Add developer documentation
And update README.
This commit is contained in:
parent
c2ac6fe7bd
commit
727d1ef1e2
2 changed files with 273 additions and 6 deletions
238
DEVELOP.md
Normal file
238
DEVELOP.md
Normal file
|
@ -0,0 +1,238 @@
|
|||
# scrcpy for developers
|
||||
|
||||
## Overview
|
||||
|
||||
This application is composed of two parts:
|
||||
- the server (`scrcpy-server.jar`), to be executed on the device,
|
||||
- the client (the `scrcpy` binary), executed on the host computer.
|
||||
|
||||
The client is responsible to push the server to the device and start its
|
||||
execution.
|
||||
|
||||
Once the client and the server are connected to each other, the server initially
|
||||
sends device information (name and initial screen dimensions), then starts to
|
||||
send a raw H.264 video stream of the device screen. The client decodes the video
|
||||
frames, and display them as soon as possible, without buffering, to minimize
|
||||
latency. The client is not aware of the device rotation (which is handled by the
|
||||
server), it just knows the dimensions of the video frames.
|
||||
|
||||
The client captures relevant keyboard and mouse events, that it transmits to the
|
||||
server, which injects them to the device.
|
||||
|
||||
|
||||
|
||||
## Server
|
||||
|
||||
|
||||
### Privileges
|
||||
|
||||
Capturing the screen requires some privileges, which are granted to `shell`.
|
||||
|
||||
The server is a Java application (with a [`public static void main(String...
|
||||
args)`][main] method), compiled against the Android framework, and executed as
|
||||
`shell` on the Android device.
|
||||
|
||||
[main]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/Server.java#L61
|
||||
|
||||
To run such a Java application, the classes must be [_dexed_][dex] (typically,
|
||||
to `classes.dex`). If `my.package.MainClass` is the main class, compiled to
|
||||
`classes.dex`, pushed to the device in `/data/local/tmp`, then it can be run
|
||||
with:
|
||||
|
||||
adb shell CLASSPATH=/data/local/tmp/classes.dex \
|
||||
app_process / my.package.MainClass
|
||||
|
||||
_The path `/data/local/tmp` is a good candidate to push the server, since it's
|
||||
readable and writable by `shell`, but not world-writable, so a malicious
|
||||
application may not replace the server just before the client executes it._
|
||||
|
||||
Instead of a raw _dex_ file, `app_process` accepts a _jar_ containing
|
||||
`classes.dex` (e.g. an [APK]). For simplicity, and to benefit from the gradle
|
||||
build system, the server is built to an (unsigned) APK (renamed to
|
||||
`scrcpy-server.jar`).
|
||||
|
||||
[dex]: https://en.wikipedia.org/wiki/Dalvik_(software)
|
||||
[apk]: https://en.wikipedia.org/wiki/Android_application_package
|
||||
|
||||
|
||||
### Hidden methods
|
||||
|
||||
Although compiled against the Android framework, [hidden] methods and classes are
|
||||
not directly accessible (and they may differ from one Android version to
|
||||
another).
|
||||
|
||||
They can be called using reflection though. The communication with hidden
|
||||
components is provided by [_wrappers_ classes][wrappers] and [aidl].
|
||||
|
||||
[hidden]: https://stackoverflow.com/a/31908373/1987178
|
||||
[wrappers]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/wrappers
|
||||
[aidl]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/aidl/android/view
|
||||
|
||||
|
||||
### Threading
|
||||
|
||||
The server uses 2 threads:
|
||||
|
||||
- the **main** thread, encoding and streaming the video to the client;
|
||||
- the **controller** thread, listening for _control events_ (typically,
|
||||
keyboard and mouse events) from the client.
|
||||
|
||||
Since the video encoding is typically hardware, there would be no benefit in
|
||||
encoding and streaming in two different threads.
|
||||
|
||||
|
||||
### Screen video encoding
|
||||
|
||||
The encoding is managed by [`ScreenEncoder`].
|
||||
|
||||
The video is encoded using the [`MediaCodec`] API. The codec takes its input
|
||||
from a [surface] associated to the display, and writes the resulting H.264
|
||||
stream to the provided output stream (the socket connected to the client).
|
||||
|
||||
[`ScreenEncoder`]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java
|
||||
[`MediaCodec`]: https://developer.android.com/reference/android/media/MediaCodec.html
|
||||
[surface]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java#L63-L64
|
||||
|
||||
On device [rotation], the codec, surface and display are reinitialized, and a
|
||||
new video stream is produced.
|
||||
|
||||
New frames are produced only when changes occur on the surface. This is good
|
||||
because it avoids to send unnecessary frames, but there are drawbacks:
|
||||
|
||||
- it does not send any frame on start if the device screen does not change,
|
||||
- after fast motion changes, the last frame may have poor quality.
