百度APP技术团队曾经发布过一篇深夜暗坑 - iOS启动图异常修复方案。
该文章分享了一些关于启动图的研究，但是遗留了一个很重要的问题，iOS 是如何获取夜间模式启动图的？
本文将通过分析系统内部文件解决一下这个问题。
方案一：通过启动文件名进行分析
我们首先对原文提供的信息进行初步的分析。
原文提供了以下2个信息：
缓存启动图的文件名具有规则，但其规则我们不得而知- 4 张启动图的文件名
├── 1FFD332B-EBA0-40C9-8EEE-BEC9AEF7C41A@3x.ktx
├── 96920D11-6312-4D69-BBDB-AFBB52DBDDB3@3x.ktx
├── 98F7B5B1-5B3B-478B-93A8-ED3DE6492AD1@3x.ktx
└── D9D48845-8565-42CE-A834-479CC9CC8BAD@3x.ktx
通过 4 个文件名，我们可以发现4张图片的命名都符合以下规则：
代码语言：javascript复制xxxxxxxx-xxxx-4xxx-xxxx-xxxxxxxxxxxx
再结合苹果官方文档 NSUUID[1] 的内容：
代码语言：javascript复制`NSUUID`conform to RFC 4122 version 4 and are created with random bytes.
我们可以得到以下结论：
- 4 个文件名的都是通过
NSUUID动态生成 - 文件名只包含版本
4，不再包含其它有效的信息
方案二：通过系统文件进行分析
方案一失败后，我们猜测 iOS 是通过其它方式保存夜间模式启动图的路径。
经过一系列的测试，作者发现了 applicationState.db 文件。
applicationState.db
系统会通过 applicationState.db 保存程序状态等各类信息，当然，也会包括夜间模式启动图的路径。
本文分析的文件位于 ~/Library/Developer/CoreSimulator/Devices/1F9B22C5-E446-4881-AFE4-3373E3513C59/data/Library/FrontBoard/applicationState.db
其中，1F9B22C5-E446-4881-AFE4-3373E3513C59代表 iOS 模拟器的设备ID。
模拟器的完整ID列表可以通过命令
plutil -p ~/Library/Developer/CoreSimulator/Devices/device_set.plist查看
测试环境
为了方便对系统文件进行分析，本文以 iOS 14 模拟器为目标进行分析。
版本信息如下所示：
代码语言：javascript复制(lldb) platform status
Platform: ios-simulator
Triple: x86_64h-apple-macosx
OS Version: 10.15.6 (19G2021)
Kernel: Darwin Kernel Version 19.6.0: Thu Jun 18 20:49:00 PDT 2020; root:xnu-6153.141.1~1/RELEASE_X86_64
Hostname: 127.0.0.1
WorkingDir: /
SDK Path: "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk"
No devices are available.
另外，后续的代码会假定
Bundle identifier是test.SplashTest
解析 applicationState.db
先尝试通过 file命令获取 applicationState.db文件类型：
file ~/Library/Developer/CoreSimulator/Devices/1F9B22C5-E446-4881-AFE4-3373E3513C59/data/Library/FrontBoard/applicationState.db
输出：
代码语言：javascript复制SQLite 3.x database, last written using SQLite version 3032003
测试成功，通过 file 命令的输出可以看到文件类型是 SQLite 3.x。
db 结构
下面，再通过 SQLite 相关的工具对该文件进行dump，我们可以得到以下信息：
sqlite> .schema
CREATE TABLE schema(version INT NOT NULL);
CREATE TABLE key_tab (id INTEGER PRIMARY KEY, key TEXT NOT NULL, UNIQUE(key));
CREATE TABLE application_identifier_tab (id INTEGER PRIMARY KEY, application_identifier TEXT NOT NULL, UNIQUE(application_identifier));
CREATE TABLE kvs ( id INTEGER PRIMARY KEY, application_identifier INT REFERENCES application_identifier_tab(id), key INT REFERENCES key_tab(id), value BLOB, UNIQUE(application_identifier, key));
CREATE INDEX kvs_keys ON kvs(key);
CREATE INDEX kvs_application_identifiers ON kvs(application_identifier);
CREATE VIEW kvs_debug AS SELECT application_identifier_tab.application_identifier, key_tab.key, value FROM application_identifier_tab, key_tab, kvs WHERE kvs.application_identifier=application_identifier_tab.id AND kvs.key=key_tab.id
/* kvs_debug(application_identifier,"key",value) */;
application_identifier_tab保存了设备安装的应用列表（id是主键，application_identifier是 APP 的Bundle identifier)key_tab负责记录常量字符串。 经过测试，夜间模式启动图的路径属于XBApplicationSnapshotManifest。
sqlite> .schema key_tab
CREATE TABLE key_tab (id INTEGER PRIMARY KEY, key TEXT NOT NULL, UNIQUE(key));
sqlite> .width 2 50
sqlite> select * from key_tab;
id key
-- --------------------------------------------------
1 SBLaunchImageIngestionInfo
2 XBApplicationSnapshotManifest
3 _SBScenes
4 SBApplicationShortcutItems
5 compatibilityInfo
6 SBApplicationRecentlyUpdated
7 SBApplicationRecentlyUpdatedTimerStartDate
kvs负责在前面两个表的基础上新增存储一份value BLOBkvs_debug负责串联上面的3个表：
classDiagram
kvs_debug --|> application_identifier_tab : application_identifier
