docs/latex/wx/tconstr.tex

   1 \section{Constraints overview}\label{constraintsoverview}
   2
   3 Classes: \helpref{wxLayoutConstraints}{wxlayoutconstraints}, \helpref{wxIndividualLayoutConstraint}{wxindividuallayoutconstraint}.
   4
   5 Objects of class wxLayoutConstraint can be associated with a window to define
   6 the way it is laid out, with respect to its siblings or the parent.
   7
   8 The class consists of the following eight constraints of class wxIndividualLayoutConstraint,
   9 some or all of which should be accessed directly to set the appropriate
  10 constraints.
  11
  12 \begin{itemize}\itemsep=0pt
  13 \item {\bf left:} represents the left hand edge of the window
  14 \item {\bf right:} represents the right hand edge of the window
  15 \item {\bf top:} represents the top edge of the window
  16 \item {\bf bottom:} represents the bottom edge of the window
  17 \item {\bf width:} represents the width of the window
  18 \item {\bf height:} represents the height of the window
  19 \item {\bf centreX:} represents the horizontal centre point of the window
  20 \item {\bf centreY:} represents the vertical centre point of the window
  21 \end{itemize}
  22
  23 The constraints are initially set to have the relationship wxUnconstrained,
  24 which means that their values should be calculated by looking at known constraints.
  25 To calculate the position and size of the control, the layout algorithm needs to
  26 know exactly 4 constraints (as it has 4 numbers to calculate from them), so you
  27 should always set exactly 4 of the constraints from the above table.
  28
  29 If you want the controls height or width to have the default value, you may use
  30 a special value for the constraint: wxAsIs. If the constraint is wxAsIs, the
  31 dimension will not be changed which is useful for the dialog controls which
  32 often have the default size (e.g. the buttons whose size is determined by their
  33 label).
  34
  35 The constrains calculation is done in \helpref{wxWindow::Layout}{wxwindowlayout}
  36 function which evaluates constraints. To call it you can either call
  37 \helpref{wxWindow::SetAutoLayout}{wxwindowsetautolayout} if the parent window
  38 is a frame, panel or a dialog to tell default OnSize handlers to call Layout
  39 automatically whenever the window size changes, or override OnSize and call
  40 Layout yourself (note that you do have to call
  41 \helpref{Layout}{wxwindowlayout} yourself if the parent window is not a
  42 frame, panel or dialog).
  43
  44 \subsection{Constraint layout: more detail}
  45
  46 By default, windows do not have a wxLayoutConstraints object. In this case, much layout
  47 must be done explicitly, by performing calculations in OnSize members, except
  48 for the case of frames that have exactly one subwindow (not counting toolbar and
  49 statusbar which are also positioned by the frame automatically), where wxFrame::OnSize
  50 takes care of resizing the child to always fill the frame.
  51
  52 To avoid the need for these rather awkward calculations, the user can create
  53 a wxLayoutConstraints object and associate it with a window with wxWindow::SetConstraints.
  54 This object contains a constraint for each of the window edges, two for the centre point,
  55 and two for the window size. By setting some or all of these constraints appropriately,
  56 the user can achieve quite complex layout by defining relationships between windows.
  57
  58 In wxWindows, each window can be constrained relative to either its {\it
  59 siblings} on the same window, or the {\it parent}. The layout algorithm
  60 therefore operates in a top-down manner, finding the correct layout for
  61 the children of a window, then the layout for the grandchildren, and so
  62 on. Note that this differs markedly from native Motif layout, where
  63 constraints can ripple upwards and can eventually change the frame
  64 window or dialog box size. We assume in wxWindows that the {\it user} is
  65 always `boss' and specifies the size of the outer window, to which
  66 subwindows must conform. Obviously, this might be a limitation in some
  67 circumstances, but it suffices for most situations, and the
  68 simplification avoids some of the nightmarish problems associated with
  69 programming Motif.
  70
  71 When the user sets constraints, many of the constraints for windows
  72 edges and dimensions remain unconstrained. For a given window,
  73 the wxWindow::Layout algorithm first resets all constraints
  74 in all children to have unknown edge or dimension values, and then iterates through the constraints,
  75 evaluating them. For unconstrained edges and dimensions, it
  76 tries to find the value using known relationships that always hold. For example,
  77 an unconstrained {\it width} may be calculated from the {\it left} and {\it right edges}, if
  78 both are currently known. For edges and dimensions with user-supplied constraints, these
  79 constraints are evaluated if the inputs of the constraint are known.
  80
  81 The algorithm stops when all child edges and dimension are known (success), or
  82 there are unknown edges or dimensions but there has been no change in this cycle (failure).
