iOS图形与动画实战：Core Graphics和Core Animation入门

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"iOS的图像和动画处理" 是一本由Vandad Nahavandipoor编写的指南，主要面向具有初步Cocoa编程经验的开发者，帮助他们创建iPhone和iPad应用中的图形和动画效果，提升应用的视觉吸引力。书中通过实例和简洁的代码，介绍了如何适应不同屏幕尺寸、构建和使用颜色、绘制文本和图像、添加阴影、创建渐变以及实现视图的动画效果如缩放、旋转和移动。在iOS的图像处理方面，书中的内容涵盖了以下几个关键知识点： 1. **适应不同屏幕尺寸**：学习基本概念，了解如何在不同的设备屏幕尺寸上正确显示图形和动画，确保用户界面在iPhone和iPad上都能保持良好的视觉效果。 2. **颜色构造与使用**：掌握如何创建和设置颜色，这在iOS应用的视觉设计中至关重要，能够为用户界面增添丰富的色彩元素。 3. **文本绘制**：学习如何在图形上下文中绘制文本，这是创建用户界面元素如按钮、标签等的基础。 4. **图像绘制**：了解如何在应用中绘制图像，包括加载和显示图片，这对于创建自定义图标或背景非常重要。 5. **线条和矩形**：掌握如何绘制直线和矩形，这些基本形状是构建复杂图形的基础。 6. **阴影效果**：学习添加阴影到形状，使图形更具立体感，提升视觉深度。 7. **渐变创建与绘制**：创建和绘制线性或径向渐变，可以用于制作平滑过渡的背景或按钮效果。 8. **路径构造**：理解如何构建和使用路径，用于绘制复杂的自定义形状。 9. **形状变形**：在图形上下文中位移、缩放和旋转形状，提供动态的视觉体验。 10. **视图动画**：学习如何实现视图的动画效果，包括平移、缩放和旋转，为用户提供更加生动和交互性的界面。 11. **视图的动画和变换**：深入探讨视图的动画技巧，如视图的平滑移动、缩放和旋转，这些都是提升用户体验的关键。这本书以易于理解的步骤和代码示例为初学者提供了实用的指导，让开发者能够在短时间内掌握iOS图形和动画的核心技术，从而提升应用的整体质量。无论是对于个人开发者还是团队，这本书都是一个宝贵的资源，有助于开发出更具吸引力的iOS应用。

Graphics and Animations

You’ve certainly seen applications with beautiful graphics effects on iPhones or iPads.

And you’ve probably also encountered impressive animations in games and other apps.

When the iOS runtime and Cocoa programming frameworks combine, they make an

amazing variety of graphics and animation effects possible with relatively simple cod-

ing. The quality of these graphics and animations depends partly, of course, on the

aesthetic sensitivities of the programmer and artistic collaborators. But in this short

book, you’ll see how much you can accomplish with modest programming skills.

I’ll dispense with conceptual background, preferring to introduce ideas such as color

spaces, transformation, and the graphics context as we go along. I’ll just mention a few

basics before leaping into code.

In Cocoa Touch, an app is made up of windows and views. An app with a UI has at

least one window that contains, in turn, one or more views. In Cocoa Touch, a window

is an instance of UIWindow. Usually, an app will open to the main window and the

programmer will then add views to the window to represent different parts of the UI:

parts such as buttons, labels, images, and custom controls. All these UI-related com-

ponents are handled and drawn by UIKit.

Some of these things might sound relatively difficult to understand, but I promise you

that as we proceed through this book, you will understand them step-by-step with the

many examples I will give.

Apple has provided developers with powerful frameworks that handle graphics and

animations in iOS and OS X. Some of these frameworks and technologies are:

UIKit

The high-level framework that allows developers to create views, windows, but-

tons, and other UI related components. It also incorporates some of the low-level

APIs into an easier-to-use high-level API.

Quartz 2D

The main engine running under the hood to facilitate drawing in iOS; UIKit uses

Quartz.

Core Graphics

A framework that supports the graphics context (more on this later), loading im-

ages, drawing images, and so on.

Core Animation

A framework that, as its name implies, facilitates animations in iOS.

Basic Concepts for Adapting to Different Screen Sizes

When drawing on a screen, one of the most important concepts to grasp is the relation

between points and pixels. I’m sure you’re familiar with pixels, but what are points?

They’re the device-independent counterpart of pixels. For instance, compare the

iPhone 3GS to the iPhone 4. Both devices have 3.5-inch displays. However, the number

of pixels that iPhone 3GS can draw in portrait mode is 320×480. The same screen size

on the iPhone 4 is capable of drawing twice as many, or 640×960, pixels in portrait

mode.

Now imagine you are writing an iPhone app that has only one screen, and that you are

simply filling the whole screen with the color green. Imagine that you naïvely specify a

rectangular area of 320×480 pixels. When iPhone 3GS users run your app, they will be

quite happy because “it does what it says it does”—fill the entire screen with the color

green. iPhone 4 users, on the other hand, will be quite unhappy: what they will see is

quite different, as shown in Figure 1.

To remedy this problem, Apple introduced device-independent drawing methods to

help developers focus on how their shapes and graphics have to appear on a device

instead of worrying about the screen sizes and resolutions of different devices that run

the same code. To fix the issue we saw in Figure 1, the developer of the app can simply

use the relevant APIs to specify the green rectangle in points instead of pixels. That will

allow the same code to run on the iPhone 3GS and the iPhone 4, ensuring that the

screen on the iPhone 4 will be filled with the rectangle. For this reason, many of the

methods that you will see in this book will rely on points (or as Apple calls them, logical

points) instead of pixels.

The origin point of the screen on an iOS device is the top-left corner.

Screens whose drawing origin is on the top-left corner are also referred

to as Upper Left Origin, or ULO, screens. This means that point (0, 0)

is the topmost and the leftmost point on the screen, and that positive

values of the x axis extend towards the right, while positive values of

the y axis extend towards the bottom. In other words, an x position of

20 is further right on the screen than a position of 10 is. On the y axis,

point 20 is further down than point 10.

2 | Graphics and Animations

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