Android TypedValue.applyDimension()的用法

這個方法的作用是 把Android系統中的非標準度量尺寸轉變爲標準度量尺寸 (Android系統中的標準尺寸是px, 即像素)

Android系統中的尺寸單位有: 

標準單位: px (px是安卓系統內部使用的單位, dp是與設備無關的尺寸單位 )

非標準單位: dp, in, mm, pt, sp

TypedValue.applyDimension()方法的功能就是把非標準尺寸轉換成標準尺寸, 如: 

dp->px:  TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 20, context.getResources().getDisplayMetrics());

in->px: TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_IN, 20, context.getResources().getDisplayMetrics());

mm->px: TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_MM, 20, context.getResources().getDisplayMetrics());

pt->px: TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_PT, 20, context.getResources().getDisplayMetrics());

sp->px: TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP, 20, context.getResources().getDisplayMetrics());

如果你還是不太理解, 請看applyDimension()方法的源碼:  

/**
 * 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) {
    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;
}