|
||||
|
||||
Both problems are [solved][repeat] by the flag
|
||||
[`KEY_REPEAT_PREVIOUS_FRAME_AFTER`][repeat-flag].
|
||||
|
||||
[rotation]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java#L89-L92
|
||||
[repeat]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java#L125-L126
|
||||
[repeat-flag]: https://developer.android.com/reference/android/media/MediaFormat.html#KEY_REPEAT_PREVIOUS_FRAME_AFTER
|
||||
|
||||
|
||||
### Input events injection
|
||||
|
||||
_Control events_ are received from the client by the [`EventController`] (run in
|
||||
a separate thread). There are 5 types of input events:
|
||||
- keycode (cf [`KeyEvent`]),
|
||||
- text (special characters may not be handled by keycodes directly),
|
||||
- mouse motion/click,
|
||||
- mouse scroll,
|
||||
- custom command (e.g. to switch the screen on).
|
||||
|
||||
All of them may need to inject input events to the system. To do so, they use
|
||||
the _hidden_ method [`InputManager.injectInputEvent`] (exposed by our
|
||||
[`InputManager` wrapper][inject-wrapper]).
|
||||
|
||||
[`EventController`]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/EventController.java#L70
|
||||
[`KeyEvent`]: https://developer.android.com/reference/android/view/KeyEvent.html
|
||||
[`MotionEvent`]: https://developer.android.com/reference/android/view/MotionEvent.html
|
||||
[`InputManager.injectInputEvent`]: https://android.googlesource.com/platform/frameworks/base/+/oreo-release/core/java/android/hardware/input/InputManager.java#857
|
||||
[inject-wrapper]: https://github.com/Genymobile/scrcpy/blob/v1.0/server/src/main/java/com/genymobile/scrcpy/wrappers/InputManager.java#L27
|
||||
|
||||
|
||||
|
||||
## Client
|
||||
|
||||
The client relies on [SDL], which provides cross-platform API for UI, input
|
||||
events, threading, etc.
|
||||
|
||||
The video stream is decoded by [libav] (FFmpeg).
|
||||
|
||||
[SDL]: https://www.libsdl.org
|
||||
[libav]: https://www.libav.org/
|
||||
|
||||
### Initialization
|
||||
|
||||
On startup, in addition to _libav_ and _SDL_ initialization, the client must
|
||||
push and start the server on the device, and open a socket so that they may
|
||||
communicate.
|
||||
|
||||
Note that the client-server roles are expressed at the application level:
|
||||
|
||||
- the server _serves_ video stream and handle requests from the client,
|
||||
- the client _controls_ the device through the server.
|
||||
|
||||
However, the roles are inverted at the network level:
|
||||
|
||||
- the client opens a server socket and listen on a port before starting the
|
||||
server,
|
||||
- the server connects to the client.
|
||||
|
||||
This role inversion guarantees that the connection will not fail due to race
|
||||
conditions, and avoids polling.
|
||||
|
||||
Once the server is connected, it sends the device information (name and initial
|
||||
screen dimensions). Thus, the client may init the window and renderer, before
|
||||
the first frame is available.
|
||||
|
||||
To minimize startup time, SDL initialization is performed while listening for
|
||||
the connection from the server (see commit [90a46b4]).
|
||||
|
||||
[90a46b4]: https://github.com/Genymobile/scrcpy/commit/90a46b4c45637d083e877020d85ade52a9a5fa8e
|
||||
|
||||
|
||||
### Threading
|
||||
|
||||
The client uses 3 threads:
|
||||
|
||||
- the **main** thread, executing the SDL event loop,
|
||||
- the **decoder** thread, decoding video frames,
|
||||
- the **controller** thread, sending _control events_ to the server.
|
||||
|
||||
|
||||
### Decoder
|
||||
|
||||
The [decoder] runs in a separate thread. It uses _libav_ to decode the H.264
|
||||
stream from the socket, and notifies the main thread when a new frame is
|
||||
available.
|
||||
|
||||
There are two [frames] simultaneously in memory:
|
||||
- the **decoding** frame, written by the decoder from the decoder thread,
|
||||
- the **rendering** frame, rendered in a texture from the main thread.
|
||||
|
||||
When a new decoded frame is available, the decoder _swaps_ the decoding and
|
||||
rendering frame (with proper synchronization). Thus, it immediatly starts
|
||||
to decode a new frame while the main thread renders the last one.
|
||||
|
||||
[decoder]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/decoder.c
|
||||
[frames]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/frames.h
|
||||
|
||||
|
||||
### Controller
|
||||
|
||||
The [controller] is responsible to send _control events_ to the device. It runs
|
||||
in a separate thread, to avoid I/O on the main thread.
