learn MQTT protocol，check demo，using mqtt on you next project。(学习MQTT，在你以后的项目中使用它) ##Offcial
##Test
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paho It's an open project ,and there are so many resources for everyone who want to learn mqtt and using it. client tool supprot so many system and devices download it.Also,you may want to build source code where you can get here,and loT website.
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IBM server ibm server for test online
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HiveMQ server hive mq for test online
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MQTT.fx client client tool for test mqtt ,support mac/windows/linux.
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mqtt-spy client client tool for test mqtt ,support linux and windows.
##3rd lib
- MQTT-Client-Framework may be it is best for iOS.
- MQTTKit This project is no longer maintained。
- Paho
##Blog
- MQTT系列
- HIVEMQ MQTT系列文章 you and find many blogs and other resources on mqtt.
- how to iOS mosquitto in background
- iOS及Android消息推送方案安装使用入门
- 知乎：WebSocket 是什么原理？为什么可以实现持久连接？
- The Seven Best MQTT Client Tools