Class

UIView

The UIView class defines a rectangular area on the screen and the interfaces for managing the content in that area.

Overview

At runtime, a view object handles the rendering of any content in its area and also handles any interactions with that content. The UIView class itself provides basic behavior for filling its rectangular area with a background color. More sophisticated content can be presented by subclassing UIView and implementing the necessary drawing and event-handling code yourself. The UIKit framework also includes a set of standard subclasses that range from simple buttons to complex tables and can be used as-is. For example, a UILabel object draws a text string and a UIImageView object draws an image.

Because view objects are the main way your application interacts with the user, they have a number of responsibilities. Here are just a few:

  • Drawing and animation

    • Views draw content in their rectangular area using technologies such as UIKit, Core Graphics, and OpenGL ES.

    • Some view Declared property can be animated to new values.

  • Layout and subview management

    • A view may contain zero or more subviews.

    • Each view defines its own default resizing behavior in relation to its parent view.

    • A view can define the size and position of its subviews as needed.

  • Event handling

    • A view is a responder and can handle touch events and other events defined by the UIResponder class.

    • Views can use the addGestureRecognizer: method to install gesture recognizers to handle common gestures.

Views can embed other views and create sophisticated visual hierarchies. This creates a parent-child relationship between the view being embedded (known as the subview) and the parent view doing the embedding (known as the superview). Normally, a subview’s visible area is not clipped to the bounds of its superview, but in iOS you can use the clipsToBounds property to alter that behavior. A parent view may contain any number of subviews but each subview has only one superview, which is responsible for positioning its subviews appropriately.

The geometry of a view is defined by its frame, bounds, and center properties. The frame defines the origin and dimensions of the view in the coordinate system of its superview and is commonly used during layout to adjust the size or position of the view. The center property can be used to adjust the position of the view without changing its size. The bounds defines the internal dimensions of the view as it sees them and is used almost exclusively in custom drawing code. The size portion of the frame and bounds rectangles are coupled together so that changing the size of either rectangle updates the size of both.

For detailed information about how to use the UIView class, see View Programming Guide for iOS.

Creating a View

To create a view programmatically, you can use code like the following:

CGRect  viewRect = CGRectMake(10, 10, 100, 100);
UIView* myView = [[UIView alloc] initWithFrame:viewRect];

This code creates the view and positions it at the point (10, 10) in its superview’s coordinate system (once it is added to that superview). To add a subview to another view, you use the addSubview: method. In iOS, sibling views may overlap each other without any issues, allowing complex view placement. The addSubview: method places the specified view on top of other siblings. You can specify the relative z-order of a subview by adding it using the insertSubview:aboveSubview: and insertSubview:belowSubview: methods. You can also exchange the position of already added subviews using the exchangeSubviewAtIndex:withSubviewAtIndex: method.

When creating a view, it is important to assign an appropriate value to the autoresizingMask property to ensure the view resizes correctly. View resizing primarily occurs when the orientation of your application’s interface changes but it may happen at other times as well. For example, calling the setNeedsLayout method forces your view to update its layout.

The View Drawing Cycle

View drawing occurs on an as-needed basis. When a view is first shown, or when all or part of it becomes visible due to layout changes, the system asks the view to draw its contents. For views that contain custom content using UIKit or Core Graphics, the system calls the view’s drawRect: method. Your implementation of this method is responsible for drawing the view’s content into the current graphics context, which is set up by the system automatically prior to calling this method. This creates a static visual representation of your view’s content that can then be displayed on the screen.

When the actual content of your view changes, it is your responsibility to notify the system that your view needs to be redrawn. You do this by calling your view’s setNeedsDisplay or setNeedsDisplayInRect: method of the view. These methods let the system know that it should update the view during the next drawing cycle. Because it waits until the next drawing cycle to update the view, you can call these methods on multiple views to update them at the same time.

For detailed information about the view drawing cycle and the role your views have in this cycle, see View Programming Guide for iOS.

