Inserting and Deleting Table Records
The form buttons each perform two different functions. When you click the Add button, your application will be placed into the insert mode. In this mode, the Add button will be changed to the Insert button, and the Delete button will be changed to the Cancel button. To accomplish this, you can create a private member variable of type bool, which signifies which mode you are in. Give this variable the name m_bInsertingRecord. You will first work on the New button.
Because the buttonNew_Click delegate needs to handle two tasks based on the Boolean value you created earlier, you will need to have two blocks of code. Whenever you click the New button, you will need to place the form into insert mode. This is done by clearing all the TextBox controls by setting each Text property for the three TextBox controls to an empty string. Next, change the text of the two buttons by changing the Text properties of these Button controls. Change the New buttons' text to "Add" and the Delete button to "Cancel," as shown on line 12 of Listing 20.7. Finally, set the Boolean flag m_bInsertingRecord to true.
If m_bInsertingRecord is true, you will add the contents of the TextBox controls to the database. The process of adding a new record involves creating a new row and adding that row to the DataTable. To begin, create a new DataRow object by calling the NewRow member function contained within the DataTable. Assign this to a local pointer to a DataRow variable, as shown on line 22 of Listing 20.7.
Now that you have created a new row, you can start setting the individual columns within that row. Any time rows are being changed, the DataRow class fires events indicating the operations being performed, such as inserting, deleting, or changing individual columns. You can temporarily disable the firing of events by wrapping the code that changes a row with BeginEdit and EndEdit function calls. Because you aren't handling events or adding large amounts of data at one time, disabling the event-firing mechanism probably doesn't buy you any performance gains. However, for the sake of illustration, you will do it anyway.
As mentioned earlier, call the BeginEdit function contained within the DataRow variable you created. Setting the individual columns within that row is similar to retrieving the data. For each column, call the set_Item function, passing the name of the database column and the value you want to set it to. Doing this for the Author column and the YearBorn column is trivial. Just set the data column to the value of the Text property of the corresponding TextBox control. The ID column you will need to set to a unique value. You can simply increment the index of the last row's ID value, as shown on line 25 of Listing 20.7. Finish the row editing by calling the EndEdit function of the DataRow object to reenable event firing.
Listing 20.7 Implementing the Button Event Handler to Add Records
1: void AuthorDBForm::buttonNew_Click(Object* sender, EventArgs* e) 2: { 3: // user wants to add a new record 4: if( !m_bInsertingRecord ) 5: { 6: // clear text boxes 7: textBoxID->Text = S""; 8: textBoxName->Text = S""; 9: textBoxYear->Text = S""; 10: 11: // change button text 12: buttonNew->Text = S"Add"; 13: buttonDelete->Text = S"Cancel"; 14: buttonDelete->Enabled = true; 15: 16: // user is now in insert mode 17: m_bInsertingRecord = true; 18: } 19: else // user has finished filling in values, add to table 20: { 21: // create new row to insert 22: DataRow* pNewRow = m_pAuthorTable->NewRow(); 23: pNewRow->BeginEdit(); 24: { 25: pNewRow->set_Item( "AU_ID", __box(Convert::ToInt32( 26: m_pAuthorTable->get_Rows()->get_Item( 27: m_pAuthorTable->get_Rows()->get_Count() 28: -1)->get_Item("AU_ID")) + 1) ); 29: 30: pNewRow->set_Item( "Author", textBoxName->Text ); 31: pNewRow->set_Item( "YearBorn", textBoxYear->Text ); 32: } 33: pNewRow->EndEdit(); 34: 35: // insert record into datatable 36: m_pAuthorTable->get_Rows()->InsertAt( 37: pNewRow, m_pAuthorTable->get_Rows()->Count ); 38: m_pDA->Update( m_pDataSet, "Authors" ); 39: 40: // update current row index 41: m_iCurRow = m_pAuthorTable->get_Rows()->Count-1; 42: 43: // update form 44: // change button text back to original 45: buttonNew->Text = S"New"; 46: buttonDelete->Text = S"Delete"; 47: 48: FillFormData( m_iCurRow ); 49: 50: // done inserting record 51: m_bInsertingRecord = false; 52: } 53: } Now that you have finished creating the row and setting the values of each of its columns, you are now ready to insert the new row into the DataTable. Because the Rows property in the DataTable is a collection, you can simply add your new row using a collection method that takes an index variable. In this case, use the InsertAt function of the DataTable, passing it an index value of one more than the current count of the number of rows. The first parameter to the InsertAt function is your new row.
