当手机没有连接PC时，手机log缓冲区仍然会保存指定大小的最新log，连接pc，通过adb logcat 仍然可以拿出来
如何查看log缓缓区的大小？
通过adb logcat -g 可以查看
C:\Users\zh>adb logcat -g
main: ring buffer is 64Kb (62Kb consumed), max entry is 5120b, max payload is 4068b
system: ring buffer is 64Kb (63Kb consumed), max entry is 5120b, max payload is 4068b
crash: ring buffer is 64Kb (1Kb consumed), max entry is 5120b, max payload is 4068b
C:\Users\zh>adb logcat -g
main: ring buffer is 256Kb (67Kb consumed), max entry is 5120b, max payload is 4068b
system: ring buffer is 256Kb (60Kb consumed), max entry is 5120b, max payload is 4068b
crash: ring buffer is 256Kb (1Kb consumed), max entry is 5120b, max payload is 4068b
C:\Users\zh>adb logcat -g
main: ring buffer is 1Mb (73Kb consumed), max entry is 5120b, max payload is 4068b
system: ring buffer is 1Mb (60Kb consumed), max entry is 5120b, max payload is 4068b
crash: ring buffer is 1Mb (1Kb consumed), max entry is 5120b, max payload is 4068b
上面的 64Kb、256Kb、1Mb 大小是可以指定的，指定路径：
Settings --- Developer options --- Logger buffer sizes，弹出一个对话框，选择设置即可
将缓冲区的log打印到屏幕并退出，使用 adb logcat -d 命令
adb logcat -d > log.txt