Note: Publishers.IsMainThread always outputs its value on the main queue (you can revise the publisher code from the beginning of the post). receive(on:) overrides that and switches to your desired scheduler regardless what queue does the publisher code use internally.
Now, in the above example, I am using httpbin.org/post to parrot back data to me under the json key. Obviously, this is not a realistic scenario, but it illustrates the pattern of chaining various publishers together (and using Combine to get out of the weeds of writing imperative code).
swift publisher 3 serial code
Serial Item and Contribution Identifier (SICI). The SICI is a variable length code that provides unique identification of serial issues (SICI) and serial contributions (SCI). The SICI consists of the ISSN, issue date, issue numbering, SICI standard version number, and a check character. The SCI adds the contribution location and a title code (if more than one title begins on a page) between the issue numbering and the SICI version number. The SICI version associated with the barcode differs from the strictly eye-readable version of the SICI, so there will often be two SICIs in a single bibliographic record.
Schedulers can also optimize code that performs commands in parallel, allowing developers to execute more commands at the same time. If code is in serial, developers can execute code one bit at a time.
In the code block above, the values are emitted from a different thread instead of the main thread. The subscribe(on) method executes tasks serially, as seen by the order of the executed instructions.
Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: developed
Maintainer: Michael Ferguson
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Package Summary Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: maintained
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Package Summary Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: maintained
Maintainer: Paul Bouchier , Mike Purvis
Author: Michael Ferguson
License: BSD
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Package Summary Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: maintained
Maintainer: Paul Bouchier , Mike Purvis
Author: Michael Ferguson
License: BSD
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Package Summary Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: maintained
Maintainer: Paul Bouchier , Mike Purvis
Author: Michael Ferguson
License: BSD
Source: git -drivers/rosserial.git (branch: jade-devel)
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Package Summary Released Continuous Integration Documented Metapackage for core of rosserial.Maintainer status: maintained
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Author: Michael Ferguson
License: BSD
Source: git -drivers/rosserial.git (branch: jade-devel)
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Package Summary Released Continuous Integration: 12 / 12Build history (last 5 of 21 builds): #21 21-Aug-2020 21:11 12 / 12 #20 17-Apr-2020 17:11 12 / 12 #19 16-Apr-2020 20:11 12 / 12 #18 15-Apr-2020 15:11 12 / 12 #17 15-Apr-2020 02:11 12 / 12 Documented Metapackage for core of rosserial.Maintainer status: maintained
Maintainer: Paul Bouchier , Mike Purvis
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Package Summary Released Continuous Integration: 14 / 14Build history (last 5 of 16 builds): #16 09-Sep-2020 16:40 14 / 14 #15 09-Sep-2020 15:40 14 / 14 #14 07-Sep-2020 14:40 14 / 14 #13 06-Sep-2020 03:40 14 / 14 #12 25-Aug-2020 23:40 14 / 14 Documented Metapackage for core of rosserial.Maintainer status: maintained
Maintainer: Paul Bouchier , Mike Purvis
Author: Michael Ferguson
License: BSD
Source: git -drivers/rosserial.git (branch: noetic-devel)
ContentsOverviewClient Libraries
ROS-side Interfaces
Examples and Use Cases
LimitationsMaximum Size of a Message, Maximum Number of Publishers/Subscribers
Float64
Strings
Arrays
ProtocolPacket Format
Topic Negotiation
Time
Report a Bug
Overviewrosserial is a protocol for wrapping standard ROS serialized messages and multiplexing multiple topics and services over a character device such as a serial port or network socket. In addition to a protocol definition, there are three types of packages found in this suite: Client LibrariesClient libraries allow users to easily get ROS nodes up and running on various systems. These clients are ports of the general ANSI C++ rosserial_client library. Currently, these packages include: rosserial_arduino support for Arduino compatible boards including UNO, Leonardo, MEGA, DUE, Teensy 3.x and LC, Spark, STM32F1, STM32Duino, ESP8266 and ESP32 rosserial_embeddedlinux support for Embedded Linux (eg, routers) rosserial_windows support for communicating with Windows applications rosserial_mbed support for mbed platforms rosserial_tivac support for TI's Launchpad boards, TM4C123GXL and TM4C1294XL rosserial_vex_v5 support for VEX V5 Robot Brain rosserial_vex_cortex support for VEX Cortex board rosserial_stm32 support for STM32 MCUs, based on STM32CubeMX HAL ros-teensy support for teensy platforms Refer to Adding Support for New Hardware for details on how to add a new hardware platform. ROS-side InterfacesDevices running rosserial code require a node on the host machine to bridge the connection from the serial protocol to the more general ROS network: rosserial_python A Python-based implementation (recommended for PC usage). rosserial_server A C++ implementation based on the ShapeShifter message, some limitations compared to rosserial_python but recommended for high-performance applications. Examples and Use CasesWe have created a number of extensions and examples: rosserial_arduino Tutorials - contains a number of examples of using various sensors and actuators with Arduino. This is the most well documented tutorial, and many of the feature depicted there can be extended into other platforms.
