【Android】屏幕适配

带着疑问学习：

drawable, drawable-mdpi, drawable-hdpi, drawable-xhdpi, drawable-xxhdpi, drawable-xxxhdpi是什么意思？
同样的图片放在不同的目录下，会有什么效果差异？内存是否会有影响
在1080x2400下调整好的布局，到1080x1920下会被堆积，如何处理？
dp值对不同分辨率手机的效果影响

相关概念：

屏幕尺寸

手机屏幕对角线的物理尺寸。单位英寸（inch），一英寸大约 2.54cm。常见的手机屏幕尺寸有 4.7 英寸、5.0英寸、5.5 英寸、6.0 英寸等。
通常需要到手机官网或者太平洋电脑网查
比如MI 10手机是6.67英寸1080x2340像素，AndroidStudio的AVD提示Density为xhdpi, 但实际中MI10采用的是xxhdpi目录下的图片。

像素

像素（英语：Picture Element），Pixel 的缩写。液晶屏显示图像，放大来看是一个个小点组成的，这些小点就是像素点。
如下图左侧为50x50，将其编辑成右边的100x100后，原来1像素的黑点变成4个像素的黑点。
如下两个图是从不同参照物来看两者区别。
参照视觉上同尺寸：

参照100*100像素尺寸:

分辨率

分辨率（英语：Image resolution），又称解析度、解像度，可以从显示分辨率与图像分辨率两个方向来分类。
在 Android 设备中指的是显示分辨率，即屏幕分辨率。也就是屏幕所能显示的像素有多少，比如：手机分辨率 1920 x 1080。
在图片中指是图像分辨率，则是单位英寸中所包含的像素点数

像素密度（Pixel densities）

像素密度是屏幕物理区域内的像素数，称为dpi（每英寸点数）。通常情况下，每英寸像素值越高，屏幕能显示的图像也越精细。

The pixel density is the number of pixels within a physical area of the screen and is referred to as dpi (dots per inch). This is different from the resolution, which is the total number of pixels on a screen.

像素无关（density-independence dip/dp）

如果不用dp,那么控件会在低分辨率的显示很大，而在高分辨率下显示很小。如上图

Your app achieves “density independence” when it preserves the physical size (from the user’s point of view) of your UI design when displayed on screens with different pixel densities (as shown in figure 2). Maintaining density independence is important because, without it, a UI element (such as a button) might appear larger on a low-density screen and smaller on a high-density screen (because when the pixels are larger—as shown in figure 2—a few pixels can go a long way).
The Android system helps you achieve density independence by providing density-independent pixels (dp or dip) as a unit of measurement that you should use instead of pixels (px).
Maintaining density independence is important because, without it, a UI element (such as a button) might appear larger on a low-density screen and smaller on a high-density screen (because when the pixels are larger—as shown in figure 2—a few pixels can go a long way).

To preserve the visible size of your UI on screens with different densities, you must design your UI using density-independent pixels (dp) as your unit of measurement. One dp is a virtual pixel unit that’s roughly equal to one pixel on a medium-density screen (160dpi; the “baseline” density). （一个dp相当于一个在中等密度屏幕上显示的像素单元，即当在mdpi中1dp=1px，以160dpi为基准）， Android translates this value to the appropriate number of real pixels for each other density.
For example, consider the two devices in figure 1. If you were to define a view to be “100px” wide, it will appear much larger on the device on the left. So you must instead use “100dp” to ensure it appears the same size on both screens.

问题解答

1. drawable, drawable-mdpi, drawable-hdpi, drawable-xhdpi, drawable-xxhdpi, drawable-xxxhdpi是什么意思？

drawable文件夹通常放xml文件，比如layer-list, sector等。如果放图片的话，效果等同于drawble-mdpi
One dp is a virtual pixel unit that’s roughly equal to one pixel on a medium-density screen (160dpi; the “baseline” density). (一个dp是一个虚拟像素单位，大致相当于中等密度屏幕上的一个像素（160dpi；“基线”密度）)
dpi是手机的像素密度，即对角线像素值除以手机物理尺寸

计算出来的像素密度对应到下面的分类中
drawable-mdpi = 1x = ~160dpi
drawable-hdpi = 1.5x = ~240dpi
drawable-xhdpi = 2x = ~ 320dpi
drawable-xxhdpi = 3x = ~ 480dpi
drawable-xxxhdpi = 4x = ~ 640dpi

2. 同样的图片放在不同的目录下，会有什么效果差异？内存是否会有影响

（1）Alternative bitmaps
To ensure your images appear at their best on all screens, you should provide alternative bitmaps to match each screen density. For example, if your app provides bitmaps only for medium density (mdpi) screens, Android scales them up when on a high-density screen so that the image occupies the same physical space on the screen. This can cause visible scaling artifacts in bitmaps. So your app should include alternative bitmaps at a higher resolution.
下图左侧是120 x 91px, 右侧是左侧图片scale成600 x 455px，会被拉模糊，这是为何要准备不同目录下的图片的原因之一

（2）内存影响：待解

3. 在10802400下调整好的布局，到10801920下会被堆积，如何处理？

待解

4. dp值对不同分辨率手机的效果影响

dp值会受手机的像素密度影响，如下，如果手机像素密度小于480dpi，那1dp=3px；如果手机像素密度小于640dpi，那么1dp=4px：
drawable-mdpi = 1x = ~160dpi
drawable-hdpi = 1.5x = ~240dpi
drawable-xhdpi = 2x = ~ 320dpi
drawable-xxhdpi = 3x = ~ 480dpi
drawable-xxxhdpi = 4x = ~ 640dpi

参考链接：
屏幕适配最佳实践
Android px density densityDpi dp 之间的关系和区别