Animations

Changes to several view properties can be animated—that is, changing the property creates an animation that conveys the change to the user over a short period of time. The UIView class does most of the work of performing the actual animations but you must still indicate which property changes you want to be animated. There are two different ways to initiate animations:

  • In iOS 4 and later, use the block-based animation methods. (Recommended)

  • Use the begin/commit animation methods.

The block-based animation methods (such as animateWithDuration:animations:) greatly simplify the creation of animations. With one method call, you specify the animations to be performed and the options for the animation. However, block-based animations are available only in iOS 4 and later. If your application runs on earlier versions of iOS, you must use the beginAnimations:context: and commitAnimations class methods to mark the beginning and ending of your animations.

The following properties of the UIView class are animatable:

For more information about how to configure animations, see View Programming Guide for iOS.

Threading Considerations

Manipulations to your application’s user interface must occur on the main thread. Thus, you should always call the methods of the UIView class from code running in the main thread of your application. The only time this may not be strictly necessary is when creating the view object itself but all other manipulations should occur on the main thread.

Subclassing Notes

The UIView class is a key subclassing point for visual content that also requires user interactions. Although there are many good reasons to subclass UIView, it is recommended that you do so only when the basic UIView class or the standard system views do not provide the capabilities that you need. Subclassing requires more work on your part to implement the view and to tune its performance.

For information about ways to avoid subclassing, see Alternatives to Subclassing.

Methods to Override

When subclassing UIView, there are only a handful of methods you should override and many methods that you might override depending on your needs. Because UIView is a highly configurable class, there are also many ways to implement sophisticated view behaviors without overriding custom methods, which are discussed in the Alternatives to Subclassing section. In the meantime, the following list includes the methods you might consider overriding in your UIView subclasses:

  • Initialization:

    • initWithFrame: - It is recommended that you implement this method. You can also implement custom initialization methods in addition to, or instead of, this method.

    • initWithCoder: - Implement this method if you load your view from an Interface Builder nib file and your view requires custom initialization.

    • layerClass Use this property only if you want your view to use a different Core Animation layer for its backing store. For example, if your view uses tiling to display a large scrollable area, you might want to set the property to the CATiledLayer class.

  • Drawing and printing:

    • drawRect: - Implement this method if your view draws custom content. If your view does not do any custom drawing, avoid overriding this method.

    • drawRect:forViewPrintFormatter: - Implement this method only if you want to draw your view’s content differently during printing.

  • Constraints:

  • Layout:

    • sizeThatFits: - Implement this method if you want your view to have a different default size than it normally would during resizing operations. For example, you might use this method to prevent your view from shrinking to the point where subviews cannot be displayed correctly.

    • layoutSubviews - Implement this method if you need more precise control over the layout of your subviews than either the constraint or autoresizing behaviors provide.

    • didAddSubview:, willRemoveSubview: - Implement these methods as needed to track the additions and removals of subviews.

    • willMoveToSuperview:, didMoveToSuperview - Implement these methods as needed to track the movement of the current view in your view hierarchy.

    • willMoveToWindow:, didMoveToWindow - Implement these methods as needed to track the movement of your view to a different window.

  • Event Handling:

Alternatives to Subclassing

Many view behaviors can be configured without the need for subclassing. Before you start overriding methods, consider whether modifying the following properties or behaviors would provide the behavior you need.

  • addConstraint: - Define automatic layout behavior for the view and its subviews.

  • autoresizingMask - Provides automatic layout behavior when the superview’s frame changes. These behaviors can be combined with constraints.

  • contentMode - Provides layout behavior for the view’s content, as opposed to the frame of the view. This property also affects how the content is scaled to fit the view and whether it is cached or redrawn.

  • hidden or alpha - Change the transparency of the view as a whole rather than hiding or applying alpha to your view’s rendered content.

  • backgroundColor - Set the view’s color rather than drawing that color yourself.

