API-reference topic

API Reference

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Every static member of FlutterPatcher, and the types they take and return.

These APIs work only in an Android or iOS app built with dart run flutter_patcher build, called a host, because build writes into the app what the device needs to check patches; anything else is in Platform support. How a host loads a patch is in How It Works; the commands that write patch files are in the CLI Reference.

import 'package:flutter_patcher/flutter_patcher.dart';

Overview

Member	Description
init	Configures crash protection; call it before runApp()
isAvailable	Whether this app can take patches
checkUpdate	Checks a URL for a newer patch file for this host
applyPatch	Downloads and installs the file checkUpdate found
patchTarget	Identifies this host, for the server in custom delivery
inspectPatch	Tells from a file's header whether installing it would change this host
applyPatchBytes	Installs a patch file the app fetched itself, as bytes
applyPatchFile	Installs a patch file the app fetched itself, from a path on the device
applyProgress	Progress of every install
currentVersion	The installed patch's seq
lastBootDiagnostic	How the latest cold start handled the patch
blacklist	Patches blacklisted on this device
appVersionCode	The host's build number
deviceAbi	The device's primary ABI
rollback	Deletes the installed patch on this device
clearBlacklist	Empties this device's blacklist

patchTarget, inspectPatch, applyPatchBytes and applyPatchFile need flutter_patcher 1.1.0 or newer, and isAvailable needs 1.1.1 or newer. Hosts built with flutter_patcher 1.0.0 are patched with flutter_patcher 1.0.0+1; see Upgrading from 1.0.

Methods and properties

init

static Future<void> init({
  int maxCrashCount = 1,
  List<String> loaderFieldCandidates = const ['flutterLoader'],
  bool loaderFallbackHeuristic = false,
  Duration verifyAfter = const Duration(seconds: 5),
})

Configures crash protection.

Parameter	Description
maxCrashCount	How many consecutive failed starts of a patch turn it off and blacklist it. A Dart error thrown once the patch's code has started is not counted: the patch is turned off at once; see After a failure. Values below 1 count as 1
verifyAfter	How long after the first frame an uncaught Dart error still counts as a failed start. Only time in the foreground counts
loaderFieldCandidates, loaderFallbackHeuristic	Android SO hosts only: how the plugin locates Flutter's loader. Keep the defaults unless you adapt a new Flutter version

Behavior

• Call it in main() before runApp(), so its error hooks are in place before the first frame. Later calls have no effect.
• Native code marks a start successful at its first frame; init keeps reporting uncaught Dart errors for verifyAfter after it. What counts as a failed start and what happens next: Crash Protection.

Throws: nothing.

isAvailable

static Future<bool> get isAvailable

Whether this app can take patches: it runs on Android or iOS and was built with dart run flutter_patcher build. It is false in an app started with flutter run or built with flutter build.

Throws: never.

checkUpdate

static Future<PatchCheckResult> checkUpdate({
  String? url,
  Duration timeout = const Duration(seconds: 10),
})

Checks url for a newer patch file for this host.

Parameter	Description	Default
url	Where to look	The address build wrote into the app: url in flutter_patcher.yaml, or http://127.0.0.1:8686, where serve offers patches, when that is not set
timeout	Limit for the connection and the header read	10 seconds

Returns: a PatchCheckResult. When there is an update, pass its patch unchanged to applyPatch.

Behavior

• Requests GET <url>/<fileName> with Range: bytes=0-145, where <fileName> is patchTarget.fileName, reads the first 146 bytes and judges them as inspectPatch does.
• hasUpdate is true only when the file is fresh: a newer patch or a newer withdrawal for this host.
• When isAvailable is false, no request is sent and there is no update.
• 404 means no update. Any status other than 200, 206 or 404 throws, so the server must answer 404 for a missing file; the full requirements are in CLI Reference: Download server.
• The address must be HTTPS, because an unsigned host installs any intact file of its build and a file replaced in transit would run in the app. Plain HTTP is accepted only to the device itself, http://127.0.0.1, http://[::1] or http://localhost, where serve is reached through adb reverse.
• Redirects are not followed and the address must not contain a user name, a password or a # fragment, so a response cannot send the download elsewhere.

Throws: PatcherException when no url is given and the app has none, and when the request fails, including a response shorter than 146 bytes.

applyPatch

static Future<PatchApplyResult> applyPatch(
  PatchInfo patchInfo, {
  void Function(PatchApplyProgress)? onProgress,
})

Downloads the file checkUpdate found and installs it as applyPatchFile does.

Parameter	Description
patchInfo	The PatchInfo checkUpdate returned
onProgress	Receives this call's PatchApplyProgress

Returns: a PatchApplyResult.

Behavior

• A patch takes effect on the next cold start, because the running process keeps its code; a withdrawal deletes the installed patch.
• The download must answer 200 with exactly the size the header announced, so an uncompressed response with a different Content-Length fails.
• It fails after 15 seconds without data or 2 minutes in total, whatever checkUpdate's timeout was. Every download failure gives network.
• A PatchInfo without the url and size that checkUpdate sets gives invalidArgs.

