Exploration of MQTT for iOS Development

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1. What is MQTT?

MQTT (Message Queueing Telemetry Transport Protocol) is the abbreviation of Message Queuing Telemetry Transport Protocol. It is a message transmission protocol based on the publish/subscribe mode of lightweight proxy launched by IBM. It runs on the TCP protocol stack and provides orderly, reliable and two-way network connection guarantee. Because of its openness, simplicity and easy implementation, it can be applied in resource-constrained environments. It is a very good choice for M2M and Internet of Things applications.

2. Why use MQTT?

The MQTT protocol is designed for the following situations:

M2M (Machine to Machine) communication, end-to-end communication between machines, such as data communication between sensors. Because it is Machine to Machine, we need to consider: Machine, or equipment, such as temperature sensors, has very weak hardware capabilities, and the protocol must consider the smallest possible resource consumption, such as computing power and storage. M2M may be a wireless connection, with unstable networks and relatively small bandwidth.

Features of MQTT:

1. Publish/subscribe message mode, providing one-to-many message publishing and decoupling applications. This is very similar to 1. Here is the list text XMPP, but the information redundancy of MQTT is much less than XMPP.

2. Message transmission with shielding of payload content.

3. Use TCP/IP to provide network connection. The mainstream MQTT is based on TCP connection for data push, but there is also a UDP-based version called MQTT-SN. Since these two versions are based on different connection methods, their advantages and disadvantages are naturally different.

4. Three message transmission modes QoS:

• 0 means "at most once", and the message publishing is completely dependent on the underlying TCP/IP network. Message loss or duplication may occur. This level can be used in the following situations: environmental sensor data, where it does not matter if a read record is lost, because a second send will be sent soon.
• 1 stands for "at least once", which ensures that the message arrives, but message duplication may occur.
• 2 stands for "only once", ensuring that the message arrives once. This level can be used in the following situations: in the billing system, message duplication or loss will lead to incorrect results. Note: Since the server is implemented using Mosca, Mosca currently only supports QoS 1
• If the message sent is temporary, for example, sending a message to all online devices of a topic, it does not matter if it is lost, so 0 is enough (the client must specify the supported QoS level when logging in, and also specify the QoS level supported by this message when sending the message). If the client needs to ensure that it can receive the message, it needs to specify QoS as 1. If it is also necessary to add that the client can receive the message even when it is not online, then the validity of the session must be specified when the client logs in. To receive offline messages, it is necessary to specify that the server retains the client's session status.
• MQTT is based on the subscriber model architecture. If clients communicate with each other, they must be in the same subscription topic, that is, they all subscribe to the same topic. Clients cannot communicate directly. The obvious advantage of the subscription model is that if you want to send group messages, you only need to publish to the topic, and all clients subscribed to this topic can receive the message.
• Messages must be sent to a topic. It is important to note that messages can be sent to a topic regardless of whether the client has subscribed to the topic. Also, if the client has subscribed to the topic, the messages it sends will also be received.

5. Small transmission, small overhead (fixed-length header is 2 bytes), minimized protocol exchange to reduce network traffic. This is why it is said in the introduction that it is very suitable for "communication between sensors and servers and information collection in the field of Internet of Things". You should know that the computing power and bandwidth of embedded devices are relatively weak, so using this protocol to transmit messages is more suitable.

6. Use the Last Will and Testament features to notify all parties of abnormal client interruptions. Last Will: This is a last will mechanism that notifies other devices under the same topic that the device sending the last will has been disconnected. Testament: This is a last will mechanism that is similar in function to Last Will.

3. How to use MQTT

Building an MQTT server on a Mac

 $ brew install mosquitto

Wait for the download to complete and the service will start automatically.

 mosquitto has been installed with a default configuration file.
 You can make changes to the configuration by editing:  
 /usr/ local /etc/mosquitto/mosquitto.conf  
 To have launchd start mosquitto now and restart at login:  
 brew services start mosquitto  
 Or , if you don't want/need a background service you can just run:  
 mosquitto -c /usr/ local /etc/mosquitto/mosquitto.conf

iOS client registration

 #import "ViewController.h"   
 
 #define kMQTTServerHost @ "iot.eclipse.org"  
 #define kTopic @ "MQTTExample/Message"   
 
 @interface ViewController ()
 @property (weak, nonatomic) IBOutlet UILabel *showMessage;
 @property (nonatomic, strong) MQTTClient *client;
 @ end   
 
 @implementation ViewController 
 
 - (void)viewDidLoad
 {
 [super viewDidLoad]; 
 
 //1. After logging in to the app, the background returns name + password + topic 
 
 //2. name + password is used to connect to the host 
 
 //3.topic is used to subscribe to topics 
 
 
    UILabel *tempShowMessage = self.showMessage; 
 
    NSString *clientID = [UIDevice currentDevice].identifierForVendor.UUIDString; 
 
    self.client = [[MQTTClient alloc] initWithClientId:clientID]; 
 
 //Connect to the server After the connection is made, the connection result code will be returned through the block, and then this code block will be executed 
 
 //This interface is a modified interface. After modification, name + password is thrown  
    [self.client connectToHost:kMQTTServerHost andName:@ "cbt" andPassword:@ "1223" completionHandler:^(MQTTConnectionReturnCode code) {
 if (code == ConnectionAccepted) // Connection successful
 {
 // Subscribe
            [self.client subscribe:kTopic withCompletionHandler:^(NSArray *grantedQos) {
                // The client is effectively subscribed to the topic when this completion handler is called
                NSLog(@ "subscribed to topic %@" , kTopic);
                NSLog(@ "return:%@" ,grantedQos);
 }];
 }
 }]; 
 
 
 //The data received in MQTTMessage is binary, and the framework encapsulates it into a string
    [self.client setMessageHandler:^(MQTTMessage* message)
 {
        dispatch_async(dispatch_get_main_queue(), ^{
 //After receiving the message, you need to switch back to the main thread when updating the interface
            tempShowMessage.text= message.payloadString;
 });
 }]; 
 
 } 
 
 
 - (void)dealloc8
 {
    // disconnect the MQTT client
    [self.client disconnectWithCompletionHandler:^(NSUInteger code)
 {
        // The client is disconnected when this completion handler is called
        NSLog(@ "MQTT is disconnected" );
 }];
 }
 @ end  

The server pushes messages to the client

 #import "ViewController.h"  
 #import "MQTTKit.h"   
 
 
 #define kMQTTServerHost @ "iot.eclipse.org"  
 #define kTopic @ "MQTTExample/Message"  
 @interface ViewController ()
 @property (weak, nonatomic) IBOutlet UITextField *pushMessage;
 @property (nonatomic, strong) MQTTClient *client;
 @ end   
 
 @implementation ViewController 
 
 - (void)viewDidLoad {
 [super viewDidLoad]; 
 
 
    NSString *clientID = [UIDevice currentDevice].identifierForVendor.UUIDString; 
 
    self.client = [[MQTTClient alloc] initWithClientId:clientID];
    [self.client connectToHost:kMQTTServerHost andName:@ "cbt" andPassword:@ "1223" completionHandler:^(MQTTConnectionReturnCode code) {
        if (code == ConnectionAccepted)
 {
 NSLog(@ "Server started successfully" );
 }
 }]; 
 
 
 } 
 
 - (IBAction)push:(id)sender {
    NSString* payload = self.pushMessage.text;
    [self.client publishString:payload
                       toTopic:kTopic
                       withQos:AtMostOnce
                        retain:YES
             completionHandler:nil];
 NSLog(@ "Push content: %@" , payload);
 }