kvs_debug --|> key_tab : key
kvs_debug --|> kvs : value
key_tab : int id
key_tab : text key
application_identifier_tab : int id
application_identifier_tab : text application_identifier
kvs : int id
kvs : int application_identifier
kvs : int key
kvs : BLOB value
kvs_debug : application_identifier
kvs_debug : key
kvs_debug : value
kvs.application_identifier=application_identifier_tab.id
kvs.key=key_tab.id
XBApplicationSnapshotManifest
通过 test.SplashTest，可以获取到4个结果，其中第2负责保存快照相关信息
sqlite> .width 15 32 8
sqlite> SELECT * FROM kvs_debug WHERE application_identifier = 'test.SplashTest';
application_ide key value
--------------- -------------------------------- --------
test.SplashTest _SBScenes bplist00
test.SplashTest XBApplicationSnapshotManifest bplist00
test.SplashTest SBApplicationRecentlyUpdated 0
test.SplashTest compatibilityInfo bplist00
导出XBApplicationSnapshotManifest
经过一番研究，我们发现 XBApplicationSnapshotManifest 对应的value 就是 SplashBoard 库 XBApplicationSnapshotManifestImpl 类的持久化结果。
所以，我们可以通过通过以下代码，对 value 的内容进行 dump。
(void)load {
void *lib = dlopen("/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/PrivateFrameworks/SplashBoard.framework/SplashBoard", RTLD_NOW);
printf("%p", lib);
[self dump:@"/Users/test/Library/Developer/CoreSimulator/Devices/1F9B22C5-E446-4881-AFE4-3373E3513C59/data/Library/FrontBoard/XBApplicationSnapshotManifest.plist"];
}
(void)dump:(NSString *)path {
NSData *data0 = [NSData dataWithContentsOfFile:path];
NSPropertyListFormat f = -1;
NSError *error = nil;
NSData *data1 = [NSPropertyListSerialization propertyListWithData:data0 options:NSPropertyListReadStreamError format:&f error:&error];
if (f==kCFPropertyListXMLFormat_v1_0) {
NSLog(@"kCFPropertyListXMLFormat_v1_0");
}
id obj = [NSKeyedUnarchiver unarchiveTopLevelObjectWithData:data1 error:&error];
NSLog(@"%@",obj);
}
dump 结果：
代码语言：javascript复制<XBApplicationSnapshotManifestImpl: 0x600000050d80; clientCount: 0> {
containerIdentity = 0x0;
snapshots = {
<XBApplicationSnapshotGroup: 0x600002e65ea0; identifier: test.SplashTest - {DEFAULT GROUP}> {
<XBApplicationSnapshot: 0x7fcb53f04d90; identifier: CE275D00-5732-4AFD-88FD-00BAE541EC12; launchInterfaceIdentifier: __from_UILaunchStoryboardName__; contentType: GeneratedDefault; referenceSize: {375, 812}; interfaceOrientation: LandscapeLeft; userInterfaceStyle: Dark> {
creationDate = September 27, 2020 at 6:08:50 PM GMT 8;
keepsImageAccessUntilExpiration = NO;
hasGenerationContext = NO;
context = {
contentType = GeneratedDefault;
fullScreen = YES;
referenceSize = {375, 812};
interfaceOrientation = LandscapeLeft;
userInterfaceStyle = Dark;
additionalContext = {
statusBarSettings = <XBStatusBarSettings: 0x600002c580f0; hidden: YES; style: 0x0; backgroundActivityEnabled: NO>;
}
}
imageContext = {
scale = 3.0;
opaque = YES;
fileRelativeLocation = default;
fileFormat = png;
}
};
<XBApplicationSnapshot: 0x7fcb57004830; identifier: B9DAB53E-29D9-47D2-873E-5772DE9220D1; launchInterfaceIdentifier: __from_UILaunchStoryboardName__; contentType: GeneratedDefault; referenceSize: {375, 812}; interfaceOrientation: Portrait; userInterfaceStyle: Light> {
creationDate = September 27, 2020 at 6:08:50 PM GMT 8;
lastUsedDate = September 27, 2020 at 6:08:50 PM GMT 8;
keepsImageAccessUntilExpiration = NO;
hasGenerationContext = NO;
context = {
contentType = GeneratedDefault;
fullScreen = YES;
referenceSize = {375, 812};
interfaceOrientation = Portrait;
userInterfaceStyle = Light;
additionalContext = {