Throws: nothing; every outcome is in the result.

final check = await FlutterPatcher.checkUpdate();
if (check.hasUpdate) {
  final result = await FlutterPatcher.applyPatch(check.patch!);
  if (!result.ok) debugPrint('${result.error?.name}: ${result.message}');
}

patchTarget

static Future<PatchTarget> get patchTarget

Identifies this host. An app that fetches patch files itself sends these values to its server, which picks the file; see Custom Delivery from Your Own Server.

Returns: a PatchTarget.

Throws: PatcherException on an app not built with flutter_patcher.

inspectPatch

static Future<PatchInspection> inspectPatch(List<int> header)

Tells from the first 146 bytes of a file whether installing it would change this host, so the app can skip downloading a file the host would reject.

Parameter	Description
header	The file's first 146 bytes; bytes beyond them are ignored

Returns: a PatchInspection.

Behavior

• It checks only magic, version, fingerprint and seq, so installing still verifies the whole file.

Throws: PatcherException on an app not built with flutter_patcher.

applyPatchBytes

static Future<PatchApplyResult> applyPatchBytes(List<int> bytes)

Installs the patch file bytes, or applies the withdrawal it is.

Parameter	Description
bytes	A whole file the CLI wrote for this host build: file in the patch output, or <fileName> in the publication store

Returns: a PatchApplyResult.

Behavior

• Installs by the rules in What a device accepts, for the next cold start.

Throws: nothing; every outcome is in the result.

applyPatchFile

static Future<PatchApplyResult> applyPatchFile(String path)

As applyPatchBytes, for a file on the device.

Parameter	Description
path	The path of the patch file on the device

Returns: a PatchApplyResult.

Behavior

• The file stays in place.
• A path that cannot be read gives ioError.

Throws: nothing; every outcome is in the result.

applyProgress

static Stream<PatchApplyProgress> get applyProgress

A broadcast stream of the progress of every install.

Returns: a stream of PatchApplyProgress.

Behavior

• It carries every phase of applyPatch, and verifying and finalizing of applyPatchBytes and applyPatchFile.
• Subscribe before installing, because a broadcast stream does not replay earlier events.

Throws: nothing.

currentVersion

static Future<String?> get currentVersion

The installed patch's seq as a decimal string, or null without one.

Behavior

• It changes as soon as an install succeeds, while the patch runs from the next cold start, so until then it describes another patch than lastBootDiagnostic.

Throws: nothing; a failed read gives null.

lastBootDiagnostic

static Future<PatchBootDiagnostic?> get lastBootDiagnostic

How the latest cold start handled the patch, or null without a record.

Returns: a PatchBootDiagnostic.

Behavior

• A start that loaded a patch is recorded at its first frame; asked earlier, the getter completes then, with this start's record.

Throws: nothing; a failed read gives null.

final diag = await FlutterPatcher.lastBootDiagnostic; // The patch this start runs.
final installed = await FlutterPatcher.currentVersion; // The patch the next cold start runs.
debugPrint('running: ${diag?.status.name} ${diag?.patchVersion}; installed: $installed');

blacklist

static Future<List<BlacklistEntry>> get blacklist

Patches blacklisted on this device.

Returns: a list of BlacklistEntry; empty without any.

Throws: nothing; a failed read gives an empty list.

appVersionCode

static Future<int?> get appVersionCode

The host's build number: Android versionCode, iOS CFBundleVersion.

Throws: nothing; null when unreadable.

deviceAbi

static Future<String> get deviceAbi

The device's primary ABI: on Android the first of Build.SUPPORTED_ABIS, such as arm64-v8a; on iOS ios-arm64.

Throws: nothing; '' when unreadable.

rollback

static Future<void> rollback()

Deletes the installed patch on this device; the next cold start runs the installed version.

Behavior

• The patch is not blacklisted.
• acceptedSeq stays, so only a file with a higher seq can install afterwards.

Throws: nothing.

clearBlacklist

static Future<void> clearBlacklist()

Empties this device's blacklist and resets every patch's failure count.

Throws: nothing.

Types

PatchCheckResult

Returned by checkUpdate.

Field	Type	Description
hasUpdate	bool	Whether there is an update to apply: a newer patch or withdrawal
patch	PatchInfo?	The update; null without one

PatchInfo

The file checkUpdate found. Pass it unchanged to applyPatch, because a PatchInfo built by hand lacks the fields the download needs and gives invalidArgs.

Field	Type	Description
version	String	The file's seq as a string
raw	Map<String, dynamic>	url, size, seq and action (publish or withdraw)

PatchApplyResult

Returned by applyPatch, applyPatchBytes and applyPatchFile.

