Read "structured/serialized data" from NSStream - ios

Question

I am developing a game app which will take structured data from a server and make responses based on these data.
I have connected the app to the internet through the NSSream. Specifically, I follow the tutorial of Apple's guide. So my code looks something like:
- (void)stream:(NSStream *)stream handleEvent:(NSStreamEvent)eventCode {
switch(eventCode) {
case NSStreamEventHasBytesAvailable:
{
if(!_data) {
_data = [[NSMutableData data] retain];
}
uint8_t buf[1024];
NSInteger len = 0;
len = [(NSInputStream *)stream read:buf maxLength:1024];
if(len) {
[_data appendBytes:(const void *)buf length:len];
// bytesRead is an instance variable of type NSNumber.
[bytesRead setIntValue:[bytesRead intValue]+len];
} else {
NSLog(#"no buffer!");
}
break;
}
// continued
My problem is: "How can I convert the _data to the format I want when it is possible". For example, my server will send two types of data. For each data, the first byte is an indicater for the data type (e.g., 1 means data type 1, 2 means data type 2). For the data type 1, an int (4 bytes) is sent as the data itself. For the data type 2, an int (4 bytes) is sent as the size of the following string bytes. For example, if this int is 10, then there will be 10 more bytes sent from the server to form a string for the client.
The code in my Android(Java) app looks like:
// dataIn = new DataInputStream(socket.getInputStream());
private void keepPacketRecving(){ // this will be executed in a separate thread
while(keepRecvThreadRunning) {
try {
byte type;
type = dataIn.readByte();
if(type == 1){
int data = dataIn.readInt();
getTye1Data(data); // callback function for receiving type 1 data (int)
} else if (type ==2) {
int dataSize = dataIn.readInt();
byte[] buf = new byte[dataSize];
// ... loop to read enough byte into buf ...
String data = new String(buf);
getType2Data(data); // callback function for receiving type 2 data (String)
}
}
}
}
I also notice I can't have a while-loop inside the switch statement to ask inputStream to read more data untile it is enougth because it will hang the thread.
I guess the best way would be always appending all the bytes into the _data (like the example) and has another function to parse the _data? Then the problem is how I can peek bytes from the data and just fetch part of it out (e.g., 10 bytes out of a 20-byte _data).
Is there any wrapper in Objective-C like the DataInputStream in Java?