Android 擷取裝置的物理尺寸

得到裝置的物理尺寸，注意，不是解析度！

從一個外國的文章上找到的答案http://stackoverflow.com/questions/2193457/is-there-a-way-to-determine-android-physical-screen-height-in-cm-or-inches

    DisplayMetrics dm = new DisplayMetrics();    getWindowManager().getDefaultDisplay().getMetrics(dm);    double x = Math.pow(dm.widthPixels/dm.xdpi,2);    double y = Math.pow(dm.heightPixels/dm.ydpi,2);    double screenInches = Math.sqrt(x+y);    Log.d("debug","Screen inches : " + screenInches);
 
還有一段：
According to the documentation:
 

  
The logical density of the display. This is a scaling factor for the Density Independent Pixel unit, where one DIP is one pixel on an approximately 160 dpi screen (for example a 240x320, 1.5"x2" screen), providing the baseline of the system's display. Thus
 on a 160dpi screen this density value will be 1; on a 120 dpi screen it would be .75; etc.
  
This value does not exactly follow the real screen size (as given by xdpi and ydpi, but rather is used to scale the size of the overall UI in steps based on gross changes in the display dpi. For example, a 240x320 screen will have a density of 1 even if
 its width is 1.8", 1.3", etc. However, if the screen resolution is increased to 320x480 but the screen size remained 1.5"x2" then the density would be increased (probably to 1.5).