touchesMoved called at irregular intervals

Question

I'm making a game for iOS where you mostly drag big objects across the screen. When I run the game on an actual iPad/iPhone for a while (continuously dragging the object in circles across the screen) every 5 minutes or so the dragged object goes all stuttery for about 10-30 seconds. Then, it goes back to moving silky-smooth.

Visually, it looks like the game's frame rate dropped to 15 fps for a while, but in actual fact it's running at rock-solid 60 fps all the time. However, I noticed that the only thing that doesn't move smoothly is the dragged object, while the rest of the game is all running perfectly smooth.

This led me to believe that the stuttering is related to the touch input in iOS. So I started looking at touchesMoved, and saw that it's normally called every 16 milliseconds (so touch input runs at 60 fps). So far so good.

Then I noticed that when the object starts stuttering, touchesMoved starts being called at weird time intervals, fluctuating wildly between 8 milliseconds and 50 milliseconds.

So while the touchscreen is in this weird state, sometimes touchesMoved will get called just 8 milliseconds after the previous call, and sometimes it won't get called until 50 ms after the previous call. Of course, this makes the dragged object look all choppy because its position is updated at irregular intervals.

Do you have any idea what could be causing touchesMoved to stop being called at regular intervals, as it normally does?

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Bonus:

-Whenever I tilt the screen to force the screen orientation to change, roughly 70% of the time the touchscreen goes into the aforementioned state where touchesMoved starts being called irregularly. Then after 10-20 seconds it goes back to normal and everything looks smooth again.

-I've tried this on two iPads and two iPhones, with iOS 6 and 7, and the issue appears in all of these devices.

-An OpenGLES view is used to display the graphics. It syncs to the display refresh rate using CADisplayLink.

-The Xcode project I'm using to test this has been generated by the unity3d game development tool, but I've found several non-unity games where the same issue appears. this appears to be a system-wide problem. note I'm measuring the timings in objective-c using CFAbsoluteTimeGetCurrent, completely outside unity.

Answer 1

This is not a bug in Unity.

Something inside the OS is getting into a bad state and the touch-drag messages stop flowing smoothly. Sometimes you'll get multiple updates in a frame and sometimes you'll get none.

The issue does not happen on iPhone4 or below, or if the game is running at 30Hz frame rate.

I experienced this bug while using an in-house engine I'd written while working at a previous company. It first manifest itself after upgrading the UI system of a scrabble genre game, where you drag the tiles about on the screen. This was utterly bizarre, and I was never able to pin down the exact reproduction steps, but they seem to be timing related, somehow.

It can also be seen on Angry Birds (unless they've fixed it by now), and a variety of other games on the market, basically anything with touch-drag scrolling or movement. For Angry Birds, just go into a level and start scrolling sideways. Most of the time it'll be silky smooth, but maybe 1 in 10 times, it'll be clunky. Re-start the app and try again.

A workaround is to drop the input update frequency to 30Hz for a few frames. This jolts the internals of the OS somehow and gives it time to straighten itself out, so when you crank it back up to 60Hz, it runs smoothly again.

Do that whenever you detect that the bad state has been entered.

Answer 2

I've also run into this. I can verify that it's a bug in Unity 4.3.x. My 4.2.x builds process touches on device at 60Hz with TouchPhase.Moved on every frame. My 4.3.x builds get 20-40Hz with TouchPhase.Stationary being emitted on dropped frames.

TestFlight history saved my sanity here.

Don't forget to file a bug with UT.

Answer 3

It's a real disaster. Sometimes it's lag and sometimes it's really smooth. It lags even when GPU and CPU utilization are below 10%. it's not Unity bug. I'm using cocos2d v3. If someone found a cure, please post it!

Answer 4

I've been running into this as well. My current hypothesis which I still have to verify is that if you ask for a given frame rate (eg. 60fps), but your actually achieved frame rate is less than that (eg. 45fps) that this causes a timing/race condition issue between Unity requesting inputs from iOS at a higher frame rate than it actually is running at. However, if you set it to 30fps (at least in my UNity 5.2 tests with iOS 9.1) then you get a solid 30Hz of input. When I disabled a chunk of my game and it was running at a very solid 60fps, then I would consistently get 60Hz of input from the touch screen. This is what I have for now, but I have to prove this in a simple project which I haven't had time to do yet. Hope this helps someone else.

Answer 5

I found a potential solution to this problem here: https://forums.developer.apple.com/thread/62315 (posting here because it took me a lot of research to stumble across that link whereas this StackOverflow question was the first Google result).

To follow up on this, I got a resonse on my bug report to Apple. This is the response:

"As long as you don't cause any display updates the screen stays in low power and therefore 30hz mode, which in turn also keeps the event input stream down at 30 hz. A workaround is to actually cause a display update on each received move, even if it is just a one pixel move of a view if input is needed while no explicit screen update will be triggered."

In my application, using a GLKView, I set its preferredFramesPerSecond to 60. Occasionally, my input rate would randomly drop to 30hz. The response from Apple doesn't fully explain why this would happen, but apparently the expected method of handling this is to call display() directly from touchesMoved() while dragging.

I've subclassed GLKView and I set preferredFramesPerSecond to 60. On touchesBegan(), I set isPaused=true, and start calling display() within touchesMoved(). On touchesEnd(), I set isPaused=false. Doing this, I'm no longer having any issues - the app is more performant than it's ever been.

Apple's example TouchCanvas.xcodeproj does all drawing from within touchesMoved() as well, so I guess this is the intended way to handle this.

Answer 6

As far as I can tell, your best bet to achieve a smooth look may be to interpolate between touch events, rather than immediately mapping your objects to your touch position.

tl;dr: It seems just having a CADisplayLink causing any OpenGL context or Metal device to draw at 60fps can cause this.

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I was repro'ing this on my iPhone 7 Plus w/ iOS 10.2.1.

I made a small sample simple app with a CADisplayLink with preferredFramesPerSecond = 60, and tried the following rendering approaches:

1. GLKView w/ display()
2. CAEAGLLayer, used as prescribed by Apple at WWDC (opaque, sole layer, fullscreen, nothing drawn above it)
3. MTLDevice

In each case, the render method would only clear the screen, not try drawing anything else.

In each case, I saw the input rate problem.

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The following "tricks" also seemed to do nothing, when called inside touchesMoved:

1. Calling glkView.setNeedsDisplay() (with enableSetNeedsDisplay set to true)
2. Moving some other view
3. Calling glkView.display() (actually, seems like it can raise your input rate to 40 events per second. But it doesn't look any better, as far as I can tell, and seems wrong to do, so I wouldn't recommend it.)

I gave up, after running all these tests. Instead, I interpolate my object between the touch positions instead. So it's what I'd recommend, too.

But I figured I'd include my research here in case somebody else takes a stab at it and finds a better solution.