Because you are working in a disconnected client scenario, none of the operations you have performed so far have updated the actual database that exists on the file system. To update the database with the contents of the DataSet, you can call the data adapter's member function Update, passing your DataSet member variable and the table within the data source to update, which in this case is the Authors table. This can be seen on line 37 of Listing 20.7.
To finish the event handler for the New button, set the current row member variable to accommodate the new row. Finally, change the form's buttons back to their original Text values and reset the inserting record Boolean member variable. Compile and run your application and then insert a new record. Don't be surprised if an exception is thrown, as shown in Figure 20.5.
Figure 20.5. Your application will throw an exception when trying to insert new records unless you create an OleDbCommandBuilder object.
In this case, an exception was thrown because the SQL command associated with the data adapter does not contain an INSERT SQL statement. However, the .NET Framework contains a very useful class you can use in an instance like this the OleDbCommandBuilder class. Any time you set the SelectCommand property of a data adapter, you will need to use this method. In the OpenDataSource member function you created earlier, place the following at the end of the function:
// create the command builder for record insertion m_pCB = new OleDbCommandBuilder( m_pDA );
Believe it or not, that is all you need to do to remove the exception. This class will automatically generate the necessary SQL commands for any changes that occur to the DataSet. If you used other data-access technologies before, you've probably struggled with command syntax. By using the OleDbCommandBuilder object, you can manipulate the data while the appropriate database commands are created for you transparently.
The last task for this hour's lesson is to implement the code for the Delete button. When you are not in insert mode, as explained earlier, the Delete button will delete the currently displayed record. When you are in insert mode, this button will allow you to cancel out of inserting a new record. Therefore, you will need to create two code blocks based on the value of the m_bInsertingRecord member variable.
If m_bInsertingRecord is true and the buttonDelete_Click event handler is called, the user is canceling out of the insert event. You must then set the form to the state it was in prior to the user entering insert mode. Change the button text for each of the buttons to New and Delete, respectively. Also, if there are no records in the database, the current row member variable is equal to -1. In this case, there is no data to display, so set the Text properties of the TextBox controls to an empty string. If there is data (in other words, m_iCurRow is set to a positive integer), set the TextBox controls by calling the FillFormData member function, as shown on line 22 of Listing 20.8.
If the form is not in insert mode and the Delete button is clicked, you will need to remove the current row from the data source. The first step is to make sure a row is currently being displayed. As was done previously, this can be determined by examining the value of the current row variable. If the variable is valid, you will remove the row. Removing a row from the DataTable is considerably easier than inserting one, because you do not have to directly access the individual columns of the row. To remove a row, you can use the collection method RemoveAt, which takes an integer as a parameter signifying which row index to remove.
After the row has been removed, you will need to update the form to display a different row. Determining which row to display is purely a design decision. For this lesson, set the form data based on the current value of your current row variable. At first, you might think this is invalid because you deleted that row, but once a row is deleted, any rows following will have their row indexes decremented by 1. If FillFormData returns false, you are attempting to access a row that doesn't exist, because it is greater than the number of rows currently in the DataTable. In this instance, fill the form data with the data from the last row in the DataTable, as shown on line 36 of Listing 20.8. If there are no more rows, set each of the TextBox controls to the empty string.
Listing 20.8 Implementing the Delete Event Handler
1: void AuthorDBForm::buttonDelete_Click(Object* sender, EventArgs* e) 2: { 3: if( m_bInsertingRecord ) 4: { 5: // user wants to cancel out of insert mode 6: m_bInsertingRecord = false; 7: buttonNew->Text = S"New"; 8: buttonDelete->Text = S"Delete"; 9: 10: // clear textboxes if not data available 11: if( m_iCurRow == -1 ) 12: { 13: // clear text boxes 14: textBoxID->Text = S""; 15: textBoxName->Text = S""; 16: textBoxYear->Text = S""; 17: 18: return; 19: } 20: 21: // set text boxes to previous record 22: FillFormData( m_iCurRow ); 23: } 24: else 25: { 26: if( m_iCurRow == -1 ) 27: return; 28: 29: // user wants to delete current record 30: m_pAuthorTable->get_Rows()->RemoveAt(m_iCurRow); 31: 32: // fill with row after deleted row 33: if( !FillFormData( m_iCurRow )) 34: { 35: // user is past last row, set to last row 36: m_iCurRow = m_pAuthorTable->get_Rows()->Count - 1; 37: 38: // check to see if last row in table has been removed 39: if( m_iCurRow == -1 ) 40: { 41: // clear text boxes 42: textBoxID->Text = S""; 43: textBoxName->Text = S""; 44: textBoxYear->Text = S""; 45: 46: return; 47: } 48: 49: FillFormData( m_iCurRow ); 50: } 51: } 52: } | Top |
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