add wait for value loop

Author: rerpha

src/main/java/org/phoebus/pvws/EpicsWebSocketServerApplication.java

@@ -68,8 +68,20 @@ public String pvget(@RequestParam String name) {
         final WebSocketPV pv = new WebSocketPV(name, null);
         String ret;
         try {
-            pv.start();
-            VType lastValue = pv.get();
+
+            // TODO make these configurable?
+            int maxAttempts = 100;
+            int retryDelay = 50;
+
+            VType lastValue = null;
+
+            for (int i = 0; i< maxAttempts; i++) {
+                Thread.sleep(retryDelay);
+                lastValue = pv.get();
+                if (lastValue != null) break;
+                System.out.println("value is" + lastValue + "attempt no " + i);
+            }
+
             System.out.println("value is" + lastValue);
             ret = Vtype2Json.toJson(name, lastValue, null, true, true);
         } catch (final Exception ex) {