| Try to understand SWT philosophy for extensibility. [message #327064] |
Tue, 08 April 2008 05:55  |
Eclipse User |
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Hi,
I'm quite new in the SWT/RCP world.
Sometimes the builtin classes don't fully meet my needs. One way of
solving this kind of problems is inheritance.
Example 1:
------------
Instead of this:
TableColumn column = new TableColumn(table, SWT.LEFT);
column.setText("Cabecera");
column.setWidth(150);
I like to write this (more compact):
new TableColumn(table, SWT.LEFT).sText("Cabecera").sWidth(150);
public class TableColumn extends org.eclipse.swt.widgets.TableColumn {
protected int index;
public TableColumn(Table parent, int style) {
super(parent, style);
index = parent.getColumnCount()-1;
}
public TableColumn sText(String string) {
super.setText(string);
return this;
}
public TableColumn sWidth(int width) {
super.setWidth(width);
return this;
}
protected void checkSubclass () {
}
}
The documentation says TableColumn cannot be derived.
So, I had to also override 'checkSubclass()'.
Example 2:
------------
SWT Table only supports ordering by one column. I want to implement
multicolumn sorting.
Problem: TableColumn#setSortDirection() isn't public. It could be
protected.
Solution (None of them I like):
1- Changing source code of my private copy of SWT
2- Using AspectJ or similar
3- Use reflection to invoke non-public methods.
It's a little hard to work with a framework not designed for extensibility.
I know it's a project used by many other ones, and the public API must be
very well delimited.
Any ideas?
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| Re: Try to understand SWT philosophy for extensibility. [message #327067 is a reply to message #327066] |
Tue, 08 April 2008 07:38  |
| Eclipse User |
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Thanks Tom for your explanation.
Example 1 is a bad example, but shows that subclassing isn't such a bad
practice.
Luckily, the classes aren't declared final, so subclassing is possible
without patching SWT.
Tom Schindl wrote:
> Well instead of subclassing you could use a Builder-Pattern:
> Builder.buildTableColumn(table,SWT.LEFT).setText("Cabecera").setWidth(150);
> public class Builder {
> public buildTableColumnn(Table table, int style) {
> return new TableColumnBuilder(new TableColumn(table,SWT.LEFT));
> }
> }
> public class TableColumnBuilder {
> private TableColumn column;
> public TableColumnBuilder(TableColumn column) {
> this.column = column;
> }
> public TableColumnBuilder setText(String text) {
> this.column.setText(text);
> return this;
> }
> public TableColumnBuilder setWidth(int width) {
> this.column.setWidth(width);
> return this;
> }
> }
> You already mentionned the reason SWT says you should not subclass it
> because keeping an API contract up with subclassing is even harder than
> without.
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