From 102bdf35b13f1b60df26052341ebc5a2833518c7 Mon Sep 17 00:00:00 2001
From: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Date: Thu, 30 May 2013 10:07:29 +0200
Subject: iOS: change accelerometer to use QTimer for polling

According to the docs (CMMotionManager class reference) there are
two ways of interacting with the accelerometer; Either through the
callback API (NSOperationQueue), or peridic sampling (polling).
Our first implementation of IOSAcceleometer used the former
technique, which turns out to have bad performance when using the
sensor together with a fine-grained QTimer. And this case is
pretty common when using sensors together with e.g QML.

Reading through the docs more carefully, they recommend using the
polling technique when creating games instead since the
NSOperationQueue introduces some overhead. So this patch does
that, change the implementation to use QTimer based polling. And
this solves the performance issues found.

Change-Id: Ifde0d2292302467afb8db90a954ef45f3238350e
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
---
 src/plugins/sensors/ios/iosaccelerometer.h  |  9 +++--
 src/plugins/sensors/ios/iosaccelerometer.mm | 52 +++++++++++------------------
 2 files changed, 24 insertions(+), 37 deletions(-)

(limited to 'src/plugins')

diff --git a/src/plugins/sensors/ios/iosaccelerometer.h b/src/plugins/sensors/ios/iosaccelerometer.h
index 5fcac19f..1a6560b1 100644
--- a/src/plugins/sensors/ios/iosaccelerometer.h
+++ b/src/plugins/sensors/ios/iosaccelerometer.h
@@ -42,29 +42,28 @@
 #ifndef IOSACCELEROMETER_H
 #define IOSACCELEROMETER_H
 
-#include <Foundation/Foundation.h>
+#include <CoreMotion/CMMotionManager.h>
 
 #include <qsensorbackend.h>
 #include <qaccelerometer.h>
 
 QT_BEGIN_NAMESPACE
 
-@class QtIoAccelListener;
-
 class IOSAccelerometer : public QSensorBackend
 {
 public:
     static char const * const id;
 
     explicit IOSAccelerometer(QSensor *sensor);
-    ~IOSAccelerometer();
+    void timerEvent(QTimerEvent *);
 
     void start();
     void stop();
 
 private:
-    NSOperationQueue *m_updateQueue;
+    CMMotionManager *m_motionManager;
     QAccelerometerReading m_reading;
+    int m_timer;
 };
 QT_END_NAMESPACE
 
diff --git a/src/plugins/sensors/ios/iosaccelerometer.mm b/src/plugins/sensors/ios/iosaccelerometer.mm
index b657b507..5f9c0f16 100644
--- a/src/plugins/sensors/ios/iosaccelerometer.mm
+++ b/src/plugins/sensors/ios/iosaccelerometer.mm
@@ -40,8 +40,6 @@
 ****************************************************************************/
 
 #include <UIKit/UIAccelerometer.h>
-#include <CoreMotion/CMMotionManager.h>
-#include <QPointer>
 
 #include "iosaccelerometer.h"
 #include "iosmotionmanager.h"
@@ -52,49 +50,39 @@ QT_BEGIN_NAMESPACE
 
 IOSAccelerometer::IOSAccelerometer(QSensor *sensor)
     : QSensorBackend(sensor)
-    , m_updateQueue([[NSOperationQueue alloc] init])
+    , m_motionManager([QIOSMotionManager sharedManager])
+    , m_timer(0)
 {
     setReading<QAccelerometerReading>(&m_reading);
     addDataRate(1, 100); // 100Hz
     addOutputRange(-22.418, 22.418, 0.17651); // 2G
 }
 
-IOSAccelerometer::~IOSAccelerometer()
-{
-    [m_updateQueue release];
-}
-
 void IOSAccelerometer::start()
 {
-    CMMotionManager *motionManager = [QIOSMotionManager sharedManager];
-    // Convert from Hz to NSTimeInterval:
     int hz = sensor()->dataRate();
-    motionManager.accelerometerUpdateInterval = (hz == 0) ? 0 : 1. / hz;
-
-    QPointer<QObject> self = this;
-    [motionManager startAccelerometerUpdatesToQueue:m_updateQueue withHandler:^(CMAccelerometerData *data, NSError *error) {
-        // NSOperationQueue is multi-threaded, so we process the data by queuing a callback to
-        // the main application queue. By the time the callback executes, IOSAccelerometer might
-        // have been deleted, so we need an extra QPointer check for that:
-        dispatch_async(dispatch_get_main_queue(), ^{
-            if (self) {
-                Q_UNUSED(error);
-                // Convert from NSTimeInterval to microseconds and G to m/s2, and flip axes:
-                CMAcceleration acc = data.acceleration;
-                const qreal G = 9.8066;
-                m_reading.setTimestamp(quint64(data.timestamp * 1000000));
-                m_reading.setX(qreal(acc.x) * G * -1);
-                m_reading.setY(qreal(acc.y) * G * -1);
-                m_reading.setZ(qreal(acc.z) * G * -1);
-                newReadingAvailable();
-            }
-        });
-    }];
+    m_timer = startTimer(1000 / (hz == 0 ? 60 : hz));
+    [m_motionManager startAccelerometerUpdates];
 }
 
 void IOSAccelerometer::stop()
 {
-    [[QIOSMotionManager sharedManager] stopAccelerometerUpdates];
+    [m_motionManager stopAccelerometerUpdates];
+    killTimer(m_timer);
+    m_timer = 0;
+}
+
+void IOSAccelerometer::timerEvent(QTimerEvent *)
+{
+    // Convert from NSTimeInterval to microseconds and G to m/s2, and flip axes:
+    CMAccelerometerData *data = m_motionManager.accelerometerData;
+    CMAcceleration acc = data.acceleration;
+    static const qreal G = 9.8066;
+    m_reading.setTimestamp(quint64(data.timestamp * 1e6));
+    m_reading.setX(qreal(acc.x) * G * -1);
+    m_reading.setY(qreal(acc.y) * G * -1);
+    m_reading.setZ(qreal(acc.z) * G * -1);
+    newReadingAvailable();
 }
 
 QT_END_NAMESPACE
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