iOS蓝牙开发（二）在设备端实现Central角色

若想在设备上实现Central角色的功能，主要有以下步骤：

1.需要创建一个 CBCentralManager 对象

2.搜索周围广播的设备

3.与一个外设进行连接，并探索外设提供的服务

4.向外设发送读写characteristic的请求，如果有需要订阅characteristic值得更新，来跟踪数据的变化。

myCentralManager =
        [[CBCentralManager alloc] initWithDelegate:self queue:nil options:nil];

[myCentralManager scanForPeripheralsWithServices:nil options:nil];

//CBCentralManager代理方法
- (void)centralManager:(CBCentralManager *)central
 didDiscoverPeripheral:(CBPeripheral *)peripheral
     advertisementData:(NSDictionary *)advertisementData
                  RSSI:(NSNumber *)RSSI {
 
    NSLog(@"Discovered %@", peripheral.name);
    self.discoveredPeripheral = peripheral;
    ...
}

选择需要的外设并建立连接，并设置外设的代理为self，用于与外设的交互

[myCentralManager connectPeripheral:peripheral options:nil];

- (void)centralManager:(CBCentralManager *)central
  didConnectPeripheral:(CBPeripheral *)peripheral {
 
    NSLog(@"Peripheral connected");
    peripheral.delegate = self;//
    ...
}

搜索外设提供的服务并读取数据

[peripheral discoverServices:nil];

- (void)peripheral:(CBPeripheral *)peripheral
didDiscoverServices:(NSError *)error {
 
    for (CBService *service in peripheral.services) {
        NSLog(@"Discovered service %@", service);
        ...
    }
    ...
}

停止搜索外设

[myCentralManager stopScan];

从服务中获取特征

NSLog(@"Discovering characteristics for service %@", interestingService);
    [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);
        ...
    }
    ...
}

从特征中读取数据

NSLog(@"Reading value for characteristic %@", interestingCharacteristic);
    [peripheral readValueForCharacteristic:interestingCharacteristic];

- (void)peripheral:(CBPeripheral *)peripheral
didUpdateValueForCharacteristic:(CBCharacteristic *)characteristic
             error:(NSError *)error {
 
    NSData *data = characteristic.value;
    // parse the data as needed
    ...
}

如果需要获取动态变化的特征，需要订阅特征变化的通知

[peripheral setNotifyValue:YES forCharacteristic:interestingCharacteristic];

- (void)peripheral:(CBPeripheral *)peripheral
didUpdateNotificationStateForCharacteristic:(CBCharacteristic *)characteristic
             error:(NSError *)error {
 
    if (error) {
        NSLog(@"Error changing notification state: %@",
           [error localizedDescription]);
    }
    ...
}

向外设写数据

NSLog(@"Writing value for characteristic %@", interestingCharacteristic);
    [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]);
    }
    ...
}

需要注意一下事项：

由于蓝牙是共享无线电硬件设备的来发送和接受数据的，可能其他无线通信也需要使用无线电硬件，例如Wifi,或者其他App也正在使用蓝牙，因此，应该尽量避免长时间使用无线电。可以通过以下方式来对其进行优化

1.只在需要时进行蓝牙外设搜索，当建立与外设的连接后，应停止搜索。调用stopScan方法

2.因为远程外设每秒会广播多次，当使用 scanForPeripheralsWithServices:options:方法搜索可以外设的时候，系统默认在一次搜索事件中忽略那些相同的外设。如果设置option为CBCentralManagerScanOptionAllowDuplicatesKey，那么每当中心设备受到一个外设的广播时，就会产生一次发现事件。因此应尽量避免使用这个option。

3.准确的获取外设的数据，而不是搜索外设提供的所有的数据，[peripheral discoverServices:@[firstServiceUUID, secondServiceUUID]];

4.当不再获取数据时，取消对数据的订阅，断开与外设的连接

setNotifyValue:NO forCharacteristic:

[myCentralManager cancelPeripheralConnection:peripheral];

重连外设

Figure 5-1 A sample reconnection workflow

iOS蓝牙开发（二）在设备端实现Central角色

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