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Implementing a Grid Layout Manager with Positionable Components

Implementing a Grid Layout Manager with Positionable Components

Why Write Custom Layout Managers? Layout manager objectifies the component layout strategy. It disentangles component layout code from the drawing code of the container of components. By isolating the implementation of a layout strategy in a separate class, a programmer can reuse it by simply assigning an instance of this layout class to the container that requires it. Without a layout manager the layout code would have to be embedded within drawing code of the container itself.

Java's AWT supports this concept in the LayoutManager interface and supplies five implementations that are sufficient in most situations. It's sometimes more efficient and effective, however, to implement one's own layout manager, designed specifically for a particular task, rather than struggling with the standard layout managers.

Let's say we need to draw components on a grid, for example, but need to specify the positions of the components arbitrarily at any time. On the first take we'd probably choose AWT's GridLayout as the layout manager - that is, until we discover that it will only position components in sequence. The programmer then would have no control over their positions. The alternative would be to use GridBagLayout, but that's way too complicated and difficult to control for such a simple task. The solution is to write our own LayoutManager. As you'll see, this isn't as difficult as it sounds and may be the best solution in many situations. This article introduces a grid layout manager with positionable components, and with flexibility and complexity somewhere between AWT's GridLayout and GridBagLayout.

Layout Manager Basics:LayoutManager Interface
AWT's LayoutManager interface declares the following methods that have to be implemented in a valid layout manager:

  • addLayoutComponent
  • removeLayoutComponent
  • layoutContainer
  • minimumLayoutSize
  • preferredLayoutSize

    AddLayoutComponent and removeLayoutComponent let the Container assigned with this layout add and remove a Component to and from the layout. LayoutContainer implements the layout strategy, changing the x and y pixel position and the dimension of the Components as necessary. MinimumLayoutSize and preferredLayoutSize let the Container find out the minimum required and preferred layout size given the Components to be laid out in it.

    AWT's Containers are assigned default LayoutManagers but a different one may be chosen using the Container's setLayout method. When you add a Component to a Container, the Container will in turn add it to its assigned LayoutManager. The same happens when you remove a Component. Whenever a Component is added or removed, the Container will call the layoutContainer of the assigned LayoutManager to update the position and size of all Components. If you need to lay out the Container again, you may invoke the Container's doLayout method to recompute the layout.

    PositionableGridLayout Class
    We call our grid layout with positionable components the PositionableGridLayout class. The minimumLayoutSize for this case should be the extent of the layout embodying the outermost component. That would be our preferredLayoutSize as well. In other words, minimumLayoutSize is the x grid position of the easternmost Component times the width of each grid and the y grid position of the southernmost Component times the height of each grid. This is shown in Listing 1.

    LayoutContainer calculates the pixel location of each Component given its grid position, and relocates it with a call to the Component's setBounds method if necessary.

    Note that we did not implement addLayoutComponent and removeLayoutComponent because nothing special needs to be done when a Component is added to or removed from the Container.

    Listing 2 shows the implementation of PositionableGridLayout for Java 1.1.

    Layout Constraints: PositionableGridConstraints Class
    An automatic LayoutManager - that is, one that would not allow the programmer to control the location and size of each Component - would normally be able to work alone. For a LayoutManager that allows the programmer to individually fix the location and size of the Components, however, an assisting class is required. Such is the case with AWT's GridBagLayout, which requires GridBagConstraints to work. In our example we need to be able to position any Component at an arbitrary location at any time. Following the example of GridBagLayout, we implement an assisting class, the PositionableGridConstraints, which allows us to specify the grid position of an added component.

    The PositionableGridConstraints class is fairly simple to implement. It mainly stores away the grid position of a Component.

    /**
    * Constructs a PositionableGridConstraints.
    */
    public PositionableGridConstraints( int gridx, int gridy, ... )
    {
    this.gridx = gridx;
    this.gridy = gridy;
    :
    }

    We need to assign the grid constraints to the Component, of course. Again, following the example set by AWT's GridBagLayout, we implement a setConstraints method in PositionableGridLayout whose purpose is to associate the grid constraints to a Component in a hashtable using the Component as the key, as shown in Listing 3. Listing 4 shows the implementation of PositionableGridConstraints.

    Using the PositionableGridLayout Class
    A good example of the use of the PositionableGridLayout class would be the familiar sliding puzzle. Initially, the tiles lie in a grid. As the user shifts the tiles' position, the application has to change the coordinate of each piece. Listing 3 shows the implementation of a simple 3x3 puzzle, the core of which follows it. Note in Listing 5 the convenience of being able to position Components by grid position instead of pixel location.

    Conclusion
    We've shown the use of LayoutManager beyond the standard AWT-supplied implementation. We've shown how customized LayoutManager can be a powerful tool for implementing unique Component layout strategy for any Container. In a future article we'll make use of the PositionableGridLayout class and the Widget interface class featured in an earlier issue of JDJ (Vol. 3, Issue 6) to implement a TreeViewer widget, a component for laying out trees in either horizontal or vertical format.

    Download Source Code
    The program in this article requires the callback and widget code from the Implementing Callback and Widget-izing AWT articles published in JDJ (Vol. 3, Issues 4 and 6, respectively). Full source code (including a complete version of Puzzle) can be downloaded free from www.wigitek.com. A more extensive version supporting programmer-defined Component size, autoadjustment and alignment for Java versions 1.0 and 1.1 is also available from Wigitek Corporation at the same Web site.

  • More Stories By Daniel Dee

    Daniel Dee has more than 10 years of experience working in the development of GUI software toolkits, starting with X Windows and then Java, since their inception. He is currently the president of Wigitek Corporation, a company providing software tools and consulting services for the development Java-based data-driven dynamic graphics software. He received an MS degree in Computer System Engineering from the University of Massachusetts.

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