FaceBook推出的Android图片加载库-Fresco
浏览数：24 /
时间：2015年06月11日
FaceBook推出的Android图片加载库-Fresco
- 原文链接:Introducing Fresco: A new image library for Android
- 译者 : ZhaoKaiQiang
- 校对者: Chaossss
- 校对者: bboyfeiyu
- 校对者: BillionWang
- 状态 : 完成
在Android设备上面，快速高效的显示图片是极为重要的。过去的几年里，我们在如何高效的存储图像这方面遇到了很多问题。图片太大，但是手机的内存却很小。每一个像素的R、G、B和alpha通道总共要占用4byte的空间。如果手机的屏幕是480*800,那么一张屏幕大小的图片就要占用1.5M的内存。手机的内存通常很小，特别是Android设备还要给各个应用分配内存。在某些设备上，分给Facebook App的内存仅仅有16MB。一张图片就要占据其内存的十分之一。
当你的App内存溢出会发生什么呢？它当然会崩溃！我们开发了一个库来解决这个问题，我们叫它Fresco。它可以管理使用到的图片和内存，从此App不再崩溃。
内存区
为了理解Facebook到底做了什么工作，在此之前我们需要了解在Android可以使用的堆内存之间的区别。Android中每个App的Java堆内存大小都是被严格的限制的。每个对象都是使用Java的new在堆内存实例化，这是内存中相对安全的一块区域。内存有垃圾回收机制，所以当App不在使用内存的时候，系统就会自动把这块内存回收。
不幸的是，内存进行垃圾回收的过程正是问题所在。当内存进行垃圾回收时，内存不仅仅进行了垃圾回收，还把 Android 应用完全终止了。这也是用户在使用 App 时最常见的卡顿或短暂假死的原因之一。这会让正在使用 App 的用户非常郁闷，然后他们可能会焦躁地滑动屏幕或者点击按钮，但 App 唯一的响应就是：在 App 恢复正常之前，请求用户耐心等待
相比之下，Native堆是由C++程序的new进行分配的。在Native堆里面有更多可用内存，App只被设备的物理可用内存限制，而且没有垃圾回收机制或其他东西拖后腿。但是c++程序员必须自己回收所分配的每一块内存，否则就会造成内存泄露，最终导致程序崩溃。
Android有另外一种内存区域，叫做Ashmem。它操作起来更像Native堆，但是也有额外的系统调用。Android 在操作 Ashmem 堆时，会把该堆中存有数据的内存区域从 Ashmem 堆中抽取出来，而不是把它释放掉，这是一种弱内存释放模式；被抽取出来的这部分内存只有当系统真正需要更多的内存时（系统内存不够用）才会被释放。当 Android 把被抽取出来的这部分内存放回 Ashmem 堆，只要被抽取的内存空间没有被释放，之前的数据就会恢复到相应的位置。
可消除的Bitmap
Ashmem不能被Java应用直接处理，但是也有一些例外，图片就是其中之一。当你创建一张没有经过压缩的Bitmap的时候，Android的API允许你指定是否是可清除的。
BitmapFactory.Options = new BitmapFactory.Options();
options.inPurgeable = true;
Bitmap bitmap = BitmapFactory.decodeByteArray(jpeg, 0, jpeg.length, options);- 1
- 2
- 3
经过上面的代码处理后，可清除的Bitmap会驻留在 Ashmem 堆中。不管发生什么，垃圾回收器都不会自动回收这些 Bitmap。当 Android 绘制系统在渲染这些图片，Android 的系统库就会把这些 Bitmap 从 Ashmem 堆中抽取出来，而当渲染结束后，这些 Bitmap 又会被放回到原来的位置。如果一个被抽取的图片需要再绘制一次，系统仅仅需要把它再解码一次，这个操作非常迅速。
这听起来像一个完美的解决方案，但是问题是Bitmap解码的操作是运行在UI线程的。Bitmap解码是非常消耗CPU资源的，当消耗过大时会引起UI阻塞。因为这个原因，所以Google不推荐使用这个特性。现在它们推荐使用另外一个特性——inBitmap。但是这个特性直到Android3.0之后才被支持。即使是这样，这个特性也不是非常有用，除非 App 里的所有图片大小都相同，这对Fackbook来说显然是不适用的。一直到4.4版本，这个限制才被移除了。但我们需要的是能够运行在 Android 2.3 - 最新版本中的通用解决方案。
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对于上面提到的“解码操作致使 UI 假死”的问题，我们找到了一种同时使 UI 显示和内存管理都表现良好的解决方法。如果我们在 UI 线程进行渲染之前把被抽取的内存区域放回到原来的位置，并确保它再也不会被抽取，那我们就可以把这些图片放在 Ashmem 里，同时不会出现 UI 假死的问题。幸运的是，Android 的 NDK 中有一个函数可以完美地实现这个需求，名字叫做 AndroidBitmap_lockPixels。这个函数最初的目的就是：在调用 unlockPixels 再次抽取内存区域后被执行。
当我们意识到我们没有必要这样做的时候，我们取得了突破。如果我们只调用lockPixels而不调用对应的unlockPixels，那么我们就可以在Java的堆内存里面创建一个内存安全的图像，并且不会导致UI线程加载缓慢。只需要几行c++代码，我们就完美的解决了这个问题。
就像《蜘蛛侠》里面说的：“能力越强，责任越大。”可清除的 Bitmap 既不会被垃圾回收器回收，也不会被 Ashmem 内置的清除机制处理，这使得使用它们可能会造成内存泄露。所以我们只能靠自己啦。
在c++中,通常的解决方案是建立智能指针类,实现引用计数。这些需要利用到c++的语言特性——拷贝构造函数、赋值操作符和确定的析构函数。这种语法在Java之中不存在，因为垃圾回收器能够处理这一切。所以我们必须以某种方式在Java中实现C++的这些保证机制。
我们创建了两个类去完成这件事。其中一个叫做“SharedReference”，它有addReference和deleteReference两个方法，调用者调用时必须采取基类对象或让它在范围之外。一旦引用计数器归零，资源处理(Bitmap.recycle)就会发生。
然而，很显然，让Java开发者去调用这些方法是很容易出错的。Java语言就是为了避免做这样的事情的！所以SharedReference之上,我们构建了CloseableReference类。它不仅实现了Java的Closeable接口,而且也实现了Cloneable接口。它的构造器和clone()方法会调用addReference()，而close()方法会调用deleteReference()。所以Java开发者需要遵守下面两条简单的的规则：
- 在分配CloseableReference新对象的时候,调用.clone()。
- 在超出作用域范围的时候，调用.close()，这通常是在finally代码块中。
这些规则可以有效地防止内存泄漏,并让我们在像Fackbook的Android客户端这种大型的Java程序中享受Native内存管理和通信。
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