This is part of the book I wrote, all original, just completed, welcome to comment.
Bitfan
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7.5.2 Move in data concentration
When you need to display multiple records in the program, we often hope to record a "record number" to facilitate location records, such as "to the first", "to the last", "jump to item 100" ... .
The record in DataTable (ie DATAROW) itself does not have a so-called "record number", and DataTable itself does not provide this feature.
prompt:
In VB6, record set object Recordset provides MoveFirst, MoveLast et al. This is achieved by ADO. In ADO.NET, DataTable and Datarow have no these methods.
At first glance, you can't move in the data set, as if it is an omission of ADO.NET design. In fact, ADO.NET extracts all the functions located in the data set, plus other features, builds the data binding mechanism of ADO.NET, compared to the limited positioning function provided by the original ADO, ADO. Net is more powerful and flexible.
Please try to run the example moveindataable in this book's condomation CD, and its running interface is shown in Figure 7-15:
Figure 7-15 Data Binding Example
When this program is running, you can click on the four buttons in the bottom right, or you can directly click on the upper grid to move in the data set, and the current row is displayed in the two text boxes in the lower part. Data can be modified directly in the text box. When moving to other rows, the records in the grid are simultaneously refreshed.
Through this example, we come to analyze the principles of data binding mechanisms in ADO.NET.
1 What is the data binding?
The so-called data binding, it is popular, that is, take the data in the data source (such as a DataTable), displayed on various controls of the form, users can view and modify data through these controls, these modifications will automatically save them. In the data source, see Figure 7-16.
Figure 7-16 Principle of data binding
Windows Forms can utilize two types of data bindings: simple binding and complex binding. These two types have different advantages.
l Simple binding
Simple data binding refers to the ability to bind a control to a single data element (such as the value in the column of the data set table). This is a typical binding type for controls such as TextBox controls or Label controls (ie, only controls that only display a single value). Two text boxes in Figure 7-15 bound to "AddressStr" and "ClientName" in DataTable.
l Complex binding
Complex data binding refers to the ability to bind a control to multiple data elements, typically binding multiple records in the database. The DataGrid in Figure 7-15 is bound to a DataTable, which can display a number of records and multiple fields at a time.
2 implementation in data concentration
Move in data concentration is a sub function in a data binding mechanism.
In order to facilitate understanding of the complex data binding mechanism in ADO.NET, we first implemented data mobile features in a simple form in the MoveindATABLE example, see Figure 7-17:
Figure 7-17 Manual implementation moves in recording
The following is analyzed
(1) The program needs to extract data, the SUB process getData implements this feature:
'Define related variables
PRIVATE CONN AS OLEDBCONNECTION
Private comm as oledbcommand
Private Da As OledbdataAdapterPrivate DS1 AS New DataSet
'retrieve data
Private sub getdata ()
'Creating a connection object connection database
CONN = New OLEDBCONNECTION
Conn.connectionstring = "provider = microsoft.jet.Oledb.4.0; data source = d: /clients.mdb; persist security info = false
Conn.open ()
'Creating a command object Used to send a SQL command to the database
Comm = New OLEDBCommand
Comm.commandtext = "SELECT * from OrderClient"
Comm.commandType = CommandType.Text
Comm.connection = conn
'Creating DataAdapter to populate DataSet
Da = New OLEDBDataAdapter (Comm)
'Data input
Da.fill (DS1)
CONN.CLOSE ()
End Sub
Note that it is assumed that the database clients.mdb is placed under "D: /". We are already very familiar with these code.
(2) Set the data binding, the SUB process setDataBing () implements this feature:
'Data Binding Management Object
Private cm as currencymanager
'Setting data binding
Private sub setDataBinding ()
'Bind DataGrid to DATATABLE
Me.DataGrid1.datasource = DS1.TABLES (0)
'Get data binding management objects
cm = me.bindingcontext (ds1.tables (0))
End Sub
In the second form of the sample program (see Figure 7-15), it is only more than the code that binds the text box to the field in the DataTable:
Me.TxtAddress.Databindings.add ("text", ds1.tables (0), "addressstr")
Me.txtName.DataBindings.add ("Text", DS1.Tables (0), "ClientName")
It can be seen that the data binding to the DataGrid is very simple, just set it directly to the DataSource property.
Set the data binding to the text box, you need to add an item to its DataBindings, which is:
"The property name to be bound", the bound DATATABLE object, "DataTable field name"
In the above code, the control is binding directly to the DataTable, in fact, you can also bind to DataSet directly, but you need some changes to the code:
'Bind DataGrid to DATATABLE
Me.DataGrid1.datasource = DS1
Me.DataGrid1.dataMember = ds1.tables (0) .tablename
'Binding text box
Me.txtaddress.databindings.add ("Text", DS1, DS1.TABLES (0) .tablename & ".addressStr")
Me.txtName.Database.dd ("text", ds1, ds1.tables (0) .tablename & ".clientname") Because there may be multiple DataTable in DataSet, you must specify a table name to DataGrid's DataMember attribute . Dynamically specify a DataTable table name in the program, you can dynamically bind the DataTable with the DataGrid corresponding to this table name.
The third parameter bound by the text box is called a "binding string", separated by English sentence, its format:
Table name. Field name
It is characterized by the larger the scope of the loop on the left.
