问题
sometime ago I read this article that shows a way to implement mouse resizable components in Swing.
The author uses a null LayoutManager in order to allow absolute component positioning. I know that a null layout should never be used, so my question is:
is there any already implemented LayoutManager that allow component's absolute positioning, or I have to implement it my own?
回答1:
A layout manager really does 3 things:
Set the location of a component. Since you need the ability to drag the component around, you would not want your layout manager to do this.
Set the size of a component. Since you need the ability to resize the component then you would not want to do this. However, you might want to give the component a default size based on the components preferred size. This way you don't need to specify the size when you create the component.
Determine the preferred size of the parent panel based on the components added to it. This will allow scroll panes to function properly as scrollbars can be added/removed as required. So you need to determine the behaviour of how dragging should work. That is, are you allowed to drag the component outside the current bounds of the panel. If so the the preferred size of the panel should automatically increase.
is there any already implemented LayoutManager that allow component's absolute positioning
I've been playing around with a layout manager that is close to your needs. It was designed to be used with the ComponentMover class from the Moving Windows link provided by trashgod.
Here is my test code for this class:
import java.awt.*;
import javax.swing.*;
import javax.swing.border.*;
/**
*/
public class DragLayout implements LayoutManager, java.io.Serializable
{
public DragLayout()
{
}
/**
* Adds the specified component with the specified name to the layout.
* @param name the name of the component
* @param comp the component to be added
*/
@Override
public void addLayoutComponent(String name, Component comp) {}
/**
* Removes the specified component from the layout.
*
* @param comp the component to be removed
*/
@Override
public void removeLayoutComponent(Component component)
{
}
/**
* Determine the minimum size on the Container
*
* @param target the container in which to do the layout
* @return the minimum dimensions needed to lay out the
* subcomponents of the specified container
*/
@Override
public Dimension minimumLayoutSize(Container parent)
{
synchronized (parent.getTreeLock())
{
return preferredLayoutSize(parent);
}
}
/**
* Determine the preferred size on the Container
*
* @param parent the container in which to do the layout
* @return the preferred dimensions to lay out the
* subcomponents of the specified container
*/
@Override
public Dimension preferredLayoutSize(Container parent)
{
synchronized (parent.getTreeLock())
{
return getLayoutSize(parent);
}
}
/*
* The calculation for minimum/preferred size it the same. The only
* difference is the need to use the minimum or preferred size of the
* component in the calculation.
*
* @param parent the container in which to do the layout
*/
private Dimension getLayoutSize(Container parent)
{
Insets parentInsets = parent.getInsets();
int x = parentInsets.left;
int y = parentInsets.top;
int width = 0;
int height = 0;
// Get extreme values of the components on the container
for (Component component: parent.getComponents())
{
if (component.isVisible())
{
Point p = component.getLocation();
Dimension d = component.getPreferredSize();
x = Math.min(x, p.x);
y = Math.min(y, p.y);
width = Math.max(width, p.x + d.width);
height = Math.max(height, p.y + d.height);
}
}
// Width/Height is adjusted if any component is outside left/top edge
if (x < parentInsets.left)
width += parentInsets.left - x;
if (y < parentInsets.top)
height += parentInsets.top - y;
// Adjust for insets
width += parentInsets.right;
height += parentInsets.bottom;
Dimension d = new Dimension(width, height);
return d;
// return new Dimension(width, height);
}
/**
* Lays out the specified container using this layout.
*
* @param target the container in which to do the layout
*/
@Override
public void layoutContainer(Container parent)
{
synchronized (parent.getTreeLock())
{
Insets parentInsets = parent.getInsets();
int x = parentInsets.left;
int y = parentInsets.top;
// Get X/Y location outside the bounds of the panel
for (Component component: parent.getComponents())
{
if (component.isVisible())
{
Point location = component.getLocation();
x = Math.min(x, location.x);
y = Math.min(y, location.y);
}
}
x = (x < parentInsets.left) ? parentInsets.left - x : 0;
y = (y < parentInsets.top) ? parentInsets.top - y : 0;
// Set bounds of each component
for (Component component: parent.getComponents())
{
if (component.isVisible())
{
Point p = component.getLocation();
Dimension d = component.getPreferredSize();
component.setBounds(p.x + x, p.y + y, d.width, d.height);
}
}
}}
/**
* Returns the string representation of this column layout's values.
* @return a string representation of this layout
*/
public String toString()
{
return "["
+ getClass().getName()
+ "]";
}
public static void main( String[] args )
{
ComponentMover cm = new ComponentMover();
cm.setEdgeInsets( new Insets(-100, -100, -100, -100) );
// cm.setEdgeInsets( new Insets(10, 10, 10, 10) );
cm.setAutoLayout(true);
JPanel panel = new JPanel( new DragLayout() );
panel.setBorder( new MatteBorder(10, 10, 10, 10, Color.YELLOW) );
createLabel(cm, panel, "North", 150, 0);
createLabel(cm, panel, "West", 0, 100);
createLabel(cm, panel, "East", 300, 100);
createLabel(cm, panel, "South", 150, 200);
createLabel(cm, panel, "Center", 150, 100);
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add( new JScrollPane(panel) );
frame.pack();
frame.setLocationRelativeTo( null );
frame.setVisible( true );
}
public static void createLabel(ComponentMover cm, JPanel panel, String text, int x, int y)
{
JLabel label = new JLabel( text );
label.setOpaque(true);
label.setBackground( Color.ORANGE );
label.setLocation(x, y);
panel.add( label );
cm.registerComponent( label );
}
}
For this layout the size is always assumed to be the preferred size. You would need to change this. Maybe set the size to be the preferred size when the size is (0, 0). You will also need to use the size of the component (not its preferred size) when determining the preferred size of the parent container.
The ComponentMover class can be configured to allow you to drag comopnents outside the bounds of the parent container or to keep the component inside the bounds. If you allow components to be moved outside the bounds, then the preferred size is automatically adjusted to take into account the new location of the component.
If you drag a component outside the top or left bounds, then all the components are shifted (right or down) do make sure no component has a negative location.
回答2:
As alternatives, also consider
How to Use Internal Frames.
Resizing Components in conjunction with Moving Windows.
An exisiting framework, such as JGraph or JUNG.
回答3:
I guess it would depend on the specifics of how you wanted it to behave.
The main reason the null layout manager is discouraged is because of the fact that interfaces built using that can only be used in the size they were designed - You can't resize the UI. If this is fine for you, use it.
Another option I know of is the AbsoluteLayout that Netbeans is distributed with. You can get more info here: http://www.java-tips.org/other-api-tips/netbeans/can-i-distribute-absolutelayout-with-my-applica.html. I think this might be exactly what you are looking for, but as you can see from that link, they recommend rather using a Null layout... I don't think it makes much of a difference either way.
If you need to be able to allow users to define how the components will resize as well, you'll end up building something like the Netbeans Matisse form designer, which is probably overkill and does not strike me as much fun :)
回答4:
The question is somewhat vague, so I might be missing the point completely. I assume that you are looking for a layout that will allow you to use absolute positioning, but will still allow you to resize the component and use all available space.
If you are handcoding it, I've had success with MIGLayout (http://www.miglayout.com/) and TableLayout (Which is less absolute but very easy to use - http://java.sun.com/products/jfc/tsc/articles/tablelayout/)
If you are using some Form designer, using GroupLayout might be a good choice, but you do not want to hand-code it. See this question : GroupLayout: Is it worth learning?
来源：https://stackoverflow.com/questions/7750538/suitable-layoutmanager-for-resizable-components