Android 各尺寸单位间的换算

~android-sdk\sources\android-17\android\util\TypedValue.java 中的函数：

 /**

     * Converts an unpacked complex data value holding a dimension to its final floating 
     * point value. The two parameters <var>unit</var> and <var>value</var>
     * are as in {@link #TYPE_DIMENSION}.
     *  
     * @param unit The unit to convert from.
     * @param value The value to apply the unit to.
     * @param metrics Current display metrics to use in the conversion -- 
     *                supplies display density and scaling information.
     * 
     * @return The complex floating point value multiplied by the appropriate 
     * metrics depending on its unit. 
     */
    public static float applyDimension(int unit, float value,
                                       DisplayMetrics metrics)
    {
        switch (unit) {
        //Value is raw pixels.
        case COMPLEX_UNIT_PX:
            return value;
        //Value is Device Independent Pixels.
        case COMPLEX_UNIT_DIP:
            return value * metrics.density;
        //Value is a scaled pixel.
        case COMPLEX_UNIT_SP:
            return value * metrics.scaledDensity;
        //Value is in points.
        case COMPLEX_UNIT_PT:
            return value * metrics.xdpi * (1.0f/72);
        //Value is in inches.
        case COMPLEX_UNIT_IN:
            return value * metrics.xdpi;
        //Value is in millimeters.
        case COMPLEX_UNIT_MM:
            return value * metrics.xdpi * (1.0f/25.4f);
        }
        return 0;
    }

~android-sdk\sources\android-17\android\util\DisplayMetrics.java 中的一些定义：
    
    /**
     * Standard quantized DPI for medium-density screens.
     */
    public static final int DENSITY_MEDIUM = 160;
    /**
     * The reference density used throughout the system.
     */
    public static final int DENSITY_DEFAULT = DENSITY_MEDIUM;
/**
     * 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.
     *  
     * <p>This value does not exactly follow the real screen size (as given by 
     * {@link #xdpi} and {@link #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).
     *
     * @see #DENSITY_DEFAULT
     */
     public float density;
//默认值 density =  DENSITY_DEVICE / (float) DENSITY_DEFAULT;


 
/**
     * A scaling factor for fonts displayed on the display.  This is the same
     * as {@link #density}, except that it may be adjusted in smaller
     * increments at runtime based on a user preference for the font size.
     */
     public float scaledDensity;
//默认值 scaledDensity = density;
 
/**The exact physical pixels per inch of the screen in the X dimension.
     */
     public float xdpi;
     //默认值xdpi = DENSITY_DEVICE;