| 1 | import Foundation |
| 2 | |
| 3 | /// Measures how long the main thread goes without answering. |
| 4 | /// |
| 5 | /// A thread of its own posts a block to the main queue every 5 ms and |
| 6 | /// times how long each takes to run. A main thread that is free answers |
| 7 | /// within a millisecond or two; one that is blocked answers only when it |
| 8 | /// is free again, and the wait is the stall. Several posts can be waiting |
| 9 | /// at once, so the longest single wait is the longest stall, to within |
| 10 | /// the 5 ms period. |
| 11 | /// |
| 12 | /// It measures the main *queue*, which is what the main actor runs on and |
| 13 | /// what a SwiftUI app's run loop drains: a beachball is this number above |
| 14 | /// about a second. |
| 15 | final class MainThreadWatchdog: @unchecked Sendable { |
| 16 | struct Stall: Sendable { |
| 17 | /// Seconds since the watchdog started. |
| 18 | var at: Double |
| 19 | var seconds: Double |
| 20 | } |
| 21 | |
| 22 | private let lock = NSLock() |
| 23 | private var running = false |
| 24 | private var longest: Stall? |
| 25 | private var stalls: [Stall] = [] |
| 26 | private let started = CFAbsoluteTimeGetCurrent() |
| 27 | private let period: Double |
| 28 | /// Waits at or above this are kept, for the report. |
| 29 | private let noteAbove: Double |
| 30 | |
| 31 | init(period: Double = 0.005, noteAbove: Double = 0.05) { |
| 32 | self.period = period |
| 33 | self.noteAbove = noteAbove |
| 34 | } |
| 35 | |
| 36 | func start() { |
| 37 | lock.lock() |
| 38 | running = true |
| 39 | lock.unlock() |
| 40 | let thread = Thread { [self] in |
| 41 | while isRunning { |
| 42 | let posted = CFAbsoluteTimeGetCurrent() |
| 43 | DispatchQueue.main.async { [self] in |
| 44 | let now = CFAbsoluteTimeGetCurrent() |
| 45 | record(Stall(at: posted - started, seconds: now - posted)) |
| 46 | } |
| 47 | Thread.sleep(forTimeInterval: period) |
| 48 | } |
| 49 | } |
| 50 | thread.qualityOfService = .userInteractive |
| 51 | thread.start() |
| 52 | } |
| 53 | |
| 54 | private var isRunning: Bool { |
| 55 | lock.lock() |
| 56 | defer { lock.unlock() } |
| 57 | return running |
| 58 | } |
| 59 | |
| 60 | private func record(_ stall: Stall) { |
| 61 | lock.lock() |
| 62 | defer { lock.unlock() } |
| 63 | guard running else { return } |
| 64 | if stall.seconds > (longest?.seconds ?? -1) { longest = stall } |
| 65 | if stall.seconds >= noteAbove { stalls.append(stall) } |
| 66 | } |
| 67 | |
| 68 | /// Forgets what was measured; the next `longest` is from now. |
| 69 | func reset() { |
| 70 | lock.lock() |
| 71 | longest = nil |
| 72 | stalls = [] |
| 73 | lock.unlock() |
| 74 | } |
| 75 | |
| 76 | /// Stops posting and gives the longest wait since the last reset, and |
| 77 | /// every wait above `noteAbove`. |
| 78 | func stop() -> (longest: Double, stalls: [Stall]) { |
| 79 | lock.lock() |
| 80 | defer { lock.unlock() } |
| 81 | running = false |
| 82 | return (longest?.seconds ?? 0, stalls) |
| 83 | } |
| 84 | |
| 85 | /// The longest wait so far, without stopping. |
| 86 | var longestSoFar: Double { |
| 87 | lock.lock() |
| 88 | defer { lock.unlock() } |
| 89 | return longest?.seconds ?? 0 |
| 90 | } |
| 91 | } |
| 92 | |
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