mipmap 目录和drawable 目录有什么区别

官方介绍：

Mipmapping for drawables

Using a mipmap as the source for your bitmap or drawable is a simple way to provide a quality image and various image scales, which can be particularly useful if you expect your image to be scaled during an animation.

Android 4.2 (API level 17) added support for mipmaps in the Bitmap class—Android swaps the mip images in your Bitmap when you've supplied a mipmap source and have enabled setHasMipMap(). Now in Android 4.3, you can enable mipmaps for a BitmapDrawable object as well, by providing a mipmap asset and setting the android:mipMap attribute in a bitmap resource file or by calling hasMipMap().

应用场景：

If you know that you are going to draw this bitmap at less than 50% of its original size, you may be able to obtain a higher quality by turning this property on. Note that if the renderer respects this hint it might have to allocate extra memory to hold the mipmap levels for this bitmap.

一个应用实例：

Nexus 6

Screen
The Nexus 6 boasts an impressive 5.96” Quad HD screen display at a resolution of 2560 x 1440 (493 ppi). This translates to ~ 730 x 410 dp (density independent pixels).

Check your assets
It has a quantized density of 560 dpi, which falls in between the xxhdpi and xxxhdpi primary density buckets. For the Nexus 6, the platform will scale down xxxhdpi assets, but if those aren’t available, then it will scale up xxhdpi assets.

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.

res/   mipmap-mdpi/      ic_launcher.png   mipmap-hdpi/      ic_launcher.png   mipmap-xhdpi/      ic_launcher.png     mipmap-xxhdpi/      ic_launcher.png   mipmap-xxxhdpi/         ic_launcher.png  # App icon used on Nexus 6 device launcher

Choosing to add xxxhdpi versions for the rest of your assets will provide a sharper visual experience on the Nexus 6, but does increase apk size, so you should make an appropriate decision for your app.

res/   drawable-mdpi/      ic_sunny.png   drawable-hdpi/      ic_sunny.png   drawable-xhdpi/         ic_sunny.png   drawable-xxhdpi/  # Fall back to these if xxxhdpi versions aren’t available      ic_sunny.png    drawable-xxxhdpi/ # Higher resolution assets for Nexus 6      ic_sunny.png

总结

这个实例总结一下是这样：

Nexus 6 有 493 ppi，它刚好在 xxhdpi和xxxhdpi之间，所以显示的时候需要对xxxhdpi的资源进行缩小，如果你用了mipmap-xxxhdpi,那么这里会对sclae有一个优化，性能更好，占用内存更少。所以现在官方推荐使用mipmap：

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.

官方介绍：

应用场景：

一个应用实例：

总结

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