Android Layout Inflating

[connglli]

Overview

In Android, each view written in xml layout is parsed and instantiated by LayoutInflater. To get LayoutInflater, one often uses the following three manners:

• Activity#getLayoutInflater()
• LayoutInflater#from(context)
• Context#getSystemService(Context.LAYOUT_INFLATER_SERVICE)

Actually, the former two APIs in the end invokes the third API

In essence, Layout Inflater Service is not actually a system service like AMS, there are no actual services in the system server, but only wants to be managed in a system-service-alike manner, i.e., managed by Context#getSystemService() (but only registered to SystemServiceRegistry, not ServiceManager, and further not register to servicemanager)

To inflate a layout, one uses LayoutInflate#inflate(). In detail, LayoutInflater#inflate():

1. firstly tries to load an precompiled view, and return that, if not exists,
2. then uses XmlResourceParser to parse the XML, reading the classes and attributes, and iterately using reflection to create Views

AttributeSet, XmlResourceParser

:warning: TO BE ADDED

References

• http://aospxref.com/android-6.0.0_r41/xref/frameworks/base/core/java/android/app/SystemServiceRegistry.java#318
• http://aospxref.com/android-6.0.0_r41/xref/frameworks/base/core/java/android/view/LayoutInflater.java#808
• http://aospxref.com/android-6.0.0_r41/xref/frameworks/base/core/java/android/view/LayoutInflater.java#796
• https://mp.weixin.qq.com/s/eJ4cvtVit10oG3IQM72aDg

Live Layout Update (DynamicLayoutInspector)

Read also #101

The Live Layout Update of Android Studio uses poll mechanism. That is, there exists an ART agent daemon installed on the device, loaded to the app's process, and continuously crawling the view hierarchy, and sending tp and caching in the profiler daemon. Android Studio polls the daemon for the new updates (see following architecture)

|AS| --poll--> |Daemon| <--send-- |Agent| --poll--> |App|

This involves several questions:

1. When and how the daemon is created, and started?
2. When and how the agent is created, and started?
3. When does Android Studio establish the connection to the daemon?
4. What does Android Studio poll?

Implementation

• /tools/adt/idea/:layout-inspector

  • LayoutInspector:50: create a client for accepting events from agent
  • DefaultInspectorClient:87: create a TransportClient with channel name TransportService.CHANNEL_NAME
  • DefaultInspectorClient:93: create a TransportEventPoller
  • DefaultInspectorClient:290: transportPoller registers an event listener
  • DefaultInspectorClient:284: whenever an event happens, issue a LayoutInspectorCommand.START
  • LayoutInspector:51: register listeners
  • when there are COMPONENT_TREE events (this event happens whenever the view hierarchy changed), load the component tree sent (the sent component is a binary stream containing the view hierarchy, and the screen bitmap)

• /tools/adt/idea/:android-transport

  • TransportEventPoller:104: schedule to poll() at a fixed delay
  • TransportEventPoller:62: each poll, send a Transport.GetEventGroupsRequest, filters out old events, and notifies its listeners if there are new events

• /tools/base/:transport

  • main/transport.cc:78: create the daemon
  • main/transport.cc:94: start the daemon server
  • daemon/daemon.cc:205: register an ATTACH_AGENT command
  • daemon/daemon.cc:216: the server is built and started
  • daemon/commands/attach_agent.cc:39: attach the agent via ART TI
  • daemon/daemon.cc:258: actually attach the agent
  • daemon/daemon.cc:161: run 'cmd activity attach-agent /code_cache/='
  • is the name of the app
  • is path under '/data/app/',
  • is 'libjvmtiagent_x86.so'
  • is '/data/local/tmp/perfd/agent.config'
  • native/agent/transport_agent.cc:81: create the agent (i.e., native/agent/agent.cc)
  • native/agent/transport_agent.cc:84: load profiler agent 'perfa.jar' (i.e., /tools/base/:profiler/native/perfa)
  • native/agent/transport_agent.cc:91: register LayoutInspectorAgentCommand
  • native/agent/transport_agent.cc:101: setup profiler agent

• /tools/base/:profiler

  • native/perfa/perfa.cc:325: initialize profiler
  • :question::question::question::question: How perfa is setup?
  • :question::question::question::question: What and when an event is send to daemon by agent

• /tools/base/:dynamic-layout-inspector - dynamic layout inspector

  • native/agent/commands/layoutinspector_agent_command.cc:33: actual register LAYOUT_INSPECTOR command
  • native/agent/commands/layoutinspector_agent_command.cc:48: when LAYOUT_INSPECTOR command is issued, method of LayoutInspectorService will be called back
  • java/.../agent/layoutinspector/LayoutInspectorService:88: when LayoutInspectorCommand::START, detect root change
  • in DetectRootChange#doInBackground(), when there are root changes, invoke LayoutInspectorService#startLayoutInspector()
  • crawl view hierarchy (by WindowInspector#getGlobalWindowViews()), compare, then sleep 1s, then crawl, then ...
  • capture the view properties in LayoutInspectorService#captureAndSendComponentTree(), ComponentTree#{writeTree, loadView}()
  • capture the bitmap in WindowInspector#getGlobalWindowViews(), LayoutInspectorService#{startLayoutInspector, captureAndSendComponentTree, performViewCapture}()
  • send the bitmap (image) along with the hierarchy (component tree) in LayoutInspectorService#sendComponentTree(), set the command type as COMPONENT_TREE
  • java/.../agent/layoutinspector/LayoutInspectorService:88: when LayoutInspectorCommand::GET_PROPERTIES, find the view and gets its properties
  • use LayoutInspectorService#findViewById()

Why ART TI?

To load the agent in the same process as the app, so that can directly use the app's inner states