Add dynamic command availability

Introduce availability concept on MethodTarget (with reason if not available)
Add bridge to @CliAvailabilityIndicator to Legacy registrar

Fixes #138

Add help for unavailable commands

Add standard API for availability

spring-shell-samples/src/main/java/org/springframework/shell/samples/legacy/LegacyCommands.java

@@ -19,6 +19,7 @@ package org.springframework.shell.samples.legacy;
 import java.lang.reflect.Method;
 
 import org.springframework.shell.core.CommandMarker;
+import org.springframework.shell.core.annotation.CliAvailabilityIndicator;
 import org.springframework.shell.core.annotation.CliCommand;
 import org.springframework.shell.core.annotation.CliOption;
 import org.springframework.stereotype.Component;
@@ -35,6 +36,13 @@ public class LegacyCommands implements CommandMarker {
 	public static final Method REGISTER_METHOD = ReflectionUtils.findMethod(LegacyCommands.class, "register", String.class, ArtifactType.class, String.class, boolean.class);
 	public static final Method SUM_METHOD = ReflectionUtils.findMethod(LegacyCommands.class, "sum", int.class, int.class);
 
+	private boolean available = true;
+
+	@CliAvailabilityIndicator("register module")
+	public boolean registerAvailable() {
+		return available;
+	}
+
 	@CliCommand(value = "register module", help = "Register a new module")
 	public String register(
 			@CliOption(mandatory = true,
@@ -57,10 +65,11 @@ public class LegacyCommands implements CommandMarker {
 		return String.format(("Successfully registered module '%s:%s'"), type, name);
 	}
 
-	@CliCommand(value = "sum", help = "adds two numbers")
+	@CliCommand(value = "sum", help = "adds two numbers. Will also toggle the 'register module' command availability")
 	public int sum(
 		@CliOption(key = "v1", unspecifiedDefaultValue = "38") int a,
 		@CliOption(key = "v2", specifiedDefaultValue = "42") int b) {
+		available = !available;
 		return a + b;
 	}
 

spring-shell-samples/src/main/java/org/springframework/shell/samples/standard/DynamicCommands.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.shell.samples.standard;
+
+import org.springframework.shell.Availability;
+import org.springframework.shell.standard.ShellComponent;
+import org.springframework.shell.standard.ShellMethod;
+import org.springframework.shell.standard.ShellMethodAvailability;
+
+/**
+ * Showcases dynamic command availability.
+ *
+ * @author Eric Bottard
+ */
+@ShellComponent
+public class DynamicCommands {
+
+    private boolean connected;
+
+    private boolean authenticated;
+
+    public Availability authenticateAvailability() {
+        return connected ? Availability.available() : Availability.unavailable("you are not connected");
+    }
+
+    @ShellMethod("Authenticate with the system")
+    public void authenticate(String credentials) {
+        authenticated = "sesame".equals(credentials);
+    }
+
+    @ShellMethod("Connect to the system")
+    public void connect() {
+        connected = true;
+    }
+
+    @ShellMethod("Disconnect from the system")
+    public void disconnect() {
+        connected = false;
+    }
+
+    @ShellMethod("Blow Everything up")
+    @ShellMethodAvailability("dangerousAvailability")
+    public String blowUp() {
+        return "Boom!";
+    }
+
+    private Availability dangerousAvailability() {
+        return connected && authenticated ? Availability.available() : Availability.unavailable("you failed to authenticate. Try 'sesame'.");
+    }
+}