Field	Type	Description
ok	bool	Whether the file was installed or the withdrawal applied; installing the installed file again also succeeds
error	PatchApplyError?	Why it failed; null when ok
message	String?	A description for developers, not written for users

PatchApplyError

Value	Meaning	What to do
invalidArgs	The file's seq is not above acceptedSeq: an older patch or withdrawal, or the same patch after rollback(). With applyPatch, also a PatchInfo that checkUpdate did not return	Deliver a file with a higher seq: run patch again, or publish --patch <dir>
blacklisted	This patch made a start fail and is blacklisted on this device	Deliver a fixed patch
network	applyPatch could not download the file	Retry later
signatureInvalid	Not this host's file (another build, flavor, channel or ABI), damaged or truncated, over 320 MiB, or not signed by the host's patch key	Do not retry, because the same file fails the same way; deliver the file named patchTarget.fileName of this build, signed by its key
ioError	The file could not be read, or a disk or file system error such as no space	Retry later
unknown	Anything else: an unsupported platform, an app not built with flutter_patcher, a patch the host rejects while installing	Report message

switch (result.error) {
  case PatchApplyError.network || PatchApplyError.ioError:
    break; // Temporary; check again later.
  case PatchApplyError.blacklisted:
    break; // This patch failed a start here; wait for a fixed one.
  default:
    debugPrint('patch rejected: ${result.error?.name} ${result.message}');
}

PatchApplyProgress

Emitted to onProgress of applyPatch and by applyProgress.

Field	Type	Description
phase	PatchApplyPhase	The current phase
bytesReceived, totalBytes	int	Meaningful only while downloading
fraction	double?	Download progress from 0.0 to 1.0; null outside downloading or when the size is unknown

PatchApplyPhase

Value	Meaning
downloading	Downloading the file; applyPatch only
verifying	Verifying the payload digest and signature
finalizing	Parsing the patch, installing its resources and committing

PatchTarget

Returned by patchTarget.

Member	Type	Description
fileName	String	The name of this host's patch files: <version_code>-<abi>.fptc for the default flavor and channel, otherwise <version_code>-<flavor>-<channel>-<abi>.fptc. See Patch files
versionCode, flavor, channel, abi	String	The build, as build recorded it
strategy	String	so or bytecode
fingerprint	String	The host build's fingerprint; files of another fingerprint are rejected
signed	bool	Whether the host accepts only files signed by its patch key. Fixed when the host is built
acceptedSeq	int	The largest seq this host accepted, of a patch or a withdrawal; 0 before any. Only files numbered above it install
scope	Map<String, String>	application_id, version_code, flavor, channel, abi, strategy and fingerprint

PatchInspection

Returned by inspectPatch.

Field	Type	Description
fresh	bool	true when the header is this host's and its seq is above acceptedSeq
action	String?	publish for a patch, withdraw for a withdrawal; null when the bytes are not a patch file header
seq	int?	The file's sequence; null when the bytes are not a patch file header
reason	String?	Why the file is not fresh: Patch file of another host, Not newer than the accepted patch, or the parse error for bytes that are not a patch file header

PatcherException

Thrown by checkUpdate, patchTarget and inspectPatch.

Field	Type	Description
message	String	The cause

PatchBootDiagnostic

Returned by lastBootDiagnostic. Its fields and the meaning of each status are in Crash Protection: Boot diagnostics.

BlacklistEntry

Returned by blacklist. Its fields are in Crash Protection: Blacklist.

What a device accepts

Every install goes through the same native check, whichever API hands over the file, so the delivery channel never decides what runs. The file format is in CLI Reference: Patch files.

File	Result
Over 320 MiB; wrong length, magic, version, host fingerprint or payload digest; or, on a signed host, a missing or invalid signature	Rejected: signatureInvalid
A blacklisted patch	Rejected: blacklisted
A patch with seq above acceptedSeq	Installed; acceptedSeq becomes its seq. It runs from the next cold start
The installed patch file again	Success; nothing changes
Any other patch with seq not above acceptedSeq	Rejected: invalidArgs
A withdrawal with seq above acceptedSeq	The installed patch is deleted and acceptedSeq becomes its seq. The running process keeps its code; the next cold start runs the installed version
A withdrawal with seq not above acceptedSeq	Rejected: invalidArgs

acceptedSeq is kept per host build, so a new build of the app (a new fingerprint) starts at 0 and the previous build's patch no longer runs.

Platform support

The plugin works only on Android and iOS. On any other platform every call does nothing and prints one warning on first use:

API	On other platforms
init, rollback, clearBlacklist	Does nothing
isAvailable	false
checkUpdate	Reports no update
applyPatch, applyPatchBytes, applyPatchFile	Returns unknown
patchTarget, inspectPatch	Throws PatcherException
applyProgress	An empty stream
currentVersion, lastBootDiagnostic, appVersionCode	null
deviceAbi	''
blacklist	An empty list

On Android and iOS, an app not built with dart run flutter_patcher build accepts no patches: isAvailable is false, checkUpdate reports no update, patchTarget and inspectPatch throw PatcherException, and applyPatch, applyPatchBytes and applyPatchFile return unknown.

Classes

FlutterPatcher

Flutter hot-update entrypoint for Android and iOS.