There are two major players involved in all Bluetooth low energy communication: the central and the peripheral. Based on a somewhat traditional client-server architecture, a peripheral typically has data that is needed by other devices. A central typically uses the information served up by peripherals to accomplish some particular task.

Peripherals broadcast some of the data they have in the form of advertising packets. An advertising packet is a relatively small bundle of data that may contain useful information about what a peripheral has to offer, such as the peripheral’s name and primary functionality. For instance, a digital thermostat may advertise that it provides the current temperature of a room. In Bluetooth low energy, advertising is the primary way that peripherals make their presence known. A central, on the other hand, can scan and listen for any peripheral device that is advertising information that it’s interested in.

The purpose of connecting to a peripheral is to begin exploring and interacting with the data it has to offer. Before you can do this, however, it helps to understand how the data of a peripheral is structured. Peripherals may contain one or more services or provide useful information about their connected signal strength. A service is a collection of data and associated behaviors for accomplishing a function or feature of a device (or portions of that device). For example, one service of a heart rate monitor may be to expose heart rate data from the monitor’s heart rate sensor. Services themselves are made up of either characteristics or included services (that is, references to other services). A characteristic provides further details about a peripheral’s service. For example, the heart rate service just described may contain one characteristic that describes the intended body location of the device’s heart rate sensor and another characteristic that transmits heart rate measurement data

After a central has successfully established a connection to a peripheral, it can discover the full range of services and characteristics the peripheral has to offer (advertising data might contain only a fraction of the available services). A central can also interact with a peripheral’s service by reading or writing the value of that service’s characteristic. For example, your app may request the current room temperature from a digital thermostat, or it may provide the thermostat with a value at which to set the room’s temperature.

Performing Common Central Role Tasks

0、Starting Up a Central Manager
myCentralManager = [[CBCentralManager alloc] initWithDelegate:self queue:nil options:nil];

1、Discovering Peripheral Devices That Are Advertising
[myCentralManager scanForPeripheralsWithServices:nil options:nil];

- (void)centralManager:(CBCentralManager *)central
 didDiscoverPeripheral:(CBPeripheral *)peripheral
     advertisementData:(NSDictionary *)advertisementData
                  RSSI:(NSNumber *)RSSI {

    NSLog(@"Discovered %@", peripheral.name);
    self.discoveredPeripheral = peripheral;

2、Connecting to a Peripheral Device After You’ve Discovered It
[myCentralManager connectPeripheral:peripheral options:nil];

- (void)centralManager:(CBCentralManager *)central
  didConnectPeripheral:(CBPeripheral *)peripheral {

    NSLog(@"Peripheral connected");
    peripheral.delegate = self;

3、Discovering the Services of a Peripheral That You’re Connected To
[peripheral discoverServices:nil];

- (void)peripheral:(CBPeripheral *)peripheral
didDiscoverServices:(NSError *)error {

    for (CBService *service in peripheral.services) {
        NSLog(@"Discovered service %@", service);
    }

4、Discovering the Characteristics of a Service
[peripheral discoverCharacteristics:nil forService:interestingService];

- (void)peripheral:(CBPeripheral *)peripheral
didDiscoverCharacteristicsForService:(CBService *)service
             error:(NSError *)error {

    for (CBCharacteristic *characteristic in service.characteristics) {
        NSLog(@"Discovered characteristic %@", characteristic);
    }

5、Retrieving the Value of a Characteristic
[peripheral readValueForCharacteristic:interestingCharacteristic];

- (void)peripheral:(CBPeripheral *)peripheral
didUpdateValueForCharacteristic:(CBCharacteristic *)characteristic
             error:(NSError *)error {

    NSData *data = characteristic.value;
    // parse the data as needed

6、Subscribing to a Characteristic’s Value
[peripheral setNotifyValue:YES forCharacteristic:interestingCharacteristic];

- (void)peripheral:(CBPeripheral *)peripheral
didUpdateNotificationStateForCharacteristic:(CBCharacteristic *)characteristic
             error:(NSError *)error {

    if (error) {
        NSLog(@"Error changing notification state: %@",
           [error localizedDescription]);
    }

7、Writing the Value of a Characteristic
[peripheral writeValue:dataToWrite forCharacteristic:interestingCharacteristic type:CBCharacteristicWriteWithResponse];

- (void)peripheral:(CBPeripheral *)peripheral
didWriteValueForCharacteristic:(CBCharacteristic *)characteristic
             error:(NSError *)error {

    if (error) {
        NSLog(@"Error writing characteristic value: %@",
            [error localizedDescription]);
    }

Refer to:
https://developer.apple.com/library/content/documentation/NetworkingInternetWeb/Conceptual/CoreBluetooth_concepts/PerformingCommonCentralRoleTasks/PerformingCommonCentralRoleTasks.html#//apple_ref/doc/uid/TP40013257-CH3-SW1 http://www.cocoachina.com/ios/20161031/17889.html
http://www.cocoachina.com/ios/20161021/17813.html
http://liuyanwei.jumppo.com/2015/07/17/ios-BLE-1.html
http://liuyanwei.jumppo.com/2015/08/14/ios-BLE-2.html