This class covers some common techniques for processing and loading
Bitmap objects in a way that keeps your user interface (UI) components responsive
and avoids exceeding your application memory limit. If you're not careful, bitmaps can quickly
consume your available memory budget leading to an application crash due to the dreaded
java.lang.OutofMemoryError: bitmap size exceeds VM budget.
There are a number of reasons why loading bitmaps in your Android application is tricky:
- Mobile devices typically have constrained system resources. Android devices can have as little as 16MB of memory available to a single application. The Android Compatibility Definition Document (CDD), Section 3.7. Virtual Machine Compatibility gives the required minimum application memory for various screen sizes and densities. Applications should be optimized to perform under this minimum memory limit. However, keep in mind many devices are configured with higher limits.
- Bitmaps take up a lot of memory, especially for rich images like photographs. For example, the
camera on the Galaxy Nexus takes photos up to 2592x1936
pixels (5 megapixels). If the bitmap configuration used is
ARGB_8888(the default from the Android 2.3 onward) then loading this image into memory takes about 19MB of memory (2592*1936*4 bytes), immediately exhausting the per-app limit on some devices.
- Android app UI’s frequently require several bitmaps to be loaded at once. Components such as
ViewPagercommonly include multiple bitmaps on-screen at once with many more potentially off-screen ready to show at the flick of a finger.
- 高效加载较大的 Bitmaps
- This lesson walks you through decoding large bitmaps without exceeding the per application memory limit.
- Bitmap processing (resizing, downloading from a remote source, etc.) should never take place
on the main UI thread. This lesson walks you through processing bitmaps in a background thread
AsyncTaskand explains how to handle concurrency issues.
- This lesson walks you through using a memory and disk bitmap cache to improve the responsiveness and fluidity of your UI when loading multiple bitmaps.
- This lesson brings everything together, showing you how to load multiple bitmaps into
GridViewusing a background thread and bitmap cache.