feat: implement PreserveStateAddon

Part of #14

Author: mvysny

src/main/java/com/flowingcode/vaadin/addons/xterm/PreserveStateAddon.java

@@ -0,0 +1,146 @@
+package com.flowingcode.vaadin.addons.xterm;
+
+import java.util.Objects;
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * Add-on which preserves the client-side state when the component is removed
+ * from the UI then reattached later on.
+ * <p></p>
+ * To use this addon, simply create the addon then make sure to call all {@link ITerminal}
+ * and {@link ITerminalOptions} methods via this addon:
+ * <pre>
+ * final XTerm xterm = new XTerm();
+ * final PreserveStateAddon addon = new PreserveStateAddon(xterm);
+ * addon.writeln("Hello!");
+ * addon.write("$ ");
+ * </pre>
+ */
+public class PreserveStateAddon implements ITerminal {
+    private final XTermBase xterm;
+    private final StringBuilder scrollbackBuffer = new StringBuilder();
+    /**
+     * All commands are properly applied before the first attach; they're just
+     * not preserved after subsequent detach/attach.
+     */
+    private boolean wasDetachedOnce = false;
+
+    public PreserveStateAddon(XTerm xterm) {
+        this.xterm = Objects.requireNonNull(xterm);
+        xterm.addAttachListener(e -> {
+            if (wasDetachedOnce) {
+                xterm.write(scrollbackBuffer.toString());
+            }
+        });
+        xterm.addDetachListener(e -> wasDetachedOnce = true);
+        xterm.addLineListener(e -> {
+            // also make sure that any user input ends up in the scrollback buffer.
+            scrollbackBuffer.append(e.getLine());
+            scrollbackBuffer.append('\n');
+        });
+    }
+
+    @Override
+    public void blur() {
+        xterm.blur();
+    }
+
+    @Override
+    public void focus() {
+        xterm.focus();
+    }
+
+    @Override
+    public CompletableFuture<Boolean> hasSelection() {
+        return xterm.hasSelection();
+    }
+
+    @Override
+    public CompletableFuture<String> getSelection() {
+        return xterm.getSelection();
+    }
+
+    @Override
+    public void clearSelection() {
+        xterm.clearSelection();
+    }
+
+    @Override
+    public void select(int column, int row, int length) {
+        xterm.select(column, row, length);
+    }
+
+    @Override
+    public void selectAll() {
+        xterm.selectAll();
+    }
+
+    @Override
+    public void selectLines(int start, int end) {
+        xterm.selectLines(start, end);
+    }
+
+    @Override
+    public void scrollLines(int amount) {
+        xterm.scrollLines(amount);
+    }
+
+    @Override
+    public void scrollPages(int pageCount) {
+        xterm.scrollPages(pageCount);
+    }
+
+    @Override
+    public void scrollToTop() {
+        xterm.scrollToTop();
+    }
+
+    @Override
+    public void scrollToBottom() {
+        xterm.scrollToBottom();
+    }
+
+    @Override
+    public void scrollToLine(int line) {
+        xterm.scrollToLine(line);
+    }
+
+    @Override
+    public void clear() {
+        xterm.clear();
+        scrollbackBuffer.delete(0, scrollbackBuffer.length());
+    }
+
+    @Override
+    public void write(String data) {
+        xterm.write(data);
+        scrollbackBuffer.append(data);
+    }
+
+    @Override
+    public void writeln(String data) {
+        xterm.writeln(data);
+        scrollbackBuffer.append(data);
+        scrollbackBuffer.append('\n');
+    }
+
+    @Override
+    public void paste(String data) {
+        xterm.paste(data);
+    }
+
+    @Override
+    public void refresh(int start, int end) {
+        xterm.refresh(start, end);
+    }
+
+    @Override
+    public void reset() {
+        xterm.reset();
+    }
+
+    @Override
+    public void resize(int columns, int rows) {
+        xterm.resize(columns, rows);
+    }
+}