ch07.htm

delphi自学的好教材！特别适合刚刚起步学习delphi的人员！同样对使用者具有参考价值！

<P>
<H2>TStrings</H2>
<P>The TStrings class is a VCL class that manages lists of strings. Several VCL components
use instances of TStrings to manage their data (usually text). For example, on Day
6 you used TStrings when you built the ScratchPad application. &quot;I don't recall
using a TStrings class,&quot; you say. Well, you did, but you just weren't aware
of it. Remember when you saved and loaded files? You used something like this:</P>
<P>
<PRE>Memo.Lines.SaveToFile(SaveDialog.FileName);
</PRE>
<P>The Lines property of TMemo is an instance of the TStrings class. The SaveToFile
method of TStrings takes the strings and saves them to a file on disk. You can use
the same technique to load a list box from a file on disk or save the contents of
a list box to disk. In the case of the TListBox class, the property that holds the
list box items is called Items. For example, try this exercise:</P>

<DL>
	<DT></DT>
	<DD><B>1. </B>Create a new application and place a ListBox component on the form.
	Size the list box as desired.
	<P>
	<DT></DT>
	<DD><B>2. </B>Change the Name property of the list box to ListBox.
	<P>
	<DT></DT>
	<DD><B>3. </B>Double-click the background of the form (not on the list box). The
	Code Editor displays the FormCreate function.
	<P>
	<DT></DT>
	<DD><B>4. </B>Modify the FormCreate function so that it looks like this:
	<P>
</DL>



<BLOCKQUOTE>
	<PRE>procedure TForm1.FormCreate(Sender: TObject);
var
  WinDir   : array [0..255] of Char;
  FileName : string;
begin
  GetWindowsDirectory(WinDir, SizeOf(WinDir));
  FileName := WinDir + `\win.ini';
  ListBox.Items.LoadFromFile(FileName);
end;</PRE>

</BLOCKQUOTE>

<PRE></PRE>

<DL>
	<DT></DT>
	<DD><B>5. </B>Click the Run button to compile and run the program.
	<P>
</DL>

<P>When the program runs, the list box will contain the contents of your WIN.INI
file. Using this method, it's easy to load a list box from any ASCII text data file.
The ComboBox component also has an Items property that works in exactly the same
way.</P>
<P>You can add, delete, insert, and move items in a list box, combo box, or memo
by calling the Add, Append, Delete, Insert, and Move methods of the TStrings class.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> How Add performs depends on the value of the Sorted property. If
	the Sorted property is set to True, Add will insert the string where it needs to
	be in the list of items. If Sorted is False, the new string will be added at the
	end of the list. 
<HR>


</BLOCKQUOTE>

<P>A component can be cleared of its contents by calling the Clear method. An individual
string can be accessed by using the array subscript operator. For example, to retrieve
the first string in a list of strings, you would use</P>
<P>
<PRE>Edit.Text := ListBox.Items[0];
</PRE>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> The strings in a TStrings class are actually contained in the Strings
	property. The Strings property is declared as the <I>default array property</I> for
	the TStrings class, so you don't specifically have to reference it when retrieving
	a specific string (although you can if you want to). Given that, then, the following
	two lines result in the same code being generated by the compiler:</P>
	<PRE>Edit.Text := ListBox.Items[0];
Edit.Text := ListBox.Items.Strings[0];</PRE>
	<P>
<HR>


</BLOCKQUOTE>

<PRE></PRE>
<P>Each string in a TStrings array contains the string itself and four bytes of extra
storage. This extra storage can be accessed through the Objects property, and you
can use the extra storage any way you like. Let's say, for example, that you create
an owner-drawn list box that displays bitmaps. You can store the string in the usual
way and store a pointer to the TBitmap object in the Objects array.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>TIP:</strong> There might be times when you need to manage a list of strings unrelated
	to a component. The TStringList class is provided for exactly that purpose. This
	class works just like TStrings but can be used outside of components. TStringList
	is particularly convenient for reading, manipulating, and storing text files. 
<HR>
<BR>
	
<HR>
<strong>NOTE:</strong> In reality, TStrings is what is called an <I>abstract base class</I>.
	An abstract base class is never used directly; it serves only as a base class from
	which to derive other classes. The Lines property is actually an instance of the
	TMemoStrings class rather than an instance of the TStrings class as I said in this
	section. This can be confusing because the Lines property <I>is</I> declared as a
	TStrings pointer but is actually an instance of TMemoStrings. The declaration and
	creation of the Lines property looks like this:</P>
	<PRE>var
  Lines : TStrings;
{ ...later }
  Lines := TMemoStrings.Create;</PRE>

</BLOCKQUOTE>

<PRE></PRE>


<BLOCKQUOTE>
	<P>This is why the Lines property appears to be a TStrings but is really not. I didn't
	mean to lead you astray, but I thought it was best to make this distinction after
	the discussion on TStrings rather than confuse you with this information during that
	discussion. 
<HR>


