android中px和dp,px和sp的轉換方法

android中px和dp,px和sp的轉換方法

在Android開發中dp和px,sp和px之間的轉換時必不可少的，網上流傳的方法

public class DisplayUtils {

    /**
     * convert px to its equivalent dp
     * 將px轉換爲與之相等的dp
     */
    public static int px2dp(Context context, float pxValue) {
        final float scale =  context.getResources().getDisplayMetrics().density;
        return (int) (pxValue / scale + 0.5f);
    }

    /**
     * convert dp to its equivalent px
     * 將dp轉換爲與之相等的px
     */
    public static int dp2px(Context context, float dipValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (dipValue * scale + 0.5f);
    }

    /**
     * convert px to its equivalent sp 
     * 將px轉換爲sp
     */
    public static int px2sp(Context context, float pxValue) {
        final float fontScale = context.getResources().getDisplayMetrics().scaledDensity;
        return (int) (pxValue / fontScale + 0.5f);
    }


    /**
     * convert sp to its equivalent px
     * 將sp轉換爲px
     */
    public static int sp2px(Context context, float spValue) {
        final float fontScale = context.getResources().getDisplayMetrics().scaledDensity;
        return (int) (spValue * fontScale + 0.5f);
    }
}

關於轉換公式中，通過類比一元一次函數，怎麼看都是有問題的，這麼明顯的問題，爲什麼沒人糾正，後來發現是自己並沒有理解，原因是float類型在強轉爲int類型是，用的是去尾法，精度上有較大差異，所以通過+0.5f的方式，將去尾法轉變成四捨五入法，提高精度。

---

調用TypedValue類實現轉換

/**
* convert dp to its equivalent px
*/
protected int dp2px(int dp){
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
         dp,getResources().getDisplayMetrics());
}

/**
* convert sp to its equivalent px
*/
protected int sp2px(int sp){
        return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_SP,
         sp,getResources().getDisplayMetrics());
}

public class TypedValue {
    ...
    /** {@link #TYPE_DIMENSION} complex unit: Value is raw pixels. */
    public static final int COMPLEX_UNIT_PX = 0;
    /** {@link #TYPE_DIMENSION} complex unit: Value is Device Independent
     *  Pixels. */
    public static final int COMPLEX_UNIT_DIP = 1;
    /** {@link #TYPE_DIMENSION} complex unit: Value is a scaled pixel. */
    public static final int COMPLEX_UNIT_SP = 2;
    /** {@link #TYPE_DIMENSION} complex unit: Value is in points. */
    public static final int COMPLEX_UNIT_PT = 3;
    /** {@link #TYPE_DIMENSION} complex unit: Value is in inches. */
    public static final int COMPLEX_UNIT_IN = 4;
    /** {@link #TYPE_DIMENSION} complex unit: Value is in millimeters. */
    public static final int COMPLEX_UNIT_MM = 5;

    /**
     * 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;
    }
    ...
}

對比兩種方式的差異，其實就在+0.5f上，去尾法和四捨五入法的區別，即精度問題。