Would it not be great configuring and switching dynamically, the layout of an application, as it were a property like a Color or a Font? Is there any layout manager that can do it?
By layout, we mean arranging and resizing the GUI components in a dialog. This can be done manually (e.g. using an editor), or automatically by a software piece called a layout manager. In both cases, laying out has always been something difficult to configure, change, and in most cases, impossible to be switched dynamically. Difficult until now, of course! In this article, we will see how easy it is to layout with EvaLayout.
Note that the flexibility and configuration of layouts, as it will be described, plays not only an important role in the look of the application, but also when modifying it, coding variants, and in the implementation of generic and rich components.
There are lots of layout managers, and also quite a different number of approaches. EvaLayout belongs to the group of grid layouts. The concepts of columns, rows, filling space, spanning columns and rows etc. is the basically the same in all these systems. What is new in EvaLayout is the easy, clear, and flexible way of representing the whole layout information in a single text and, not less important, the decoupling between layout information and physical components that makes possible the advantages mentioned in the introduction.
Layout Info and Text Format in EvaLayout
Let us see the rules for defining an EvaLayout.
EvaLayout places all components within the cells of a grid - or table - of n columns and m rows. For the whole grid, the following can be defined:
- A symmetric (same for left and right) horizontal margin
- A symmetric (same for top and bottom) vertical margin
- A horizontal gap (space between two adjacent columns)
- A vertical gap (space between two adjacent rows)
For columns and rows, a header for each one determines its behaviour. There are three possibilities for these headers:
- A (default): The size of the column/row will be adapted to the minimum required by the components in this column/row. That means, the maximum of the default sizes.
- X: For expandable column/row. All expandable columns/rows will share the remaining space equally.
- Number: Representing the width/height in pixels that the column/row has to have.
And finally, each cell in the grid may contain one of the following things:
- blank, nothing is specified.
- a name, it represents a logical name of a component, the cell where this name appears is the upper-left cell of the component.
- sign -, a component that wants to occupy more cells at its right side (spanning columns) has to use this symbol in those cells.
- sign +, a component that wants to occupy more cells at lower rows (spanning rows) has to use this symbol in that cells.
All this is given to the layout manager object (
EvaLayoutManager) in a single text where the information is separated by commas. In the first line, the general margins and gaps are given, and the rest is for specifying the grid or the table. Note that the white spaces or blanks are only to make it more readable, they are actually not needed by
EvaLayoutManager. Let us see an example. The text:
EvaLayout, 10, 10, 5, 5
grid, 75 , X , A ,
A, boton1 , memo , - ,
A, boton2 , + , ,
A, boton3 , + , ,
X, , + , ,
A, text1 , - , boton4 ,
describes a layout with:
- 10 pixels of left, right, upper, and bottom margins
- 5 pixels of horizontal and vertical gaps
- A first column of 75 pixels, a second one expandable, and a third one adaptable
- Five rows, all adaptable except the one that is expandable
- A memo component that spans vertically 3 rows and horizontally 1 column
- A text component that spans horizontally 1 column
The whole design could look like:
and the resultant dialog is:
Now, we will see how to implement it in a C++ Windows application. We will show how to do it using the Windows API as it is done in the demo project, but it is also possible to do it in a MFC application and there is a how-to text that explains it.
Using the Code
For the implementation, we will need the following objects (note that except
EvaLayoutInfo, the rest could be local objects):
We divide the steps to use
EvaLayoutManager, in four:
- Preparing or loading the layout Info
- Setting the layout info into the manager
- Declaring the real components to handle
- Doing the layout (positioning and resizing the components)
Preparing or Loading the Layout Info
The preparation can be done, for instance, in the message
WM_INITDIALOG, and consists in setting at least an
Eva object or, in order to load more layouts, an
EvaUnit object that can be used as if it were an array of Evas. This preparation can be done in two ways:
BOOL CALLBACK MainDlgProc(HWND hDlg, UINT uMsg, WPARAM wParam, LPARAM lParam)
layInfo.addLine (EvaLine ("EvaLayout, 10, 10, 5, 5"));
layInfo.addLine (EvaLine (" grid, 75 , X , A ,"));
layInfo.addLine (EvaLine (" A, boton1 , memo , - ,"));
layInfo.addLine (EvaLine (" A, boton2 , + , ,"));
layInfo.addLine (EvaLine (" A, boton3 , + , ,"));
layInfo.addLine (EvaLine (" X, , + , ,"));
layInfo.addLine (EvaLine (" A, edit1 , - , boton4 ,"));
- From a file (Eva format, see the file WinLayouts.eva in the demo project):
BOOL CALLBACK MainDlgProc(HWND hDlg, UINT uMsg, WPARAM wParam, LPARAM lParam)
eFile.load (eUnit, "WinLayouts.eva", "data");
layInfo = eUnit;
Setting the Layout Info into the Manager
This is just a call to the method
setLayout of the layout manager. The message
WM_INITDIALOG is a good place for that too, but note that this can also be performed wherever and whenever, to dynamically change the layout. After doing that, you should force a repaint, or better, a resize message. In the demo project, it is done somehow "tricky" but I think effectively.
Declaring the Real Components to Handle
This step associates the physical window handle of the component with its logical name given in the layout info. This is thought to be done once but it could be carried out more times. The components that appear in our layout info, and all the components associated with the dialog are to be added here. The layout manager shows or hides the components according to the layout info, but it has to know all of them.
manager.addComponent("memo", GetDlgItem(hDlg, ID_MEMO));
manager.addComponent("boton1", GetDlgItem(hDlg, ID_BUTTON_1));
manager.addComponent("boton2", GetDlgItem(hDlg, ID_BUTTON_2));
manager.addComponent("boton3", GetDlgItem(hDlg, ID_BUTTON_3));
Doing the Layout (Positioning and Resizing the Components)
It should be enough to do it in the
WM_SIZE message handler.
BOOL CALLBACK MainDlgProc(HWND hDlg, UINT uMsg,
WPARAM wParam, LPARAM lParam)
Conclusions and Future Versions
In spite of its simplicity, this layout can do almost everything that is reasonably needed in normal applications.
This layout is available in java, C++ (this article) and now also in java script! (more about this very soon)
Version 4 Composition and Masking components
This version add two new features to the C++ EvaLayout library that makes much more easier defining complex layouts and permits changing dynamically not only all but also a part of the layout .
Composition of layouts
Given that we pass to the manager several layouts (or layout info). By composition it is meant that a layout can reference not only components but also another layout and this will be resolved by the manager automatically. This facilitates a lot defining complex layout by dividing it into smaller pieces.
Component Mask Mechanism
A new function in the class EvaLayoutManager called maskElement permits the replacement of one component (widget or layout) by another one. In other words the ability of changing a part of the window dynamically in a straightforward way.
Both features are shown in the demo included in the EvaLayoutV4 download.
- 22.04.2006 - First release
- 02.01.2008 - Fixed some bugs and added a cleaner MFC example
- 21.01.2016 - Version4 Layout composition and Mask mechanism