最近在看AOSP,发现mipmaps, 百度 了一下，发现有各种说法，最后还是google出来的比较正确。

引用官方文档中：
http://developer.android.com/tools/projects/index.html#mipmap

[size=medium]Managing Launcher Icons as mipmap Resources[/size]

Different home screen launcher apps on different devices show app launcher icons at various resolutions. When app resource optimization techniques remove resources for unused screen densities, launcher icons can wind up looking fuzzy because the launcher app has to upscale a lower-resolution icon for display. To avoid these display issues, apps should use the mipmap/ resource folders for launcher icons. The Android system preserves these resources regardless of density stripping, and ensures that launcher apps can pick icons with the best resolution for display.

Make sure launcher apps show a high-resolution icon for your app by moving all densities of your launcher icons to density-specific res/mipmap/ folders (for example res/mipmap-mdpi/ and res/mipmap-xxxhdpi/). The mipmap/ folders replace the drawable/ folders for launcher icons. For xxhpdi launcher icons, be sure to add the higher resolution xxxhdpi versions of the icons to enhance the visual experience of the icons on higher resolution devices.

Note: Even if you build a single APK for all devices, it is still best practice to move your launcher icons to the mipmap/ folders.

这里面有说到app resource optimization techniques remove resources for unused screen densities 这句是重点，说 app资源优化技术会将不需要的解析度的屏幕的资源移除掉，这样 Launcher App会使用低密度的 icon放大显示而导致了 icon 模糊。如果把icon放到mipmap/ folders 所有all densities 的都会被保留，这样匹配的时候会用高密度的 icon去缩小显示，这样就避免了模糊。

这里提到到app resource optimization techniques，我还没有搜索到相关信息。

有一篇文章说明了mipmap的历史：
https://androidbycode.wordpress.com/2015/02/14/goodbye-launcher-drawables-hello-mipmaps/

Why use mipmaps for your launcher icons?

Using mipmaps for your launcher icon is described as best practice by the Android team. The advantage you get is that you can keep resources in the mipmap folders for all device densities and then strip out other resources from the drawable folders that are not relevant to the specific users device density.

Here is an example, say a user has a device that is classified as xxhdpi. The drawable resources in your apk for all other densities, such as xxxhdpi, are not required and can be stripped out. The launcher icon is different to other resources as it is possible that a higher resolution icon is displayed in the users launcher. If that higher resolution image has been stripped from your drawables folder, then a lower density icon will programmatically be up-scaled. This may cause an unattractive blurry icon.

You may not be producing multiple apk files with stripped out resources to minimize apk size yet. Therefore making the change to mipmaps for your launcher icon will currently not make any difference. That does not mean you should not do it. The best reason for using mipmaps folder is that the Android team want you to and describe it as best practice. They know what new features are in the pipeline for Android and who knows, but maybe they are planning to introduce a new feature where unused resources are automatically stripped for the user when they download apps.

History of launcher mipmaps in Android

The mipmaps resource folders in Android started appearing long before they were introduced to the wider Android development community to use, which started in Android Jelly Bean 4.3. Very little official information came from the Android team about why to use them. Around the time of the release of 4.3, Android framework developer @hackbod announced:

    …if you are building different versions of your app for different densities, you should know about the “mipmap” resource directory.  This is exactly like “drawable” resources, except it does not participate in density stripping when creating the different apk targets.

Since that time if you have taken a close look at the apps produced out of Google you will notice that some apps were doing exactly that.  Shipping only the required density but including all densities in the mipmap folders for the launcher icons.

As time went on still there was not much noise out of the Android team about using launcher mipmaps, that was until Lollipop hit AOSP and the new Nexus devices hit the market.  The Android Developers blog posted about preparing your apps for the new Nexus 6 and 9.  This detailed a compelling reason you should use mipmap folders for your app icon and announced the shift of best practice to using mipmaps:

    …Provide at least an xxxhdpi app icon because devices can display large app icons on the launcher. It’s best practice to place your app icons in mipmap- folders (not the drawable- folders) because they are used at resolutions different from the device’s current density. For example, an xxxhdpi app icon can be used on the launcher for an xxhdpi device.