目录
- What is launch
- How to properly measure your launch
- Use Instrument to profile your launch
- Track your launch over time
1、What is launch
Why Launch Is Important
- First experience with your app should be delightful
- Indicative of your codes overall performance
- Impacts the system performance and battery
Example: Involves a lot of CPU work and a lot of memory work.
Launch Type
Cold launch
Cold launches occur after reboot, or when your app has not been launched for very long time. In order to launcher app, we need to bring it from disk into memory, startup system-side services that support your app, and then spawn your process.
Warm launch
Once it's happened once, you'll experience a warm launch. In this case, your app still needs to be spawned, but we've already brought your app into memory and started up some of those system-side services.
So, this will be a little bit faster and a little bit more consistent.
Resume
This occurs when a user reenters your app from either the home screen or the app switcher.
Phases of App Launch（App运行阶段）
As you know, launch generally starts when the user taps your icon on your home screen.Then over the next 100 or so milliseconds, iOS will do the necessary system-side work in order to initialize your app.
That leaves you as developers about 300 milliseconds to create your views, load your content, and generate your first frame.
These six phases cover everything from system initialization to the app initialization to view creation and layout, and then depending on your app, potentially a asynchronous loading phase for your data, the extended phase.
- System Interface
- Runtime Init
- UIKit Init
- Application Init
- Initial Frame Render
- Extended
System Interface
DYLD3
- Dynamic Linker loads shared libraries and frameworks
- Introduces caching of runtime dependencies to improve warm launch
App Startup Time: Past, Present, and Future WWDC 2017
Some steps to optimizing for DYLD3
- Avoid linking unused frameworks（this can have hidden costs）
- Avoid dynamic library loading during launch（such as DLOpen or NSbundleLoad）
- Hard link all your dependencies
For libSystem Init
- Initializes the interfaces with low level system components
- System side work with a fixed cost
Static Runtime Initialization
Steps to do:
- Initializes the language runtime（Objective-C and Swift run times）
- Invokes all class static load methods
Ways to optimizing:
- Expose delicate initialization API in frameworks
- Reduce impact to launch by avoiding
+ [Class load] - Use
+ [Class initialize]to lazily conduct static initialization
UIKit Initialization
Steps to do:
- Instantiates the
UIApplicationAndUIApplicationDelegate - Begins event processing and integration with the system
Ways to optimizing:
- Minimize work in
UIApplicationsubclass - Minimize work in
UIApplicationDelegateinitialization - Application Initialization
Lifecycle Callbacks
Invokes UIApplicationDelegate app lifecycle callbacks
application:willFinishLaunchingWithOptions:
application:didFinishLaunchingWithOptions:
Invokes UIApplicaitonDelegate UI lifecycle callbacks
applicationDidBecomaActive:
Invokes UIScreenDelegate UI lifecycle callbacks for each scene
scene:willConnectToSession:options:
scene:willEnterForeground:
sceneDidBecomeActive:
Ways to optimizing:
- Defer unrelated work
by either pushing it to the background queues or just doing it later entirely
- share resources between scenes
First Frame Render
Creates, performs layout for, and draws views
Commits and renders first frame
loadView
viewDidLoad
layoutSubviews
Ways to optimizing:
- Flatten view hierarchies （reducing the number of views in your hierarchy）and lazily load views
- Optimize auto layout usage（reduce the number of constraints）
Extended
- App-specific period after first frame
- Displaya asynchronously loaded data
- App should be interactive and responsive
- Leverage os_signpost to messure work
Measuring Performance Using Logging WWDC 2018
2、How to properly measure your launch
Trading Representativeness for Consistency
- Remove sources of variance to produce mode consistent results
- May result in launch times that are not representative
- Use consistent results to evaluate progress
Test in a Clean and Consistent Environment
- Reboot then let system quiesce for 2-3 minutes
- Enable Airplane Mode or mock the network
- Use unchanging or no iCloud Account
- Use release build of your app
- Measure warm launches
Measuring Launch with XCTest
Improving Battery Life and Performance WWDC 2019
3、Use Instruments to profile your launch
Tips and Tricks
Minimize work
- Defer work unrelated to first frame
- Move blocking work off main thread
- Reduce memory usage
Prioritize work
- Identify the right QoS for your task
- Utilize scheduler optimizations for app launch
- Preserve the priority with the right primitives
Modernizing Grand Central Dispatch Usage WWDC 2017
Optimize work
- Simplify or limit existing work
- Optimize algorithms and data structures
- Cache resources and computations
4、Track Your Launch Over Time
Make performance a development-time priority
Plot it and have a target
Monitor Customer Launches with Xcode Organizer
Adopt MetricKit for More Statistics
- Collect custom power and performance metrics
- Aggregated results delivered every 24 hours
Summary
- Start understanding your launch today
- Measure---don't estimate---performance
- Track performance in all phases of development
