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Introduce "Ready flag".

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This commit is contained in:
Matthew Chen 2020-05-08 09:34:07 -03:00
parent bb67c1e04e
commit ea9db2359e
1 changed files with 28 additions and 8 deletions
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36
SignalServiceKit/src/Util/ReadyFlag.swift
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@ -4,6 +4,24 @@
import Foundation
// We often want to enqueue work that should only be performed
// once a certain milestone is reached. e.g. when the app
// "becomes ready" or when the user's account is "registered
// and ready", etc.
//
// This class provides the following functionality:
//
// * A boolean flag whose value can be consulted from any thread.
// * "Will become ready" and "did become ready" blocks that are
// performed immediately if the flag is already set or later
// when the flag is set.
// * Additionally, there's support for a "polite" flavor of "did
// become ready" block which are performed with slight delays
// to avoid a stampede which could block the main thread. One
// of the risks there is 0x8badf00d crashes.
// * The flag can be used in various "queue modes". "App readiness"
// blocks should be enqueued and performed on the main thread.
// Other flags will want to do their work off the main thread.
@objc
public class ReadyFlag: NSObject {
@ -20,16 +38,18 @@ public class ReadyFlag: NSObject {
public enum QueueMode: UInt {
// Like the "app is ready" flag:
//
// * The flag should only be accessed on the main thread.
// * The flag should only be set on the main thread.
// * All blocks are performed on the main thread.
// * The non-polite blocks are performed sync when
// the flag is set.
case mainThreadOnly
// * The flag can be accessed from any thread.
// * The flag can be set from any thread.
// * All blocks are performed _sync_ on the serial thread
// except "polite" blocks which are performed async.
// * There is the risk of deadlock if any block
// accesses the flag.
case serialQueueSync
// * The flag can be accessed from any thread.
// * The flag can be set from any thread.
// * All blocks are performed _async_ on the serial thread.
case serialQueueAsync
}
@ -66,7 +86,7 @@ public class ReadyFlag: NSObject {
@objc
public func runNowOrWhenWillBecomeReady(_ readyBlock: @escaping ReadyBlock) {
perform {
performInternal {
if self.isSet {
readyBlock()
} else {
@ -77,7 +97,7 @@ public class ReadyFlag: NSObject {
@objc
public func runNowOrWhenDidBecomeReady(_ readyBlock: @escaping ReadyBlock) {
perform {
performInternal {
if self.isSet {
readyBlock()
} else {
@ -88,7 +108,7 @@ public class ReadyFlag: NSObject {
@objc
public func runNowOrWhenDidBecomeReadyPolite(_ readyBlock: @escaping ReadyBlock) {
perform {
performInternal {
if self.isSet {
readyBlock()
} else {
@ -99,7 +119,7 @@ public class ReadyFlag: NSObject {
@objc
public func setIsReady() {
perform {
performInternal {
guard !self.isSet else {
assert(self.willBecomeReadyBlocks.isEmpty)
assert(self.didBecomeReadyBlocks.isEmpty)
@ -131,7 +151,7 @@ public class ReadyFlag: NSObject {
}
}
private func perform(_ block: @escaping () -> Void) {
private func performInternal(_ block: @escaping () -> Void) {
switch queueMode {
case .mainThreadOnly:
AssertIsOnMainThread()