rosserial_xbee - tools for creating sensor networks using XBEE devices and Arduino.
rosserial_embeddedlinux Tutorials contains a number of examples of various sensors and actuators with an embedded linux system which cannot run full-blown ROS.
rosserial_mbed Tutorials - contains a number of examples of using various sensors and actuators with and Mbed platform.
rosserial_tivac Tutorials - Various examples for programming TI's TivaC Launchpad with Energia IDE or catkinized projects.
LimitationsMaximum Size of a Message, Maximum Number of Publishers/SubscribersThe number of Publishers and Subscribers are limited at 25, and the size of serialization and deserialization buffers are limited at 512 bytes by default for rosserial_client. However, those numbers and sizes are too big for microcontroller with limited SRAM. The buffer sizes, and numbers of Publisher/Subscriber for rosserial_arduino now vary depending on the chip used: AVR Model Input/Output buffer sizes Publishers/Subscribers ATMEGA168 150/150 bytes 6/6 ATMEGA328P 280/280 bytes 25/25 All others 512/512 bytes 25/25 You can change these numbers and sizes, refer to tutorial NodeHandle and ArduinoHardware for more informaiton. typedef NodeHandle_ NodeHandle;You should, however, be careful not to consume too much of the limited SRAM found in the Arduino. If the Arduino ran out of SRAM, it will simply hang with no debug information. Messages larger than the buffer size is not transmitted. A ROS error message will be relayed telling whether the message was coming from, or going to, the device. Float64The Arduino does not support 64-bit float datatypes. The serialization/deserialization code generated by make_library will automatically convert 64-bit floats into 32-bit datatypes, however, it should be noted that a loss of precision may occur! StringsTo conserve precious AVR memory, strings are not stored inside a message instance, instead an unsigned char * is stored. This has two impacts: When publishing, you must assign store the string data elsewhere and set the pointer: function isnumbered(obj) return obj.childNodes.length && obj.firstChild.childNodes.length && obj.firstChild.firstChild.className == 'LineNumber';function nformat(num,chrs,add) var nlen = Math.max(0,chrs-(''+num).length), res = ''; while (nlen>0) res += ' '; nlen-- return res+num+add;function addnumber(did, nstart, nstep) var c = document.getElementById(did), l = c.firstChild, n = 1; if (!isnumbered(c)) if (typeof nstart == 'undefined') nstart = 1; if (typeof nstep == 'undefined') nstep = 1; var n = nstart; while (l != null) if (l.tagName == 'SPAN') var s = document.createElement('SPAN'); var a = document.createElement('A'); s.className = 'LineNumber'; a.appendChild(document.createTextNode(nformat(n,4,''))); a.href = '#' + did + '_' + n; s.appendChild(a); s.appendChild(document.createTextNode(' ')); n += nstep; if (l.childNodes.length) l.insertBefore(s, l.firstChild); else l.appendChild(s); l = l.nextSibling; return false;function remnumber(did) var c = document.getElementById(did), l = c.firstChild; if (isnumbered(c)) while (l != null) if (l.tagName == 'SPAN' && l.firstChild.className == 'LineNumber') l.removeChild(l.firstChild); l = l.nextSibling; return false;function togglenumber(did, nstart, nstep) var c = document.getElementById(did); if (isnumbered(c)) remnumber(did); else addnumber(did,nstart,nstep); return false;document.write('Toggle line numbers'); 1 std_msgs::String str_msg; 2 unsigned char hello[13] = "hello world!"; 3 4 str_msg.data = hello;When subscribing to a message containing a string datatype, the string itself will not be copied from the deserialization buffer. Therefore, while it will be valid during the callback function, it will disappear when any other message is deserialized. If you need to keep the value of the string outside the callback, you must manually copy the string elsewhere. ArraysArrays have similar limitations to strings, however, since there is no easy way to