|
||||
|
||||
On SDL event, received on the main thread, the [input manager][inputmanager]
|
||||
creates appropriate [_control events_][controlevent]. It is responsible to
|
||||
convert SDL events to Android events (using [convert]). It pushes the _control
|
||||
events_ to a blocking queue hold by the controller. On its own thread, the
|
||||
controller takes events from the queue, that it serializes and sends to the
|
||||
client.
|
||||
|
||||
[controller]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/controller.h
|
||||
[controlevent]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/controlevent.h
|
||||
[inputmanager]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/inputmanager.h
|
||||
[convert]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/convert.h
|
||||
|
||||
|
||||
### UI and event loop
|
||||
|
||||
Initialization, input events and rendering are all [managed][scrcpy] in the main
|
||||
thread.
|
||||
|
||||
Events are handled in the [event loop], which either updates the [screen] or
|
||||
delegates to the [input manager][inputmanager].
|
||||
|
||||
[scrcpy]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/scrcpy.c
|
||||
[event loop]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/scrcpy.c#L38
|
||||
[screen]: https://github.com/Genymobile/scrcpy/blob/v1.0/app/src/screen.h
|
||||
|
||||
|
||||
## Hack
|
||||
|
||||
For more details, go read the code!
|
||||
|
||||
If you find a bug, or have an awesome idea to implement, please discuss and
|
||||
contribute ;-)
|
41
README.md
41
README.md
|
@ -1,8 +1,8 @@
|
|||
# scrcpy
|
||||
|
||||
This project displays screens of Android devices connected on USB, and allow to
|
||||
control them using the host mouse and keyboard. It does not require any _root_
|
||||
access. It works on _GNU/Linux_, _Windows_ and _Mac OS_.
|
||||
This application provides display and control of Android devices connected on
|
||||
USB. It does not require any _root_ access. It works on _GNU/Linux_, _Windows_
|
||||
and _Mac OS_.
|
||||
|
||||
![screenshot](assets/screenshot-debian-600.jpg)
|
||||
|
||||
|
@ -50,8 +50,15 @@ Install the required packages from your package manager (here, for Debian):
|
|||
|
||||
#### Windows
|
||||
|
||||
For Windows, for simplicity, a prebuilt package with all the dependencies
|
||||
(including `adb`) is available: TODO.
|
||||
For Windows, for simplicity, a prebuilt archive with all the dependencies
|
||||
(including `adb`) is available:
|
||||
|
||||
- [`scrcpy-windows-with-deps-v1.0.zip`][direct-windows-with-deps].
|
||||
_(SHA-256: TODO)_
|
||||
|
||||
[direct-windows-with-deps]: https://github.com/Genymobile/scrcpy/releases/download/v1.0/scrcpy-windows-with-deps-v1.0.zip
|
||||
|
||||
_(It's just a portable version including _dll_ copied from MSYS2.)_
|
||||
|
||||
Instead, you may want to build it manually. You need [MSYS2] to build the
|
||||
project. From an MSYS2 terminal, install the required packages:
|
||||
|
@ -128,7 +135,12 @@ Just remove them to "uninstall" the application.
|
|||
|
||||
Since the server binary, that will be pushed to the Android device, does not
|
||||
depend on your system and architecture, you may want to use the prebuilt binary
|
||||
instead: [`scrcpy-server.jar`](TODO).
|
||||
instead:
|
||||
|
||||
- [`scrcpy-server-v1.0.jar`][direct-scrcpy-server].
|
||||
_(SHA-256: TODO)_
|
||||
|
||||
[direct-scrcpy-server]: https://github.com/Genymobile/scrcpy/releases/download/v1.0/scrcpy-server-v1.0.jar
|
||||
|
||||
In that case, the build does not require Java or the Android SDK.
|
||||
|
||||
|
@ -186,6 +198,23 @@ If several devices are listed in `adb devices`, you must specify the _serial_:
|
|||
| enable/disable FPS counter (on stdout) | `Ctrl`+`i` |
|
||||
|
||||
|
||||
## Why _scrcpy_?
|
||||
|
||||
A colleague challenged me to find a name as unpronounceable as [gnirehtet].
|
||||
|
||||
[`strcpy`] copies a **str**ing; `scrcpy` copies a **scr**een.
|
||||
|
||||
[gnirehtet]: https://github.com/Genymobile/gnirehtet
|
||||
[`strcpy`]: http://man7.org/linux/man-pages/man3/strcpy.3.html
|
||||
|
||||
|
||||
## Developers
|
||||
|
||||
Read the [developers page].
|
||||
|
||||
[developers page]: DEVELOP.md
|
||||
|
||||
|
||||
## Licence
|
||||
|
||||
Copyright (C) 2018 Genymobile
|
||||
|
|
Loading…
Reference in a new issue