Technical inside:
In fact, data binding is not limited to DataSet and DataTable, which can also bind text boxes and DataGRIDs to other objects, such as arrays and arraylist, at this time, binding strings are very important, interested readers can be in MSDN By searching for "Data Binding" keywords how to bind to non-Dataset and DataTable data.
(3) It is now possible to move in the record set, which is implemented by assigning the current property of the CurrencyManager, and the key code is listed below:
'Move to the first
cm.position = 0
'Move to the previous one
IF cm.position> 0 THEN
cm.position - = 1
END IF
'Move to the next
IF cm.position cm.position = 1 END IF 'Move to the last cm.position = cm.count - 1 It can be seen that movement is implemented by the CurrencyManager object in the data set. Although the code is simple, if it does not understand the mechanism behind this, it is still unable to fully utilize the data binding mechanism. 3 data binding principle ADO.NET's data binding mechanism is mainly composed of the following classes: Figure 7-18 Data Binding Class Figure 7-18 shows the core architecture of data binding in ADO.NET (note that all classes are not integrated) in the UML montage. prompt: UML is referred to as Unified Modeling Language (Unified Modeling Language), which uses graphical ways to express object-oriented software system architectures, and have been widely accepted by software industry and become international standards. Part IV Part of this book introduces a large number of UML clanographed programming programming in object-oriented programming. There is a UML basic tutorial in the appendix of this book, which is available for readers. Now introduce each class one by one. From the example we already know, the controls such as text box can be bound to the field, this fact itself contains the following information: The own attribute to be bound (such as Text Attribute) Available in data sources (such as DataTable) Bind navigation string .......... This information is encapsulated and forms a Binding class. Each control that implements data binding has at least one Binding object, indicating its binding information. There may be multiple controls on a form (such as a plurality of text boxes) bind to a data source, which means that there are multiple Binding objects. Therefore, there is a class to manage these objects. This class is a BindingManagerBase class, but this class is an abstract class. You cannot create an object directly. CurrencyManager directly inherit from the BindingManagerBase class, implement all the basic classes that are not implemented, can be used directly . The function moved in the data set is implemented by the CurrencyManager object. Now I know, I want to move in the data set, I have to get the CurrencyManager object corresponding to this dataset. How do you do this? A form will not only have multiple controls (such as multiple text boxes) bind to a data source, and there may be a case where multiple controls are bound to multiple data sources (assuming two DataSets in the form, respectively Provides data for two sets of controls, there are two CurrencyManager objects in the form), so a class is designed to manage these CurrencyManager objects, which is the function of the BindingContext class. There will be at least one BindingContext object on each form of data binding. Summary now: Gets the BindingContext object à by the BindingContext object à through the CurrencyManager object to implement the function in the recordset This is the insider that implements the mobile function in the data set. Said so much, in fact, only one sentence in the sample code: 'Data Binding Management Object Private cm as currencymanager '.......... 'Get data binding management objects cm = me.bindingcontext (ds1.tables (0)) Self-exploration: A container control, such as Form, GroupBox, and TabControl, you can have your own BindingContext object. The following code sets two Groupbox's BindingContext object: DIM BCG1 AS New BindingContext () DIM BCG2 As New BindingContext () Groupbox1.bindingContext = BCG1 Groupbox2.bindingContext = BCG2 Please design an app with two GroupBox, and some data binding controls are available in each GroupBox, which is programmed to implement data from different locations in the same data source. 3 Design data binding auxiliary class As can be seen from the above, the data binding function provided by ADO.NET is flexible, but it is indeed too complicated and is not easy to use, and the function of moving in the data set is very common if in each form. Repeat these codes, it is really boring and boring. Therefore, these code can be encapsulated as a data binding assisted class. The DataBindingHelper class is created for this purpose. First define some of the members variables of the class: 'Internal binding manager Private _cm as currencymanager = nothing 'The internal binding control container is usually the form Private _Container as Control = Nothing 'Revealing the currencymanager object to the outside world Public Readonly Property cm () AS CurrencyManager Get Return_cm END GET End Property 'Container Control Public property container () AS Control Get Return_Container END GET Set (ByVal Value As Control) _Container = Value End set End Property Then you need to provide a method to specify the data source: 'Data source properties, you can specify DataMember, or you can omit, for DataTable Public SUB setDataSource (byval DataSource As Object, Optional Byval DataMember AS String = Nothing) IF _Container is nothing then MSGBOX ("Please specify control containers first") EXIT SUB END IF If DataMember is nothing then _cm = _Container.BindingContext (Datasource) Else _cm = _Container.BindingContext (DataSource, DataMember) END IF End Sub Now you can achieve your mobile function, and then move a record as an example: Public Sub Movenext () '_cm is not created or empty, all exit IF_cm is nothing then EXIT SUB END IF IF _CM.COUNT = 0 THEN EXIT SUB END IF IF _CM.Position <_cm.count - 1 THEN _cm.position = 1 END IF End Sub With the DataBindingHelper class, it is very simple to move in the data set. The following is the code in the sample project: Dim DBH1 As New DataBindingHelper 'Set the container object as the form DBH1.CONTAINER = ME 'Setting the data source DBH1.SetDataSource (ds1.tables (0)) The mobile code is as follows: Private sub btnmovefirst1_click (...) Handles btnmovefirst1.click DBH1.MOVEFIRST () End Sub PRIVATE SUB BTNMOVEPREV_CLICK (...) Handles BtnmovePrev.click DBH1.MOVEPREV () End Sub Is it very simple? I believe that you can understand the benefits of object-oriented technology!