</BLOCKQUOTE>

<P><strong>New Term:</strong> An <I>abstract base class</I> is a class that cannot be used
directly. A descendent class must be created using the abstract base class, and an
instance of the descendent class is used instead.</P>
<P>
<H2><A NAME="Heading17"></A>Standard Windows Control Components</H2>
<P>Back in the Jurassic age, there was something called Windows 3.0. Windows 3.0
gave you options such as edit controls (single line and multiline), list boxes, combo
boxes, buttons, check boxes, radio buttons, and static controls. These controls must
have been fairly well designed because they are very prevalent in Windows programs
today--even considering all the new Win32 controls.</P>
<P>I'm not going to go over every Windows control and its corresponding VCL component.
There are a few points, though, that you should know regarding the standard components,
which are covered in the next sections.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> I will refer to components in one of two ways: by the component's
	name or by the name of the VCL class that defines the component. I might say, &quot;The
	Label component is used for...&quot; or I might say, &quot;TLabel is used for....&quot;
	In either case, I am talking about the same component. 
<HR>


</BLOCKQUOTE>

<P>
<H2><A NAME="Heading18"></A>Edit Controls</H2>
<P>Delphi comes with four edit-control components. The Edit, Memo, and MaskEdit components
are based on the standard Windows edit control. The RichEdit component is based on
the Win32 rich edit control, which is not one of the standard Windows controls. Still,
I will discuss RichEdit here because it has many features in common with the other
edit controls.</P>
<P>
<H4>The Edit Component</H4>
<P>The Edit component encapsulates the basic single-line edit control. This component
has no Align or Alignment property. It has no Alignment property because the text
in a single-line edit control can only be left-justified. The Edit component has
no Align property because it cannot (or more accurately, should not) be expanded
to fill the client area of a window.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>TIP:</strong> If you need text in an edit component to be right-justified or centered,
	use a Memo component but make its height the height of a standard Edit component.
	Then set the Alignment property as needed. 
<HR>
</P>
	<P>
<HR>
<strong>NOTE:</strong> Keep your forms standard whenever possible. Although you can make
	an Edit component as tall as you like, it will confuse users if you make its height
	greater than a standard Windows edit control (it might appear to the user to be a
	multiline edit). 
<HR>


</BLOCKQUOTE>

<H4><BR>
The MaskEdit Component</H4>
<P>The MaskEdit component is an Edit component with an input filter, or mask, attached.
The MaskEdit does not represent a Windows control per se, but rather is just a VCL
extension of a standard edit control. A mask is used to force input to a specific
range of numbers or characters. In addition, the mask can contain special characters
that are placed in the edit control by default. For example, a date is commonly formatted
as follows:</P>
<P>
<PRE>03/21/98
</PRE>
<P>An edit mask for a date can already have the slashes in place so the user only
has to enter the numbers. The edit mask would specify that only numbers can be entered
to avoid the possibility of the user entering a nonnumeric character.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> The DateTimePicker component (found on the Win32 tab) enables you
	to pick a date or a time from a specialized edit component. When the Kind property
	is set to dtkDate, the component displays a drop-down calendar from which the user
	can choose a date. When Kind is set to dtkTime, the DateTimePicker displays a multi-field
	edit control that enables the user to set the hours, minutes, seconds, and AM or
	PM. The DateTimePicker is preferred over the MaskEdit for date and time entry. 
<HR>


</BLOCKQUOTE>

<P>The EditMask property controls the mask that is used. When you click the ellipsis
button in the Value column for the EditMask property, the Input Mask Editor is displayed.
This dialog box enables you to choose from one of the predefined masks or to create
your own. You can choose prebuilt masks from several countries. Figure 7.3 shows
the Input Mask Editor displaying the United States' set of predefined input masks.</P>
<P><A HREF="javascript:popUp('28670703.gif')"><B>FIGURE 7.3.</B></A><B> </B><I>The
Input Mask Editor.</I></P>
<P>For more information on building your own masks, see the Delphi online help.</P>
<P>
<H4>The Memo Component</H4>
<P>The Memo component encapsulates a multiline edit control. The Lines property is
the most significant property in a Memo component. As I mentioned earlier in the
discussion on TStrings, the Lines property enables you to save the contents of the
Memo component to disk, load the Memo with text from a file, or access the memo's
lines individually.</P>
<P>The ScrollBars property is unique to the Memo component. This property enables
you to specify whether your component has a horizontal scrollbar, a vertical scrollbar,
or both. You used the ScrollBars property on Day 6 when you wrote the ScratchPad
application. The Memo component is a very versatile component that you will probably
find yourself using frequently.</P>
<P>
<H4>The RichEdit Component</H4>
<P>The RichEdit component is the biggest and the best of all the edit components;
it is based on the Win32-rich edit control. The RichEdit component enables you to
change fonts, use indentation, set text to bold, italic, or underlined, and much
more. Basically, the RichEdit component is a mini word processor in one neat package.
RichEdit has surprisingly few design-time properties over what the Memo component
has.</P>
<P>Key runtime properties include SelAttributes and Paragraph. The RichEdit component
is complex but easy to use, considering its complexities. See the Delphi online help
for full details on the RichEdit component.</P>
<P>
<H4>Common Edit Control Properties</H4>
<P>Table 7.4 lists the properties specific to components based on edit controls.</P>
<P>
<H4>TABLE 7.4. PROPERTIES FOR EDIT CONTROLS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Item</I></TD>
		<TD ALIGN="LEFT"><I>Applies To</I></TD>
		<TD ALIGN="LEFT"><I>Description</I></TD>
	</TR>
	<TR ALIGN="CENTER" VALIGN="TOP">
		<TD ALIGN="CENTER" COLSPAN="3">
			<P ALIGN="CENTER"><B>Properties</B>
		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">AutoSelect</TD>
		<TD ALIGN="LEFT">Edit, MaskEdit</TD>