Two Quick questions:
Are the seconds value of the clock/time on all iPhones around the world the same? If not, are they close at least & how close?
Do the seconds value of the clock/time on all iPhones around the world change/increment at the exact same time?
Upon request, I'm editing this post and adding the purpose for asking such questions:
I'm trying to make a corporate app that can play a video on multiple iPhones around the world at the exact same time (or as close as possible, ideally the exact same moment). Could you please guide me on how to do this?
Much thanks in advance!
To answer your actual question per se,
Are the seconds value of the clock/time on all iPhones around the world the same?
The fact is, yes, 99.9999% of iPhone users simply use the "get time from a server" system which is of course built in to any phone now.
(Indeed, this simply applies to any Windows, Mac, Android, iOS, etc etc device.)
Yes, they are all "about the same".
You could rely on it being within a second or two, probably even closer.
You cannot rely on it being closer than that.
It seems that you
I'm trying to make a corporate app that can play a video on multiple iPhones around the world at the exact same time
Synchronization in general and streaming video (is it streaming?) is a well-explored (and rather technical) branch of software engineering. (For example a massive amount of game engineering, which is a huge field, relates to inside-the-frame synchronization.)
This is not something you can learn how to do in five minutes, and it requires a stack of device-cloud stuff. Go ahead and ask new questions about this, or just google to get started! Good luck.
Regarding using push notifications.
With the push notification you would send a time. You would not send a "command to play it".
So say right now it is (example) 09:13:28. You would pick a time in the future by a couple minutes. So let's say 09:14:30.
Then using a push notification you would send that information "09:14:30" (and a video file name) to everyone connected. (You'd be sending a "command" as it were, to play video X at 09:14:30.)
Then every device would in fact play the video at 09:14:30 (simply using the local clock, as asked in your question).
Be aware that sending push notifications is extremely sloppy and slow. It can take any amount of time from 5 seconds to a minute, AND quite often there are delays beyond that (ie ten minutes or the like).
I personally would not even bother starting to experiment with push notifications, for the project you describe.
These days, making apps is entirely about using device-cloud services, such as Firebase. Everything is about "OCC" - occasionally connected computing.
(So, you can't get a job "making apps" anymore - i.e. if you know how to move buttons around on an iPhone screen. You get a job because you can make a total, live, device-cloud system - indeed such as you are making.)
Indeed your example project is the perfect such "demo" project for learning about how to do modern apps.
Simply use Firebase to sync everything up.
You'll essentially put a piece of information on Firebase ("play video X at 09:14:30") and that information will be communicated fairly quickly/reliably to everyone connected.
For the particular task you describe, I personally would use PubNub which is faster than Firebase and basically made for game-like problems precisely like you describe.
http://pubnub.com
If you truly needed performance/reliability better than pubnub, you are really talking major engineering. So, the (buildings of) engineers who make live games at Nintendo, Warcraft etc, would tackle such an issue as "being even faster than PubNub".
So, the answer in brief!
The very short answer then to your question posed is:
Learn to use the various device-cloud services, which are at the heart of all apps today. (Knowing how to make "an Android or iOS app", as such, is of no consequence today.) For your particular problem, you'll want to use PubNub specifically, as it is built for precisely realtime problems such as this. (Firebase more leans towards "OCC" type data problems.)
Really that's it.
Related
It is more or less asking how Whattsapp, Instagram, Facebook etc works.
How does my Phone know i got a new message even if the app is closed
(Please dont focus that much on the examples above, it is more a general question.)
I can think of a couple of solutions:
1.My Phone asks (in the background) an api every couple of seconds and fetches the data.
2.My Phone has an on going connection over the Web(i heard of technologies like: WebSocket, WebRTC, WebTransport, (standard) sockets, TCP)
My Phone is running a Webserver 24/7 and gets the signal like that
So in general the question is how can my PC/Phone etc. wait for a signal/data (Im talking 1, 2 or 3 bytes) in the background efficiently
it Highly Depends on the Operating System that the Program is Running on and also each OS has its own specific Security & Privacy Policy. However, all of them have a Common Agreement that this Types of Operation should be Handled in Background Processes as Secure Lightweight as Possible. so for your listed solution the closest one is 1th case.