  83
  84 It then sets all the window positions and sizes according to the values it has found.
  85
  86 Because the algorithm is iterative, the order in which constraints are considered is
  87 irrelevant, however you may reduce the number of iterations (and thus speed up
  88 the layout calculations) by creating the controls in such order that as many
  89 constraints as possible can be calculated during the first iteration. For example, if
  90 you have 2 buttons which you'd like to position in the lower right corner, it is
  91 slightly more efficient to first create the second button and specify that its
  92 right border IsSameAs(parent, wxRight) and then create the first one by
  93 specifying that it should be LeftOf() the second one than to do in a more
  94 natural left-to-right order.
  95
  96 \subsection{Window layout examples}\label{layoutexamples}
  97
  98 \subsubsection{Example 1: subwindow layout}
  99
 100 This example specifies a panel and a window side by side,
 101 with a text subwindow below it.
 102
 103 \begin{verbatim}
 104   frame->panel = new wxPanel(frame, -1, wxPoint(0, 0), wxSize(1000, 500), 0);
 105   frame->scrollWindow = new MyScrolledWindow(frame, -1, wxPoint(0, 0), wxSize(400, 400), wxRETAINED);
 106   frame->text_window = new MyTextWindow(frame, -1, wxPoint(0, 250), wxSize(400, 250));
 107
 108   // Set constraints for panel subwindow
 109   wxLayoutConstraints *c1 = new wxLayoutConstraints;
 110
 111   c1->left.SameAs       (frame, wxLeft);
 112   c1->top.SameAs        (frame, wxTop);
 113   c1->right.PercentOf   (frame, wxWidth, 50);
 114   c1->height.PercentOf  (frame, wxHeight, 50);
 115
 116   frame->panel->SetConstraints(c1);
 117
 118   // Set constraints for scrollWindow subwindow
 119   wxLayoutConstraints *c2 = new wxLayoutConstraints;
 120
 121   c2->left.SameAs       (frame->panel, wxRight);
 122   c2->top.SameAs        (frame, wxTop);
 123   c2->right.SameAs      (frame, wxRight);
 124   c2->height.PercentOf  (frame, wxHeight, 50);
 125
 126   frame->scrollWindow->SetConstraints(c2);
 127
 128   // Set constraints for text subwindow
 129   wxLayoutConstraints *c3 = new wxLayoutConstraints;
 130   c3->left.SameAs       (frame, wxLeft);
 131   c3->top.Below         (frame->panel);
 132   c3->right.SameAs      (frame, wxRight);
 133   c3->bottom.SameAs     (frame, wxBottom);
 134
 135   frame->text_window->SetConstraints(c3);
 136 \end{verbatim}
 137
 138 \subsubsection{Example 2: panel item layout}
 139
 140 This example sizes a button width to 80 percent of the panel width, and centres
 141 it horizontally. A listbox and multitext item are placed below it. The listbox
 142 takes up 40 percent of the panel width, and the multitext item takes up
 143 the remainder of the width. Margins of 5 pixels are used.
 144
 145 \begin{verbatim}
 146   // Create some panel items
 147   wxButton *btn1 = new wxButton(frame->panel, -1, "A button") ;
 148
 149   wxLayoutConstraints *b1 = new wxLayoutConstraints;
 150   b1->centreX.SameAs    (frame->panel, wxCentreX);
 151   b1->top.SameAs        (frame->panel, wxTop, 5);
 152   b1->width.PercentOf   (frame->panel, wxWidth, 80);
 153   b1->height.PercentOf  (frame->panel, wxHeight, 10);
 154   btn1->SetConstraints(b1);
 155
 156   wxListBox *list = new wxListBox(frame->panel, -1, "A list",
 157                                   wxPoint(-1, -1), wxSize(200, 100));
 158
 159   wxLayoutConstraints *b2 = new wxLayoutConstraints;
 160   b2->top.Below         (btn1, 5);
 161   b2->left.SameAs       (frame->panel, wxLeft, 5);
 162   b2->width.PercentOf   (frame->panel, wxWidth, 40);
 163   b2->bottom.SameAs     (frame->panel, wxBottom, 5);
 164   list->SetConstraints(b2);
 165
 166   wxTextCtrl *mtext = new wxTextCtrl(frame->panel, -1, "Multiline text", "Some text",
 167                         wxPoint(-1, -1), wxSize(150, 100), wxTE_MULTILINE);
 168
 169   wxLayoutConstraints *b3 = new wxLayoutConstraints;
 170   b3->top.Below         (btn1, 5);
 171   b3->left.RightOf      (list, 5);
 172   b3->right.SameAs      (frame->panel, wxRight, 5);
 173   b3->bottom.SameAs     (frame->panel, wxBottom, 5);
 174   mtext->SetConstraints(b3);
 175 \end{verbatim}
 176
 177