Apple

On Sat, 7 Mar 2015, Dave O'Neill wrote:

>
> ---------- Forwarded message ----------
> From: Dave O'Neill <email@hidden>
> Date: Sat, Mar 7, 2015 at 9:44 AM
> Subject: Re: How does one update a view position during a core audio render callback?
> To: "Patrick J. Collins" <email@hidden>
>
>
> The simplest way to do is to just set a variable to your current time in your render callback.
> And then employ a repeating timer to check that value and update your view on the main thread.
> @implementation MyObject{
> float currentTime;
> }
>
> OSStatus myRenderCallback{
> MyObject *object = (MyObject *)inRefCon;
> object->currentTime = CalculateTimeOnRenderThread(); //with no Obj-C messaging!
> }
>
> -(void)myTimerCallback{
> self.playhead.position = currentTime;
> }
>
> If you want it to refresh the currentTime faster than the render callback you should get the
> mach_absolute_time() at the start of playing and again in your timer and calculate the difference.
>
> @implementation MyObject{
> UInt64 startMachTime;
> }
> // if you need accuracy get the mHostTime from your render callback
> // otherwise do this
>
> -(void)startAudio{
> startMachTime = mach_absolute_time();
> }
>
> -(void)myTimerCallback{
> UInt64 ticksSinceStart = mach_absolute_time() - startMachTime;
> self.playhead.position = convertTicksToSeconds(ticksSinceStart);
> }
>
> A CADisplyLink is an excellent timer for this
>
> On Sat, Mar 7, 2015 at 9:17 AM, Patrick J. Collins <email@hidden> wrote:
> Hi,
>
> So I have plotted out a waveform, and simply want to have a vertical
> line represent a playhead which will move across the waveform's X-axis
> as it plays.
>
> I am using an NSBox as my playhead, and in my callback proc, I tried doing inside the
> loop:
>
>    player->sampler.playheadView.position = player->sampler.playheadView.containerWidth
> / player->buffer.size * currentSampleIndex
>
> my playheadView's position setter just does this:
>
> -(void)setPosition:(NSUInteger)position {
>    if (position == self.position) return;
>    [self setFrameOrigin:NSMakePoint(position, 0)];
> }
>
> However, this causes everything to slow down to the point that the audio plays
> with clicks and gaps inbetween the frames..
>
> I changed this to do a performSelectorOnMainThread, where this operation
> occurs, but it seems like the playhead only gets updated a handful of times
> during playback, so it does not look good..
>
> What is the ideal way to get visual feedback like this during playback?
>
> Thanks!
>
> Patrick J. Collins
> http://collinatorstudios.com
>
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