Home > Articles > Programming > General Programming/Other Languages

This chapter is from the book

Drawing Within a UIKit Context

UIKit simplifies the task of creating and managing contexts. It takes just one line to establish a new image or PDF context, and another to wrap things up. Between those lines, you are free to apply any drawing requests. These are applied to the current context.

Listing 1-6 applies the same drawing functions as in Listing 1-5, and it produces an identical image result. However, instead of drawing to a custom Quartz context, these updates are drawn to a newly established image context. In order to bridge between Quartz and UIKit, you call UIGraphicsGetCurrentContext(). This returns a CGContextRef, enabling you to use that value in your Core Graphics drawing calls.

Once again, when you compare the number of lines of code in Listing 1-6 with the combination of Listings 1-4 and 1-5, you see how much simpler UIKit drawing can be.

Listing 1-6 Drawing an Ellipse Within a UIKit Image Context

// Establish the image context
    targetSize, isOpaque, 0.0);

// Retrieve the current context
CGContextRef context = UIGraphicsGetCurrentContext();

// Perform the drawing
CGContextSetLineWidth(context, 4);
    [UIColor grayColor].CGColor);
CGContextStrokeEllipseInRect(context, rect);

// Retrieve the drawn image
UIImage *image =

// End the image context

You can call UIGraphicsGetCurrentContext() after starting any image or PDF context. That context persists until you call UIGraphicsEndImageContext() or UIGraphicsEndPDFContext().

Similarly, you can establish a context outside these routines by calling UIGraphicsPushContext(context). Supply a manually built Quartz context like the one created in Listing 1-4. You end that context with UIGraphicsPopContext(). These calls establish the context you’re drawing to inside the UIKit context stack. Pushing and popping the stack enables you to temporarily switch between drawing destinations as needed.

Otherwise, outside an explicit context environment, the current context is nil—with one exception. Upon calling drawRect:, views push a context onto the UIKit graphics context stack. So if you are implementing the drawRect: method, you can assume that there’s always a valid context that you can retrieve:

- (void) drawRect: (CGRect) rect
    // Perform drawing here
    // If called, UIGraphicsGetCurrentContext()
    // returns a valid context

UIKit Current Context

In Quartz, nearly every drawing function typically requires a context parameter. You pass it explicitly to each function. For example, you might set the stroke color to gray:

CGContextSetStrokeColorWithColor(context, [UIColor grayColor].CGColor);

In UIKit drawing, context is established from runtime conditions. Consider Listing 1-7. This code once again builds the same 4-pixel-wide ellipse shown in Figure 1-5. However, nowhere does this code explicitly reference a context.

This listing creates an elliptical path. It sets the line width to 4, establishes a gray color for stroking, and then strokes the path. In each step, the context is accessed on your behalf. As with Listing 1-6, the same gray color is applied to the current context, but that context is not specified. Listing 1-7 does this without mentioning the context.

Listing 1-7 Drawing an Ellipse Within a UIKit Image Context

// Stroke an ellipse using a Bezier path
UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:rect];
path.lineWidth = 4;
[[UIColor grayColor] setStroke];
[path stroke];

What’s happening is that UIKit owns a stack of graphics contexts. It applies drawing operations to whatever context is at the top of that stack. The settings that update the context state to 4 for line width and gray for stroke color apply to that topmost context.

So how do you integrate Core Graphics contexts with UIKit? UIKit provides two key functions, which were briefly mentioned earlier in the chapter:

  • You manually push a context by calling UIGraphicsPushContext(context). This function pushes the context onto the UIKit stack and sets the active context you’ll be drawing to.
  • You balance this by calling UIGraphicsPopContext(). This pops the current context off the UIKit stack, resetting the active context either to the next item on the stack or to nil.

By surrounding Listing 1-7 with these calls, you can embed the Bezier path-based drawing code from Listing 1-7 into the Quartz-based context in Listing 1-3. This approach bridges UIKit drawing into Core Graphics context creation and management.

To summarize, the steps for mixing a Core Graphics context with UIKit drawing are as follows:

  1. Create a Core Graphics context.
  2. Push the context using UIGraphicsPushContext().
  3. Use a combination of UIKit drawing calls that infer the current context and Quartz calls that use the context explicitly.
  4. (Optionally retrieve the context contents as an image.)
  5. Pop the context with UIGraphicsPopContext().
  6. Release the context’s memory.

If you try to draw with UIKit with no active context, you’ll receive warnings that the context is invalid.

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.


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.


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.


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.


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


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


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.


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.


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