Android DisplayMetrics 获取和屏幕相关的信息

Android源码中是这样来描述DisplayMetrics的。

/**
 * A structure describing general information about a display, such as its
 * size, density, and font scaling.
 * <p>To access the DisplayMetrics members, initialize an object like this:</p>
 * <pre> DisplayMetrics metrics = new DisplayMetrics();
 * getWindowManager().getDefaultDisplay().getMetrics(metrics);</pre>
 */

按照DisplayMetrics注释中的那样，我们直接写个例子来测试下，就什么都明白了。我用的是小米3：

        DisplayMetrics metrics = new DisplayMetrics();
        getActivity().getWindowManager().getDefaultDisplay().getMetrics(metrics);

        /**
         * 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).
         *
         * density 是逻辑上的屏幕密度，约定在160dpi的设备上，1px=1dip。
         * density = densityDpi / 160
         *
         * @see #DENSITY_DEFAULT
         */
        float density = metrics.density;
        /**
         * The screen density expressed as dots-per-inch.  May be either
         * {@link #DENSITY_LOW}, {@link #DENSITY_MEDIUM}, or {@link #DENSITY_HIGH}.
         * densityDpi 表示每英寸的点数（并不是像素数）
         */
        int densityDpi = metrics.densityDpi;
        /**
         * The absolute height of the display in pixels.
         * 屏幕的绝对高度，以像素为单位。
         */
        int heightPixels = metrics.heightPixels;
        /**
         * The absolute width of the display in pixels.
         * 同样，这个是屏幕的绝对宽度，以像素为单位。
         */
        int widthPixels = metrics.widthPixels;

        /**
         * The exact physical pixels per inch of the screen in the X dimension.
         * 横向每一英寸确切的物理像素，我们可以尝试通过这个值和widthPixels 来计算屏幕的宽度英寸。
         */
        float xdpi = metrics.xdpi;
        /**
         * The exact physical pixels per inch of the screen in the Y dimension.
         * 横向每一英寸确切的物理像素，我们可以尝试通过这个值和heightPixels 来计算屏幕的高度英寸。
         */
        float ydpi = metrics.ydpi;
        /**
         * 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.
         *  scaledDensity 这个值从上面的注释看，是和字体相关的factor，通常情况下这个值和density是相等的，但是假如用户手动的调整了
         *  系统字体的大小，那么这个值就有可能改变（以小米3为例，标准字体，这个值=3，最大字体这个值=3.25）。
         *  因此现在很多情况下，我们把字体单位写成dp，尽管Google建议的字体让写成sp。
         */
        float scaledDensity = metrics.scaledDensity;
        LogUtil.logd(TAG, "metrics.density = " + density);
        LogUtil.logd(TAG, "metrics.densityDpi = " + densityDpi);
        LogUtil.logd(TAG, "metrics.heightPixels = " +heightPixels);
        LogUtil.logd(TAG, "metrics.widthPixels = " +widthPixels);
        LogUtil.logd(TAG, "metrics.xdpi = " +xdpi);
        LogUtil.logd(TAG, "metrics.ydpi = " +ydpi);
        LogUtil.logd(TAG, "metrics.scaledDensity = " +scaledDensity);

        //来计算手机是几英寸的
        float pixelsToDipWidth  = widthPixels / xdpi;
        float pixelsToDipHeight = heightPixels / ydpi;
        LogUtil.logd(TAG, "pixelsToDipWidth = " + pixelsToDipWidth);
        LogUtil.logd(TAG, "pixelsToDipHeight = " + pixelsToDipHeight);
        double mobileInch = Math.sqrt(pixelsToDipHeight * pixelsToDipHeight + pixelsToDipWidth * pixelsToDipWidth);
        //我用的小米3，得到的值是4.917646062686045
        LogUtil.logd(TAG, "mobileInch = " + mobileInch);

通过这个例子，我们明白了dip及px代表着什么，我们就可以来写出dip与px相互转换的方法。（dip = px / density）

    /**
     * 根据手机的分辨率从 dp 的单位 转成为 px(像素)
     */
    public static int dip2px(Context context, float dpValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (dpValue * scale + 0.5f);
    }

    /**
     * 根据手机的分辨率从 px(像素) 的单位 转成为 dp
     */
    public static int px2dip(Context context, float pxValue) {
        final float scale = context.getResources().getDisplayMetrics().density;
        return (int) (pxValue / scale + 0.5f);
    }

上面 加的这个0.5f的原因，Google官方文档中其实有介绍：

Then add 0.5f to round the figure up to the nearest whole number, when converting to an integer.

说白了就是为了四舍五入。。。

比如我上面的例子：

mobileInch = 4.917646062686045 ，我+0.5f（=5.417646062686045），然后再取整之后，就得到5。