Home > Articles > Programming > General Programming/Other Languages

  • Print
  • + Share This
This chapter is from the book

Context State

In Listing 1-8, the set method, when called on a color instance, specified the color of subsequent fill and stroke operations within the current context. In that listing, the purple color was set first, and then the green color. After each color was specified, it applied to all subsequent drawing operations.

Two related methods specify whether a color is used for just fill (setFill) or just stroke (setStroke) operations. A fill is used to completely color the interior of a shape. A stroke is used to outline that shape. In Figure 1-9, the fill color is green, and the stroke color is purple.

All three of these methods (set, setFill, and setStroke) update a current drawing state, specifying the active fill and stroke colors.

Figure 1-9

Figure 1-9 The context stroke and fill colors apply to all subsequent drawing operations.

Applying State

Consider the following lines of code, which were used to create the graphic shown in Figure 1-9:

[greenColor setFill];
[purpleColor setStroke];
[bunnyPath fill];
[bunnyPath stroke];

They set fill and stroke colors and then apply them to a Bezier path. The big question is this: Who is the target of these calls, storing the fill and stroke color states, allowing them to be applied by later operations?

The answer is the current context. The object returned by UIGraphicsGetCurrentContext() stores the fill and stroke colors. That context is inferred by each setting and drawing method.

All contexts store graphic state information, which acts as drawing operation parameters. Fill and stroke color are just two types of state saved to a context. As you’re about to discover, contexts can store quite a lot of information. The graphics state affects drawing operations by tweaking the ways each operation is realized.

Pushing and Popping Graphic State

Every context owns a stack of graphic state settings. Each time you create a new context, the stack starts with a fresh state. You can then modify that state and, if needed, push and pop copies of that state onto and off of a graphics state (GState) stack.

This stack is different from the context stack maintained by UIKit. That stack stores drawing destinations, letting you move between contexts by pushing and popping the stack. A drawing destination is like a canvas. When you change the context stack, you choose which canvas to draw to. The state stack is specific to each context. It holds sets of drawing preferences that apply to this context alone, changing how drawing operations apply to each “canvas.” Both approaches use stacks, but they affect different parts of the graphics system.

Each graphics state remembers any changes made to it. For example, if you push a new state onto the stack and adjust the default line width to 10, that context state persists until it’s popped off the stack. After that, the default line width returns to whatever value it was before that state was created.

Listing 1-9 demonstrates the process of managing the graphics state stack. It starts by setting the fill and stroke colors to the same green and purple you saw used in Figure 1-9. It draws a bunny and then “saves” the current state by calling CGContextSaveGState(). This pushes a copy of the state onto the context’s GState stack. Any changes to the context will now apply to that new copy of the graphics state.

If you kept drawing without making any changes to that state, you’d keep creating green bunnies with purple outlines. However, Listing 1-9 updates its colors before drawing. These new colors are orange and blue. And they override any previous color settings for the current state. When the second bunny is drawn, it displays in orange and blue.

Finally, the listing restores the previous graphics state by calling CGContextRestoreGState(). This pops the stack, discarding any changes made to the newer state copy. The final bunny therefore falls back to the original color states, namely purple and green. Figure 1-10 shows the result of the drawing operations detailed in Listing 1-9.

Listing 1-9 Managing State

UIGraphicsBeginImageContext(size);
CGContextRef context = UIGraphicsGetCurrentContext();

// Set initial stroke/fill colors
[greenColor setFill];
[purpleColor setStroke];

// Draw the bunny
[bunnyPath fill];
[bunnyPath stroke];

// Save the state
CGContextSaveGState(context);

// Change the fill/stroke colors
[[UIColor orangeColor] setFill];
[[UIColor blueColor] setStroke];

// Move then draw again
[bunnyPath applyTransform:
    CGAffineTransformMakeTranslation(50, 0)];
[bunnyPath fill];
[bunnyPath stroke];

// Restore the previous state
CGContextRestoreGState(context);

// Move then draw again
[bunnyPath applyTransform:
    CGAffineTransformMakeTranslation(50, 0)];
[bunnyPath fill];
[bunnyPath stroke];

UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
Figure 1-10

Figure 1-10 The color changes used to draw the second bunny are discarded upon restoring the graphic state to its previous settings.

State Types

A context saves many kinds of state, not just fill and stroke settings. Each of these states expresses a persistent aspect of the current context. Table 1-1 lists customizable state attributes that you can adjust using Core Graphics context calls and provides visual examples that roughly demonstrate the kinds of changes you can make to these settings.

Table 1-1 Context States

Technology

Explanation

01.jpg

Color—Color states consist of the fill and stroke settings that specify how items are drawn to the context.

02.jpg

Transformation matrices—These apply geometric transformations to contexts, allowing you to rotate, scale, and translate the canvas you’re drawing to, in order to create sophisticated geometric results.

03.jpg

Clipping—When you clip a context, you create a shape that automatically excludes content. This enables you to build content limited to circles, rectangles, or any other shape you can imagine.

04.jpg

Line parameters—Line states describe how Quartz draws your lines. These states include width (the thickness of the line), dash patterns (the pattern used to draw the line), miter limits (how pointy angles are), join styles (how corners are expressed; styles include miter, round, or bevel), and caps (the ends of lines, drawn as butt, round, and square).

05.jpg

Flatness—This is a factor that determines how accurate each curved path segment can be, specifying the maximum permissible distance between a point on the mathematical curve and the rendered point. The default is 0.6. Larger values produce more jagged curves but they are rendered faster as they require fewer computations.

06.jpg

Antialiasing—This determines whether Quartz mathematically smoothes jagged lines on curves and diagonals by averaging values between pixels. Antialiasing renders more slowly than normal drawing, but its results are visually superior. Quartz defaults to using antialiasing.

07.jpg

Alpha levels—These control the transparency of the material drawn to the context. As the alpha level decreases from 1 (fully opaque) to 0 (fully invisible), drawn material becomes more and more transparent.

08.jpg

Text traits—Text states include font, font size, character spacing, and text drawing modes. Modes specify how the text is drawn (by stroking, filling, etc.). Other details control font smoothing and subpixel positioning.

09.jpg

Blend modes—Blend modes use color and alpha levels to determine how to blend each new layer of color into the material already in the destination. Quartz supplies numerous blend modes. Appendix A explores these modes in exhaustive detail.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020