2th Case almost Impossible because it is in Contradiction to being Lightweight
3th Case totally wrong way
My question involves keeping an app that monitors user interactions in the background, for example time spent in one or the app. The issue arises when you can not have a background process run for more than 10 min or violate Apple's sandbox restrictions. Since I am relatively new to the Apple API, and could not find a direct answer that didn't involve location services or VOIP (which are both interesting options, but my application will not be able to use either viably), I come to ask for options in which I can understand when another app opens, when it closes, when it fetches data, and when user holds phone in certain orientation (ie when people hold their phone at certain angles to read text and etc.) for certain amount of time.
The purpose of this analyzation is to predict an attention span for the user, so I need to be able to run in the background, as the user will not be using my app while it predicts attention span.
My thoughts on this are possibly accessing the system log, and somehow parse previous statements (I don't think sandbox will allow), but inevitably the iOS system will suspend my processes after some point unless I put a timer. There is also the option of having the system wake up my app via opportunistic fetching, but that won't be useful if I don't collect the data.
Keep in mind this is within IOS 11, so it is much more restrictive than previous iterations. I understand this may seem like a complex problem, but even a direction in which to head could be useful to me.
This solution might work, (not recommended since it drains the battery quicker).
Just update your current location, every 10 mins. It will reset the background thread timer.
I am attempting to stream audio files from a server to iOS devices and play them completely synchronized. For example on my phone I might be 20 secs into a song and then my friend next to me should also be 20 secs into the song as well. I know this is not an easy problem to solve, but I am attempting to do so.
I can currently get them within one second of each other by calculating the difference in time between the devices and then have them sync up, however that is not good enough because the human ear can detect a major difference in a second and this is over WIFI.
My next approach is going to be to unicast the one file from the server and then have the all devices pick it up directly from the server and then implement some type of buffer system similar to netflix so that network connectivity would be a limiting factor. http://www.wowza.com/ is what I would use to help with that.
I know this can be done, because http://lysn.in/ is does it with their app and I want to be able to do something similar.
Any other recommendations after I try my unicast option?
Would implementing firebase help solve a lot of the heavy lifting problems?
(1) In answer to ONE of your questions (the final one):
Firebase is not "realtime" in "that sense" -- PubNub is probably (almost certainly) the fastest "realtime" messaging for and between apps/browser/etc.
But they don't mean real-time in the sense of real-time, say, as race game engineers mean it or indeed in your use-case.
So firebase is not relevant to you here and won't help.
(2) Regarding your second general question: "how to synchronise time on two or more devices, given that we have communications delays."
Now, this is a really well-travelled problem in computer science.
It would be pointless outlining it here, because it is fully explained here http://www.ntp.org/ntpfaq/NTP-s-algo.htm if you click on "How is time synchronised"?
So in fact, to get a good time base on both machines, you should use that! Have both machines really accurately set a time to NTP using the existing (perfected for decades) NTP synchronisation.
(So for example https://stackoverflow.com/a/6744978/294884 )
In fact are you doing this?
It's possible that doing that will solve all your problems; then just agree to start at a certain exact time.
Hope it helps!
I would recommend against using the data movement to synchronize the playback. This should be straightforward to do with a buffer and a periodic "sync" signal that is sent at a period of < 1/2 the buffer size. Worst case this should generate a small blip on devices that get ahead or behind relative to the sync signal.
So I'm making an iphone application that needs to scrape a website once everyday.
What I'm going to scrape is a table of upcoming games for that same day for a soccer division. Thats why i need the app to scrape from the same page and same table once everyday to keep the upcoming games updated.
I was referred to import.io but they didn't have something like a schedule re-crawl.
I would love to get some ideas and tips to how i should do this since I'm stuck now.
You might take a look at https://www.kimonolabs.com/
I played around with the service a while back and was impressed with how easy it way to set up. They have a "free" option so long as the APIs you create are not private.
Oh, and I agree with Paul, screen scraping is not something the iOS client should be doing. Too fragile, and when (not if) something breaks, you will need to go through an Apple review process to fix it.
This doesn't seem like something an app should do, your server should do it (so that the scraping is only performed once), and your clients can retrieve it from your server. That also means you could send out push notifications for important fixtures etc. Maybe that's what you meant, anyway.
