屏幕适配多个dimens
由于Android手机屏幕的各异，导致我们在开发过程中，为保证ui效果的统一，往往需要针对性的进行屏幕适配。
我们先看几个概念：
px：pixel，像素Android原生API，UI设计计量单位，如获取屏幕宽高。
屏幕分辨率：指在纵向和横向方向上的像素点数，单位是px，一般显示方式是纵向像素数量*横向像素数量，如1920*1080。
屏幕尺寸：一般是屏幕对角线长度，单位是英寸，常见尺寸有3.5，4.0，4.3，4.7，5.0，6.0等。
屏幕像素密度：ppi pixel per inch的缩写，意思是每英寸屏幕上的像素数，因为屏幕尺寸是商家生产时就规定好的，屏幕尺寸一样的手机，屏幕宽高却不一定一样，所以通常取屏幕对角线像素数量和屏幕尺寸（屏幕对角线长度）来计算屏幕像素密度，计算公式就是通过勾股定理和分辨率计算得到屏幕对角线像素数量，再除以屏幕尺寸。手机参数上也会有这个数值。
dpi：dot per inch点像素密度，它的计算方法也和ppi一样，但从很多方面上，dpi是和ppi有区别的，ppi是用在设备上的单位，比如显示器；而dpi是用在印刷品上的单位，比如要打印一幅地图；在图像上的像素看起来是一个个点或者方块，这时候通常就将两者混用。要注意的是，我们并不能在Android适配时直接使用手机参数上的ppi值，而是使用dpi，Android对dpi根据大小做了规定的分档，以160dpi为基准，分为ldpi（120dpi）、mdpi（160dpi）、hdpi（240dpi）、xhdpi（320dpi）、xxhdpi（480dpi）等，当然现在手机的分辨率和尺寸更加多，像560dpi，600dpi也有，举个例子，Nexus 6 ppi是493，那按道理我们计算出来的dpi也是493，但Android的分档里面并没有493dpi，所以实际上它的dpi是560，而这个值我们是可以在手机的系统文件里面修改的。一定程度上，我们甚至可以说分辨率和适配没有关系，我们需要考虑的是dpi值，而这个值是可以与分辨率没有关系的，是手机厂家设的。
/**
* 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;通过设置多个dimens适配
比如values-w320dp，values-w360dp
通过上面的例子得到对应的dp值为360,
wdp = widthPixels / density
附dp和px互转：
TypeValue
/**
* Converts a complex data value holding a dimension to its final value
* as an integer pixel size. This is the same as
* {@link #complexToDimension}, except the raw floating point value is
* converted to an integer (pixel) value for use as a size. A size
* conversion involves rounding the base value, and ensuring that a
* non-zero base value is at least one pixel in size.
* The given data must be structured as a
* {@link #TYPE_DIMENSION}.
*
* @param data A complex data value holding a unit, magnitude, and
* mantissa.
* @param metrics Current display metrics to use in the conversion --
* supplies display density and scaling information.
*
* @return The number of pixels specified by the data and its desired
* multiplier and units.
*/
public static int complexToDimensionPixelSize(int data,
DisplayMetrics metrics)
{
final float value = complexToFloat(data);
final float f = applyDimension(
(data>>COMPLEX_UNIT_SHIFT)&COMPLEX_UNIT_MASK,
value,
metrics);
final int res = (int)(f+0.5f);
if (res != 0) return res;
if (value == 0) return 0;
if (value > 0) return 1;
return -1;
}TypedArray
/**
* Retrieve a dimensional unit attribute at index for use
* as a size in raw pixels. This is the same as
* {@link #getDimension}, except the returned value is converted to
* integer pixels for use as a size. A size conversion involves
* rounding the base value, and ensuring that a non-zero base value
* is at least one pixel in size.
*
* @param index Index of attribute to retrieve.
* @param defValue Value to return if the attribute is not defined or
* not a resource.
*
* @return Attribute dimension value multiplied by the appropriate
* metric and truncated to integer pixels, or defValue if not defined.
*
* @see #getDimension
* @see #getDimensionPixelOffset
*/
public int getDimensionPixelSize(int index, int defValue) {
if (mRecycled) {
throw new RuntimeException("Cannot make calls to a recycled instance!");
}
index *= AssetManager.STYLE_NUM_ENTRIES;
final int[] data = mData;
final int type = data[index+AssetManager.STYLE_TYPE];
if (type == TypedValue.TYPE_NULL) {
return defValue;
} else if (type == TypedValue.TYPE_DIMENSION) {
return TypedValue.complexToDimensionPixelSize(
data[index+AssetManager.STYLE_DATA], mMetrics);
} else if (type == TypedValue.TYPE_ATTRIBUTE) {
throw new RuntimeException("Failed to resolve attribute at index " + index);
}
throw new UnsupportedOperationException("Can't convert to dimension: type=0x"
+ Integer.toHexString(type));
}