19 款的 mbp 还用的是 5500M 独立显卡，外接显示器必须使用独立显卡（基于个人折腾经验），只用集成显卡没办法外接屏幕，你可以通过命令行开关是否使用独立显卡
# Always use the integrated graphics card while running on battery power
sudo pmset -b gpuswitch 0
# Always use the discrete graphics card while running on battery power
sudo pmset -b gpuswitch 1
# Switch between discrete and integrated graphics cards automatically while running on battery power
sudo pmset -b gpuswitch 2
The -a, -b, -c, -u flags determine whether the settings apply to battery ( -b ), charger (wall power) ( -c ), UPS ( -u ) or all ( -a ).
