Android总结之drawable(hdpi,mdpi,ldpi)文件夹的使用

做Android有段时间了,但是好多细节还没有深入理解,关于Android中drawable文件夹的使用理解的就不是很深入。

Android为开发者提供了两种解决适配问题的方法,第一种方式是使用dip的单位,另一种方法是使用不同的drawable文件夹,今天就详细说下不同文件夹的区别

Android中提供了三个存放图片的文件夹,分别为hdpi,mdpi,ldpi,google文档上说,Andriod系统会根据手机屏幕的大小及屏幕密度去选择不同文件夹下的图片资源,以此来实现在不同大小不同屏幕分辨率下适配的问题。

比如在一个低分辨率的手机上,Android就会选择ldpi文件夹下的图片,但是如果没有在ldpi的文件夹下找见相关的资源文件,Android系统会首先从hdpi文件夹中选择文件,然后对图片资源进行缩放处理,显示在屏幕上;如果hdpi文件夹下也没有的话,会在默认的drawable文件夹中寻找。

明白以上内容,就很好解释图片放错文件夹,显示大小不同的问题了。如图

图1

图2

出现图二的原因是将放在hdpi下的图片放到了默认的文件夹下

本身图片是同一张图片,由于没有在hdpi文件夹中找到对应图片,系统在默认文件夹下找见了图片资源,但是这时系统会认为改图适用于中等分辨率的屏幕上,如果直接放到高分辨率的手机上不能适配,所以系统会自动的将图片放大,所以虽然是同一张图片,但是现实出来就一个正常效果,一个放大效果。

同理,如果同一张图片,放置在ldpi的文件夹下,在低分辨率的手机上显示正常,但是如果放在hdpi文件夹中,系统认为该图是为高分辨率显示的,要将图片缩小处理,所以显示出来的效果就是图变小了。

所以我们在平时要注意,明明都是一张图片,只是放在不同的文件夹中,显示出来的效果就很不同,原因就是我们对Android还不够了解。

对了,我还有个疑问,就是布局文件中设置wrap_content时,view的大小的单位是dip还是px呢!!!

以下为google文档的描述

At runtime, the system ensures the best possible display on the current screen with the following procedure for any given resource:

  1. The system uses the appropriate alternative resource

    Based on the size and density of the current screen, the system uses any size- and density-specific resource provided in your application. For example, if the device has a high-density screen and the application requests a drawable resource, the system looks for a drawable resource directory that best matches the device configuration. Depending on the other alternative resources available, a resource directory with thehdpiqualifier (such asdrawable-hdpi/) might be the best match, so the system uses the drawable resource from this directory.

  2. If no matching resource is available, the system uses the default resource and scales it up or down as needed to match the current screen size and density

    The "default" resources are those that are not tagged with a configuration qualifier. For example, the resources indrawable/are the default drawable resources. The system assumes that default resources are designed for the baseline screen size and density, which is a normal screen size and a medium density. As such, the system scales default density resources up for high-density screens and down for low-density screens, as appropriate.

    However, when the system is looking for a density-specific resource and does not find it in the density-specific directory, it won't always use the default resources. The system may instead use one of the other density-specific resources in order to provide better results when scaling. For example, when looking for a low-density resource and it is not available, the system prefers to scale-down the high-density version of the resource, because the system can easily scale a high-density resource down to low-density by a factor of 0.5, with fewer artifacts, compared to scaling a medium-density resource by a factor of 0.75.

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