Core Animation 是对现有图片的操作函数。
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreAnimation_guide/Introduction/Introduction.html
Core Animation Manages Your App’s Content
Core Animation is not a drawing system itself. It is an infrastructure for compositing and manipulating your app’s content in hardware. At the heart of this infrastructure are layer objects, which you use to manage and manipulate your content. A layer captures your content into a bitmap that can be manipulated easily by the graphics hardware. In most apps, layers are used as a way to manage the content of views but you can also create standalone layers depending on your needs.
All you have to do is configure a few animation parameters (such as the start and end points) and tell Core Animation to start. Core Animation does the rest, handing most of the actual drawing work off to the onboard graphics hardware to accelerate the rendering. This automatic graphics acceleration results in high frame rates and smooth animations without burdening the CPU and slowing down your app.
The Layer-Based Drawing Model
Most layers do not do any actual drawing in your app. Instead, a layer captures the content your app provides and caches it in a bitmap, which is sometimes referred to as the backing store. When you subsequently change a property of the layer, all you are doing is changing the state information associated with the layer object. When a change triggers an animation, Core Animation passes the layer’s bitmap and state information to the graphics hardware, which does the work of rendering the bitmap using the new information, as shown in Figure 1-1. Manipulating the bitmap in hardware yields much faster animations than could be done in software.
Figure 1-1 How Core Animation draws content
