android.util.TypedValue.applyDimension

先看一个例子：

int size = (int)TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 3,
　　context.getResources().getDisplayMetrics());

这行代码是把在当前屏幕分辨率的状态下将dip为3的值转换成单位为px值　　

下面是API介绍：

public static float applyDimension (int unit, float value, DisplayMetrics metrics)

Since: API Level 1

Converts an unpacked complex data value holding a dimension to its final floating point value. The two parameters unit and value are as in TYPE_DIMENSION.

Parameters

unit	The unit to convert from.
value	The value to apply the unit to.
metrics	Current display metrics to use in the conversion -- supplies display density and scaling information.

Returns

• The complex floating point value multiplied by the appropriate metrics depending on its unit. 

查看其在framework中的源码

    public static float applyDimension(int unit, float value,
                                       DisplayMetrics metrics)
    {
        switch (unit) {
        case COMPLEX_UNIT_PX:
            return value;
        case COMPLEX_UNIT_DIP:
            return value * metrics.density;
        case COMPLEX_UNIT_SP:
            return value * metrics.scaledDensity;
        case COMPLEX_UNIT_PT:
            return value * metrics.xdpi * (1.0f/72);
        case COMPLEX_UNIT_IN:
            return value * metrics.xdpi;
        case COMPLEX_UNIT_MM:
            return value * metrics.xdpi * (1.0f/25.4f);
        }
        return 0;
    }

如果传入的unit为COMPLIEX_UNIT_PX，则返回值与传入的value值保持一致；反之会根据一定的计算方式转换成px并返回。