【发布时间】：2021-04-05 10:55:50
【问题描述】：
我正在构建一个 iphone 应用程序，它使用 MQTT_Client_Foundation(https://github.com/novastone-media/MQTT-Client-Framework) 订阅和发布到 Adafruit.com 上的主题。 我让它在服务器上更新值时连接并查找更新，并在点击按钮时发布到服务器。 我的问题是我想在打开应用程序时获取服务器上的当前主题值，而不必等待更新。 这是我现在的代码：
#import "ViewController.h"
#import <MQTTClient/MQTTClient.h>
@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *batteryValueLabel;
@property (strong, nonatomic) IBOutlet UIImageView *tempValueLabel;
@property (strong, nonatomic) IBOutlet UILabel *temperatureValueLabel;
@property (weak, nonatomic) IBOutlet UILabel *levelValueLabel;
@property (weak, nonatomic) IBOutlet UILabel *tripValueLabel;
@property (weak, nonatomic) IBOutlet UILabel *dateValueLabel;
@property(nonatomic, strong, readwrite) NSArray *certificates;
@property(nonatomic, weak) IBOutlet UIButton *Fill;
-(IBAction)Turnon:(id)sender;
@end
NSString *trip = @"0";
int trigger =1;
int B = 50;
int T = 90;
MQTTSession *session;
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.tempValueLabel.frame = CGRectMake(20, 250-(T+45),65, T+45);
_batteryValueLabel.frame = CGRectMake(114, 287, B*1.7, 63);
NSString* myNewString = [NSString stringWithFormat:@"%d", T];
self.temperatureValueLabel.text = myNewString;
MQTTCFSocketTransport *transport = [[MQTTCFSocketTransport alloc] init];
transport.host = @"io.adafruit.com";
transport.port = 1883;
session = [[MQTTSession alloc] init];
session.userName = @"xxxxxxxxx";
session.password = @"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
session.transport = transport;
session.delegate = self;
//此处为 /GET 修改代码
[session connectWithConnectHandler:^(NSError *error) {
if(!error){
[session subscribeToTopic:@"XXXXXX/feeds/+" atLevel:1 subscribeHandler:^(NSError *error, NSArray *gQoss){
if (error) {
NSLog(@"Subscription failed %@", error.localizedDescription);
} else {
NSLog(@"Subscription sucessfull! Granted Qos: %@", gQoss);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
//在我看来他们传递的是数字而不是字符串
trip = @"0";
NSData* data = [trip dataUsingEncoding:NSUTF8StringEncoding];
[session publishData:data onTopic:@"gbenna/feeds/temperature/get" retain:NO qos:1 publishHandler:nil];
NSLog(@"%@", trip);
NSLog(@"%d",trigger);
});}
}];
}
else {NSLog(@"[connectWithConnectHandler]Error Connect %@", error.localizedDescription);}
}];
}
//this is continuously called and gets data when server values are updated
- (void)newMessage:(MQTTSession *)session
data:(NSData *)data
onTopic:(NSString *)topic
qos:(MQTTQosLevel)qos
retained:(BOOL)retained
mid:(unsigned int)mid {
NSString *dataString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
NSLog(@"topic: %@", topic);
NSLog(@"data:%@", dataString);
if ([topic isEqualToString:@"xxxxxxx/feeds/temperature"]){
T = [dataString intValue];
self.temperatureValueLabel.text = dataString;
self.tempValueLabel.frame = CGRectMake(20, 250-(T+45),65, T+45);
if (T <= 69) {
_tempValueLabel.image = [UIImage imageNamed:@"dk blue rectangle.png"];
}
else if ((69< B)&& (B <= 90)) {
_tempValueLabel.image = [UIImage imageNamed:@"blue rectangle.png"];
}
else if (B > 90) {
_tempValueLabel.image = [UIImage imageNamed:@"red rectangle.png"];
}
}
if ([topic isEqualToString:@"xxxxxxxx/feeds/level"]){
self.levelValueLabel.text = dataString;}
if ([topic isEqualToString:@"xxxxxxxx/feeds/date"]){
self.dateValueLabel.text = dataString;}
if ([topic isEqualToString:@"xxxxxxxx/feeds/battery"]){
B = [dataString intValue];
_batteryValueLabel.frame = CGRectMake(114, 287, B*1.7, 63);
if (B <= 20) {
_batteryValueLabel.image = [UIImage imageNamed:@"red rectangle.png"];
}
else if ((20< B)&& (B <= 100)) {
_batteryValueLabel.image = [UIImage imageNamed:@"green rectangle.png"];
}
else if (B > 100) {
_batteryValueLabel.image = [UIImage imageNamed:@"red rectangle.png"];
}
}
if ([topic isEqualToString:@"xxxxxxxxx/feeds/trip"]){
self.tripValueLabel.text = dataString;}
}
//when I tap this button the value on the server is updated
-(IBAction)Turnon:(id)sender;{
if(trigger ==1){
trip = @"0";
trigger = 2;
}
else if(trigger==2){
trip = @"100";
trigger = 1;
}
NSData* data = [trip dataUsingEncoding:NSUTF8StringEncoding];
[session publishData:data onTopic:@"xxxxxxxxx/feeds/trip" retain:NO qos:1 publishHandler:nil];
NSLog(@"%@", trip);
NSLog(@"%d",trigger);
}
@end
有谁知道什么代码和放在哪里，这样当我打开应用程序时，它会在服务器上获取当前主题值而无需等待更新？
【问题讨论】：
-
Adafruit 代理是否支持保留消息？
-
无法确定很难从他们那里得到任何答案。你能建议一种方法来做我想做的事吗？我会试试的。
-
发布一条设置了保留位的消息
-
我找不到如何发布保留标志，但他们确实有 /get 修饰符。这基本上将发布发送到您想要当前值的主题，并使其更新为相同的值。因此，当您订阅相同的主题时，它会更新为相同的值。这里有一个例子：gist.github.com/abachman/f6c2c4e1600517f339b32e10ac580c0d 和 /get 的解释：io.adafruit.com/api/docs/mqtt.html#adafruit-io-39-s-limitations 在“使用 *get 主题”下。我已经更新了我原来的帖子，试图让它发挥作用。似乎使用了#tho
-
保留位是您发布消息时的一个选项，而不是您在订阅客户端执行的操作。我建议您阅读以下内容：hivemq.com/blog/mqtt-essentials-part-8-retained-messages