diff --git a/tidevice/__main__.py b/tidevice/__main__.py
index e304e9c..79a1a82 100644
--- a/tidevice/__main__.py
+++ b/tidevice/__main__.py
@@ -373,22 +373,14 @@ def cmd_launch(args: argparse.Namespace):
         logger.warning("skip_running is deprecated, always kill app now")
 
     d = _udid2device(args.udid)
-
     env = {}
     for kv in args.env or []:
         key, val = kv.split(":", 1)
         env[key] = val
     if env:
         logger.info("App launch env: %s", env)
-
-    try:
-        with d.connect_instruments() as ts:
-            pid = ts.app_launch(args.bundle_id,
-                                        app_env=env,
-                                        args=args.arguments)
-            print("PID:", pid)
-    except ServiceError as e:
-        sys.exit(e)
+    pid = d.app_start(args.bundle_id, args=args.arguments, env=env)
+    print("PID:", pid)
 
 
 def cmd_kill(args: argparse.Namespace):
diff --git a/tidevice/_ssl.py b/tidevice/_ca.py
similarity index 100%
rename from tidevice/_ssl.py
rename to tidevice/_ca.py
diff --git a/tidevice/_device.py b/tidevice/_device.py
index d84f581..27849a7 100644
--- a/tidevice/_device.py
+++ b/tidevice/_device.py
@@ -194,7 +194,7 @@ def pair(self):
         wifi_address = self.get_value("WiFiAddress", no_session=True)
 
         try:
-            from ._ssl import make_certs_and_key
+            from ._ca import make_certs_and_key
         except ImportError:
             #print("DevicePair require pyOpenSSL and pyans1, install by the following command")
             #print("\tpip3 install pyOpenSSL pyasn1", flush=True)
@@ -702,13 +702,15 @@ def app_kill(self, *args, **kwargs) -> int:
 
     def app_start(self,
                   bundle_id: str,
-                  args: Optional[list] = []) -> int:
+                  args: Optional[list] = [],
+                  env: typing.Mapping = {}) -> int:
         """
         start application
         
         Args:
             bundle_id: com.apple.Preferences
-            args: ['-AppleLanguages', '(en)']
+            args: eg ['-AppleLanguages', '(en)']
+            env: eg {'MYPATH': '/tmp'}
 
         Returns:
             pid
@@ -716,7 +718,7 @@ def app_start(self,
         if args is None:
             args = []
         with self.connect_instruments() as ts:
-            return ts.app_launch(bundle_id, args=args)
+            return ts.app_launch(bundle_id, args=args, app_env=env)
 
     def app_install(self, file_or_url: Union[str, typing.IO]) -> str:
         """
diff --git a/tidevice/_instruments.py b/tidevice/_instruments.py
index 324579e..d64110b 100644
--- a/tidevice/_instruments.py
+++ b/tidevice/_instruments.py
@@ -520,16 +520,17 @@ def recv_dtx_message(self) -> Tuple[Any, bytearray]:
         >> Body 部分
         00000030: F0 1B 00 00 00 00 00 00  61 1B 00 00 00 00 00 00  ........a.......
         \\\\
-        # 前面8个字节 0X1BF0 据说是Magic word
+        # 前面8个字节 0X1BF0 据说是Magic word  TODO
         # 后面的 0x1B61 是整个序列化数据的长度
         # 解析的时候可以直接跳过这部分
 
+        
+        00000040: 0A 00 00 00 02 00 00 00  55 1B 00 00 62 70 6C 69  ........U...bpli
         # 序列化的数据部分, 使用OC的NSKeyedArchiver序列化
         # 0A,00,00,00: 起始头
         # 02,00,00,00: 2(obj) 3(u32) 4(u64) 5(u32) 6(u64)
         # 55,1B,00,00: 序列化的数据长度
-        00000040: 0A 00 00 00 02 00 00 00  55 1B 00 00 62 70 6C 69  ........U...bpli
-        .....
+        # 62,70,6C,69: bplist00这些就是plistlib的dumps后的数据了
 
         # 最后面还跟了一个NSKeyedArchiver序列化后的数据，没有长度字段
 
diff --git a/tidevice/struct2.py b/tidevice/struct2.py
index f9d078d..d51af20 100644
--- a/tidevice/struct2.py
+++ b/tidevice/struct2.py
@@ -6,6 +6,7 @@
 import struct
 from collections import namedtuple
 from functools import partial
+import typing
 
 
 class Field(object):
@@ -64,6 +65,7 @@ def __init__(self, typename: str, *fields, byteorder="<"):
             if f.name in self._field_names:
                 raise ValueError("Struct has duplicated name", f.name)
             self._field_names.append(f.name)
+        self._size = sum([f.size for f in self._fields])
 
     @property
     def size(self):
@@ -75,6 +77,9 @@ def _convert_field(self, fvalue):
         else:
             raise ValueError("Unknown type:", fvalue)
 
+    def parse_stream(self, reader: typing.BinaryIO):
+        return self.parse(reader.read(self.size))
+                          
     def parse(self, buffer: bytes):
         values = struct.unpack(self._fmt, buffer)
         return namedtuple(self._typename, self._field_names)(*values)