WWDC2019 - Optimizing App Launch

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

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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）

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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.

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• 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 UIApplication And UIApplicationDelegate
• Begins event processing and integration with the system

Ways to optimizing:

• Minimize work in UIApplication subclass
• 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

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Improving Battery Life and Performance WWDC 2019

3、Use Instruments to profile your launch

Tips and Tricks

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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

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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

Optimizing App Launch