From 8d26afc2c58d40806d741b07c2bb954d0b321397 Mon Sep 17 00:00:00 2001
From: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Date: Thu, 9 Jun 2016 13:45:01 -0400
Subject: [PATCH] NPEs can happen

Boo, hiss; the ordering of onStop and the canvas being destroyed appear
to be nondeterministic.  Just check for null in cycler thread.
---
 .../com/acmetensortoys/watchviz/MainActivity.java    | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/app/src/main/java/com/acmetensortoys/watchviz/MainActivity.java b/app/src/main/java/com/acmetensortoys/watchviz/MainActivity.java
index d1d947b..a2b5806 100644
--- a/app/src/main/java/com/acmetensortoys/watchviz/MainActivity.java
+++ b/app/src/main/java/com/acmetensortoys/watchviz/MainActivity.java
@@ -105,9 +105,6 @@ public class MainActivity extends WearableActivity
                     while (!Thread.interrupted()) {
                         final RenderCB rcb;
                         ar.read(samples, 0, samples.length, AudioRecord.READ_BLOCKING);
-                        synchronized(this) {
-                            rcb = cyclercb;
-                        }
                         /*
                          * Debug: triangle wave
                          */
@@ -119,7 +116,16 @@ public class MainActivity extends WearableActivity
                         */
                         fft.realForward(samples);
 
+                        synchronized(this) {
+                            rcb = cyclercb;
+                        }
+
                         Canvas cv = h.lockCanvas();
+                        if (cv == null) {
+                            // the surface must have been destroyed out from under us;
+                            // just stop here.
+                            break;
+                        }
                         cv.drawColor(Color.BLACK);
                         rcb.render(cv,samples);
                         h.unlockCanvasAndPost(cv);
-- 
2.50.1