移动开发的过程中，在程序中打印日志是最有效的调试手段。Android系统提供了一套很好用日志API（android.util.Log），可以在代码中添加不同级别的日志。
在开发过程中logcat可以通过adb使用也可以在设备上直接使用，先看下帮助内容：
Usage: logcat [options] [filterspecs]
options include:
-s Set default filter to silent.
Like specifying filterspec '*:s'
-f Log to file. Default to stdout
-r [] Rotate log every kbytes. (16 if unspecified). Requires -f
-n Sets max number of rotated logs to , default 4
-v
Sets the log print format, where
is one of:
brief process tag thread raw time threadtime long
-c clear (flush) the entire log and exit
-d dump the log and then exit (don't block)
-g get the size of the log's ring buffer and exit
-b request alternate ring buffer
('main' (default), 'radio', 'events')
-B output the log in binary
filterspecs are a series of
[:priority]
where is a log component tag (or * for all) and priority is:
V Verbose
D Debug
I Info
W Warn
E Error
F Fatal
S Silent (supress all output)
'*' means '*:d' and by itself means :v
If not specified on the commandline, filterspec is set from ANDROID_LOG_TAGS.
If no filterspec is found, filter defaults to '*:I'
If not specified with -v, format is set from ANDROID_PRINTF_LOG
or defaults to "brief"