  • Subviews - Rather than draw your content using a drawRect: method, embed image and label subviews with the content you want to present.

  • Gesture recognizers - Rather than subclass to intercept and handle touch events yourself, you can use gesture recognizers to send an Target-Action to a target object.

  • Animations - Use the built-in animation support rather than trying to animate changes yourself. The animation support provided by Core Animation is fast and easy to use.

  • Image-based backgrounds - For views that display relatively static content, consider using a UIImageView object with gesture recognizers instead of subclassing and drawing the image yourself. Alternatively, you can also use a generic UIView object and assign your image as the content of the view’s CALayer object.

Animations are another way to make visible changes to a view without requiring you to subclass and implement complex drawing code. Many properties of the UIView class are animatable, which means changes to those properties can trigger system-generated animations. Starting animations requires as little as one line of code to indicate that any changes that follow should be animated. For more information about animation support for views, see Animations.

For more information about appearance and behavior configuration, see About Views in UIKit User Interface Catalog.

Symbols

Initializing a View Object

- initWithFrame:

Initializes and returns a newly allocated view object with the specified frame rectangle.

Configuring a View’s Visual Appearance

backgroundColor

The view’s background color.

hidden

A Boolean value that determines whether the view is hidden.

alpha

The view’s alpha value.

opaque

A Boolean value that determines whether the view is opaque.

tintColor

The first nondefault tint color value in the view’s hierarchy, ascending from and starting with the view itself.

tintAdjustmentMode

The first non-default tint adjustment mode value in the view’s hierarchy, ascending from and starting with the view itself.

clipsToBounds

A Boolean value that determines whether subviews are confined to the bounds of the view.

clearsContextBeforeDrawing

A Boolean value that determines whether the view’s bounds should be automatically cleared before drawing.

maskView

An optional view whose alpha channel is used to mask a view’s content.

layerClass

Returns the class used to create the layer for instances of this class.

layer

The view’s Core Animation layer used for rendering.

Configuring the Event-Related Behavior

userInteractionEnabled

A Boolean value that determines whether user events are ignored and removed from the event queue.

multipleTouchEnabled

A Boolean value that indicates whether the receiver handles multi-touch events.

exclusiveTouch

A Boolean value that indicates whether the receiver handles touch events exclusively.

Configuring the Bounds and Frame Rectangles

frame

The frame rectangle, which describes the view’s location and size in its superview’s coordinate system.

bounds

The bounds rectangle, which describes the view’s location and size in its own coordinate system.

center

The center of the frame.

transform

Specifies the transform applied to the receiver, relative to the center of its bounds.

Managing the View Hierarchy

superview

The receiver’s superview, or nil if it has none.

subviews

The receiver’s immediate subviews.

window

The receiver’s window object, or nil if it has none.

- addSubview:

Adds a view to the end of the receiver’s list of subviews.

- bringSubviewToFront:

Moves the specified subview so that it appears on top of its siblings.

- sendSubviewToBack:

Moves the specified subview so that it appears behind its siblings.

- removeFromSuperview

Unlinks the view from its superview and its window, and removes it from the responder chain.

- insertSubview:atIndex:

Inserts a subview at the specified index.

- insertSubview:aboveSubview:

Inserts a view above another view in the view hierarchy.

- insertSubview:belowSubview:

Inserts a view below another view in the view hierarchy.

- exchangeSubviewAtIndex:withSubviewAtIndex:

Exchanges the subviews at the specified indices.

- isDescendantOfView:

Returns a Boolean value indicating whether the receiver is a subview of a given view or identical to that view.

Configuring the Resizing Behavior

autoresizingMask

An integer bit mask that determines how the receiver resizes itself when its superview’s bounds change.

autoresizesSubviews

A Boolean value that determines whether the receiver automatically resizes its subviews when its bounds change.

contentMode

A flag used to determine how a view lays out its content when its bounds change.

contentStretch

The rectangle that defines the stretchable and nonstretchable regions of a view.

