添加 if/else 语句后，异常消失，但无论如何它不起作用并返回 null

Answer 1

问题出在 ContentProvider 中。我已经更改了代码，现在它可以工作了。

  @Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs,
                    String sortOrder) {
    // Get readable database
    SQLiteDatabase database = mDbHelper.getReadableDatabase();

    // This cursor will hold the result of the query
    Cursor cursor;

    // Figure out if the URI matcher can match the URI to a specific code
    int match = sUriMatcher.match(uri);
    switch (match) {
        case PETS:
            // For the PETS code, query the pets table directly with the given
            // projection, selection, selection arguments, and sort order. The cursor
            // could contain multiple rows of the pets table.
            cursor = database.query(PetEntry.TABLE_NAME, projection, selection, selectionArgs,
                    null, null, sortOrder);
            break;
        case PET_ID:
            // For the PET_ID code, extract out the ID from the URI.
            // For an example URI such as "content://com.example.android.pets/pets/3",
            // the selection will be "_id=?" and the selection argument will be a
            // String array containing the actual ID of 3 in this case.
            //
            // For every "?" in the selection, we need to have an element in the selection
            // arguments that will fill in the "?". Since we have 1 question mark in the
            // selection, we have 1 String in the selection arguments' String array.
            selection = PetEntry._ID + "=?";
            selectionArgs = new String[] { String.valueOf(ContentUris.parseId(uri)) };

            // This will perform a query on the pets table where the _id equals 3 to return a
            // Cursor containing that row of the table.
            cursor = database.query(PetEntry.TABLE_NAME, projection, selection, selectionArgs,
                    null, null, sortOrder);
            break;
        default:
            throw new IllegalArgumentException("Cannot query unknown URI " + uri);
    }
    return cursor;
}