statusBarSettings = <XBStatusBarSettings: 0x600002c5c1a0; hidden: NO; style: 0x0; backgroundActivityEnabled: NO>;
}
}
imageContext = {
scale = 3.0;
opaque = YES;
fileRelativeLocation = default;
fileFormat = png;
}
};
<XBApplicationSnapshot: 0x7fcb57004b60; identifier: 6B84614D-0867-4048-BE04-8E22E6742DDF; launchInterfaceIdentifier: __from_UILaunchStoryboardName__; contentType: GeneratedDefault; referenceSize: {375, 812}; interfaceOrientation: Portrait; userInterfaceStyle: Dark> {
creationDate = September 27, 2020 at 6:08:50 PM GMT 8;
keepsImageAccessUntilExpiration = NO;
hasGenerationContext = NO;
context = {
contentType = GeneratedDefault;
fullScreen = YES;
referenceSize = {375, 812};
Portrait;
userInterfaceStyle = Dark;
additionalContext = {
statusBarSettings = <XBStatusBarSettings: 0x600002c5c2d0; hidden: NO; style: 0x0; backgroundActivityEnabled: NO>;
}
}
imageContext = {
scale = 3.0;
opaque = YES;
fileRelativeLocation = default;
fileFormat = png;
}
};
<XBApplicationSnapshot: 0x7fcb57005140; identifier: D3E8D00C-EE33-466B-98A6-7E60865D8001; launchInterfaceIdentifier: __from_UILaunchStoryboardName__; contentType: GeneratedDefault; referenceSize: {375, 812}; interfaceOrientation: LandscapeLeft; userInterfaceStyle: Light> {
creationDate = September 27, 2020 at 6:08:50 PM GMT 8;
keepsImageAccessUntilExpiration = NO;
hasGenerationContext = NO;
context = {
contentType = GeneratedDefault;
fullScreen = YES;
referenceSize = {375, 812};
interfaceOrientation = LandscapeLeft;
userInterfaceStyle = Light;
additionalContext = {
statusBarSettings = <XBStatusBarSettings: 0x600002c5c400; hidden: YES; style: 0x0; backgroundActivityEnabled: NO>;
}
}
imageContext = {
scale = 3.0;
opaque = YES;
fileRelativeLocation = default;
fileFormat = png;
}
};
};
}
}
SplashBoard 部分类图
通过类信息，整理如下所示（只包含关键属性）：
classDiagram
XBApplicationSnapshotManifestImpl *-- XBApplicationSnapshotGroup : _snapshotGroupsByID.value
XBApplicationSnapshotManifestImpl *-- NSString : _snapshotGroupsByID.key
XBApplicationSnapshotManifestImpl : _snapshotGroupsByID
XBApplicationSnapshotGroup : NSString identifier
XBApplicationSnapshotGroup : NSMutableSet _snapshots
XBApplicationSnapshotGroup *-- XBApplicationSnapshot : _snapshots
XBApplicationSnapshot : NSString _identifier
XBApplicationSnapshot : NSString _groupID
XBApplicationSnapshot : NSString _relativePath
XBApplicationSnapshot : NSDate _creationDate
XBApplicationSnapshot : int _userInterfaceStyle
XBApplicationSnapshot : XBStatusBarSettings statusBarSettings
XBApplicationSnapshot *-- XBStatusBarSettings:statusBarSettings
结论
通常上面的内容，我们可以对 iOS 获取夜间模式启动的流程进行合理的猜测
启动时，会先通过 XBApplicationSnapshotGroup的identifier 获取test.SplashTest - {DEFAULT GROUP} 的启动图列表
再通过 XBApplicationSnapshot 的 userInterfaceStyle = Dark; 和 interfaceOrientation = Portrait;等信息，判断启动时应该使用
<XBApplicationSnapshot: 0x7fcb57004b60; identifier: 6B84614D-0867-4048-BE04-8E22E6742DDF; launchInterfaceIdentifier: __from_UILaunchStoryboardName__; contentType: GeneratedDefault; referenceSize: {375, 812}; interfaceOrientation: Portrait; userInterfaceStyle: Dark>
最后再通过 XBApplicationSnapshot的 _relativePath 拼接启动图的真实路径
~/Library/Developer/CoreSimulator/Devices/1F9B22C5-E446-4881-AFE4-3373E3513C59/data/Containers/Data/Application/FA902232-17D2-495F-B23E-410349A9921C/Library/SplashBoard/Snapshots/test.SplashTest - {DEFAULT GROUP}/6B84614D-0867-4048-BE04-8E22E6742DDF@3x.ktx
总结
本文通过对 applicationState.db 进行一系列的分析，最终实现了获取夜间模式启动图的诉求。
参考资料
[1]
NSUUID: https://developer.apple.com/documentation/foundation/nsuuid?language=objc
