Access a widget from an event handler in Java SWT

问题

I'm trying to design a small Java application with an UI using Java SWT: in Eclipse, I created a new Application Window and I added a button and a label. What I want is to make it so when I click the button, the label's text changes from "Not Clicked" to "Clicked". For this, I added an event handler for the button's SelectionEvent.

However, I found that I cannot access the label from inside the event handler, so that I can change it's text.

protected void createContents() {
    shell = new Shell();
    shell.setSize(450, 300);
    shell.setText("SWT Application");

    Button btnClickMe = new Button(shell, SWT.NONE);
    btnClickMe.addSelectionListener(new SelectionAdapter() {
        @Override
        public void widgetSelected(SelectionEvent arg0) {
            lblStatus.setText("Clicked"); // the compiler can't find lblStatus
        }
    });
    btnClickMe.setBounds(10, 10, 75, 25);
    btnClickMe.setText("Click Me");

    Label lblStatus = new Label(shell, SWT.NONE);
    lblStatus.setBounds(10, 47, 75, 15);
    lblStatus.setText("Not clicked.");

}

I realize this is probably a dumb question, but I've been searching for a fix to no avail. I'm quite new to using Java widgets (only worked with C# in VS until now).

回答1:

You have to declare lblStatus before referencing it. There is no hoisting like in JavaScript. Right now, your are declaring the label after the event handler.

回答2:

To have access to lblStatus you should declare it as the class instance variable.

public class MyClass {
    Label lblStatus;

protected void createContents() {
    shell = new Shell();
    shell.setSize(450, 300);
    shell.setText("SWT Application");

    Button btnClickMe = new Button(shell, SWT.NONE);
    btnClickMe.addSelectionListener(new SelectionAdapter() {
        @Override
        public void widgetSelected(SelectionEvent arg0) {
            lblStatus.setText("Clicked"); // the compiler is aware of lblStatus
        }
    });
    btnClickMe.setBounds(10, 10, 75, 25);
    btnClickMe.setText("Click Me");

    lblStatus = new Label(shell, SWT.NONE);
    lblStatus.setBounds(10, 47, 75, 15);
    lblStatus.setText("Not clicked.");

}
}

来源：https://stackoverflow.com/questions/33241905/access-a-widget-from-an-event-handler-in-java-swt

标签

java

swt