find the termination of an array (analogous to the \0 found at the end of a string), we need to specify the size of the array. An extra variable is added to the class definition to accomplish this. For instance, the geometry_msgs/PoseArray is declared as: Header headergeometry_msgs/Pose[] posesOn the Arduino, this will translate to a class: document.write('Toggle line numbers'); 1 class PoseArray 2 3 Header header; 4 int poses_length; 5 Pose * poses; 6 Therefore, to send an array message, we have to set the length and pointer. When deserializing, we cannot deserialize in-place like the string (since the bytes of the message are actually packed, unlike a string which is passed in plain form). Therefore, the deserialization function will automatically allocate enough storage using realloc(), attempting to reuse the memory location whenever possible and only expanding it when the new message received is larger than the largest previous message. Some array types containing other array types will not deserialize correctly, as all elements of the child type will point to the same memory. This limitation may be addressed in the future. ProtocolThe rosserial protocol is aimed at point-to-point ROS communications over a serial transmission line. We use the same serialization/de-serialization as standard ROS messages, simply adding a packet header and tail which allows multiple topics to share a common serial link. This page describes the low-level details of the packet header and tail, and several special topics used for synchronization. Packet Format 1st Byte - Sync Flag (Value: 0xff) 2nd Byte - Sync Flag / Protocol version 3rd Byte - Message Length (N) - Low Byte 4th Byte - Message Length (N) - High Byte 5th Byte - Checksum over message length 6th Byte - Topic ID - Low Byte 7th Byte - Topic ID - High Byte x Bytes - Serialized Message Data Byte x+1 - Checksum over Topic ID and Message DataThe Protocol version byte was 0xff on ROS Groovy, 0xfe on ROS Hydro, Indigo, and Jade. Topics ID 0-100 are reserved for system functions, as defined in the rosserial_msgs/TopicInfo message. The checksums on the length and data are used to make sure that a particular packet has not been corrupted. The checksum over the message length is computed as follows: Message Length Checksum = 255 - ((Message Length High Byte + Message Length Low Byte) % 256 )The checksum over the Topic ID and data is computed as follows: Message Data Checksum = 255 - ((Topic ID Low Byte + Topic ID High Byte + Data byte values) % 256)Topic NegotiationBefore data transfer can begin, the PC/Tablet side must query the Arduino or other embedded device for the names and types of topics which will be published or subscribed to. Topic negotiation consists of a query for topics, a response with the number of topics, and packets to define each topic. The request for topics uses a topic ID of 0. The query for topics will look like: 0xff 0xfe 0x00 0x00 0xff 0x00 0x00 0xffA series of response packets (message type rosserial_msgs/TopicInfo, each containing information about a particular topic, with the following data in place of the serialized message: uint16 topic_id string topic_name string message_type string md5sum int32 buffer_sizeHere, the topic name is the name of the topic, for instance "cmd_vel", and message type is the type of the message, for instance "geometry_msgs/Twist". If a response packet is not received correctly, another query may be sent. MD5 checksums are transmitted to verify that both the sender and receiver are using the same message. TimeTime synchronization is handled by sending a std_msgs::Time in each direction. The embedded device can request the current time from the PC/Tablet by sending an empty Time message. The returned time is used to find clock offset. Report a BugReport bugs, ask questions in the issues list on github/rosserial Wiki: rosserial (last edited 2018-10-01 21:17:03 by RomainReignier)
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