If it's on the server you can just setup a scheduler (in Java, for example) to run once every x hours (probably a smaller number than 24 assuming you don't know when the website is to be updated). Then your app can just get the latest list of fixtures from your server on startup, pull-to-refresh, etc. Presumably someone will open your app, look at the fixtures, then come out of your app - so it doesn't seem like you need to cover the case where someone is in your app all day, but if you did you could use NSTimer to run every x minutes after the initial on-startup server call.
Basically, for my team's app, we need to be able to synchronize music across multiple iOS devices. The first way we did this was by having the music on all the devices already and just sending a play command to all the devices. Some would get it later than others, so that method did not work. There was an idea mentioned to calculate the latency between all the devices and send the commands at the appropriate times based on the latency.
The second way proposed would be to stream the music. If we were to implement streaming, how should we go about doing it. Should Audio Units be used, OpenAL, etc.? Also, if streaming was being done, how would we go about making sure that each device's stream was in sync.
Basically, the audio has to be in sync so that the person hearing it cannot differentiate between the devices. A few milliseconds off should not be a problem (unless the listener has super-human hearing).
You'd be amazed at how good the human ear us at spotting audio anomalies...
Sync the time of day
Effectively your trying to meet a real time requirement with a whole load if very variable things in the way (wifi, etc). I strongly suspect the only way you're going to get close to doing this is to issue a 'play' instruction that includes a particular time to start playing. Of course that relies on all the clocks being accurately set.
NTP
I don't know how iPhones get their time of day. If they use (or could use) NTP then you'll be getting close. NTP is designed to convey accurate time of day information over a network despite variable network delays. I've had a quick look and it seems that most NTP clients for iOS are the simple ones, not the full NTP that measures and tunes out network delays, clock drifts, etc.
GPS
Alternatively GPS is also a very good source of time information. Again I don't know if iPhones can or do use GPS for setting their clock but if it could be done then that would likely be pretty good. On Solaris (and I think Linux too) the 1 pulse per second that most GPS chips generate from the GPS signal can be used to condition the internal OS clock, making it very accurate indeed (sub microsecond accuracy).
I fear that iPhones don't do either of these things natively; both involve using a fair bit of electricity, so I wouldn't be surprised if they did something else less sophisticated.
Cell Time Service
Some Cell networks provide a time service too, but I don't think it's designed for accurate time setting. Also it tends not to be available everywhere. You often find it at major airports so that recent arrivals get their phones set to something close to local time.
Play at time X
So if one of those could be used to ensure that all the iPhones are set to exactly the same time of day then all you have to do is write your software to start playing at a specific time. That will probably involve polling the clock in a very tight loop waiting for it to tick over; most OSes don't provide a means of sleeping until a specific time. They do at least allow for sleeping for a period of time, which can be used to sleep until close to the appointed time. You'd then start polling the clock until the right time is reached.
Delay Measurement and Standard Deviation
Your first method is doomed I think. You might be able to measure average delays and so forth but that doesn't mean that every message has exactly the same latency. The standard deviation in the latency will tell you what you can expect to achieve, and I don't think that's going to be particularly small. If so then the message has got to include a timestamp.
NTP can work because it's only interested in the average delay measured over a period of time (hours sometimes), whereas you're interested in instantaneous delay.
Streaming with RTP
Your second method may work if you can time sync the devices as discussed above. The RTP protocol was designed for use in these circumstances; it doesn't help with achieving sync, but it does help a lot with the streaming. It tells you whereabouts in the stream any one piece of received data fits, allowing you to play it at the right time.
Clock Drift
Another problem to deal with is how long you're playing for. If it's a long time then you may discover that the 44kHz (or whatever) audio clock rate on each device isn't quite the same. So, whilst you might find a way of starting to play all at the same time, the separate devices will then start diverging ever so slightly. Over a long period of time they may be noticeably out.
BlueTooth
It might be possible to do something with BlueTooth. It has many weird and wonderful profiles, and it might be that one of those would serve to send an accurate 'start now' message.
Audio Trigger
You might also use sound as a means of conveying a start signal. One device can play a particular sound whilst your software in the others is listening with the mic. When a particular feature is detected in the sound, that's the time for everyone to start playing. Sort of a computerised "1, 2, a 1 2 3 4".
Camera Flash
Should be easy to spot in software...
I think your first way would work if you expand it a little bit. Assuming all the clocks on the devices are in sync you could include a timestamp in your play command. Then each device would calculate the time between the timestamp and when it received the command. You would then play the music and offset it by the time difference.