Deprecated
- sizeThatFits:

Asks the view to calculate and return the size that best fits the specified size.

- sizeToFit

Resizes and moves the receiver view so it just encloses its subviews.

Laying out Subviews

- layoutSubviews

Lays out subviews.

- setNeedsLayout

Invalidates the current layout of the receiver and triggers a layout update during the next update cycle.

- layoutIfNeeded

Lays out the subviews immediately.

requiresConstraintBasedLayout

A Boolean value that indicates whether the receiver depends on the constraint-based layout system.

translatesAutoresizingMaskIntoConstraints

A Boolean value that determines whether the view’s autoresizing mask is translated into Auto Layout constraints.

Creating Constraints Using Layout Anchors

bottomAnchor

A layout anchor representing the bottom edge of the view’s frame.

centerXAnchor

A layout anchor representing the horizontal center of the view’s frame.

centerYAnchor

A layout anchor representing the vertical center of the view’s frame.

firstBaselineAnchor

A layout anchor representing the baseline for the topmost line of text in the view.

heightAnchor

A layout anchor representing the height of the view’s frame.

lastBaselineAnchor

A layout anchor representing the baseline for the bottommost line of text in the view.

leadingAnchor

A layout anchor representing the leading edge of the view’s frame.

leftAnchor

A layout anchor representing the left edge of the view’s frame.

rightAnchor

A layout anchor representing the right edge of the view’s frame.

topAnchor

A layout anchor representing the top edge of the view’s frame.

trailingAnchor

A layout anchor representing the trailing edge of the view’s frame.

widthAnchor

A layout anchor representing the width of the view’s frame.

Managing the View’s Constraints

constraints

The constraints held by the view.

- addConstraint:

Adds a constraint on the layout of the receiving view or its subviews.

- addConstraints:

Adds multiple constraints on the layout of the receiving view or its subviews.

- removeConstraint:

Removes the specified constraint from the view.

- removeConstraints:

Removes the specified constraints from the view.

Working with Layout Guides

- addLayoutGuide:

Adds the specified layout guide to the view.

layoutGuides

The array of layout guide objects owned by this view.

layoutMarginsGuide

A layout guide representing the view’s margins.

readableContentGuide

A layout guide representing an area with a readable width within the view.

- removeLayoutGuide:

Removes the specified layout guide from the view.

Measuring in Auto Layout

- systemLayoutSizeFittingSize:

Returns the size of the view that satisfies the constraints it holds.

- systemLayoutSizeFittingSize:withHorizontalFittingPriority:verticalFittingPriority:

Returns the size of the view that satisfies the constraints it holds.

intrinsicContentSize

The natural size for the receiving view, considering only properties of the view itself.

- invalidateIntrinsicContentSize

Invalidates the view’s intrinsic content size.

- contentCompressionResistancePriorityForAxis:

Returns the priority with which a view resists being made smaller than its intrinsic size.

- setContentCompressionResistancePriority:forAxis:

Sets the priority with which a view resists being made smaller than its intrinsic size.

- contentHuggingPriorityForAxis:

Returns the priority with which a view resists being made larger than its intrinsic size.

- setContentHuggingPriority:forAxis:

Sets the priority with which a view resists being made larger than its intrinsic size.

Aligning Views in Auto Layout

- alignmentRectForFrame:

Returns the view’s alignment rectangle for a given frame.

- frameForAlignmentRect:

Returns the view’s frame for a given alignment rectangle.

alignmentRectInsets

The insets from the view’s frame that define its alignment rectangle.

- viewForBaselineLayout

Returns a view used to satisfy baseline constraints.

Deprecated
viewForFirstBaselineLayout

Returns a view used to satisfy first baseline constraints.

viewForLastBaselineLayout

Returns a view used to satisfy last baseline constraints.

Triggering Auto Layout

- needsUpdateConstraints

A Boolean value that determines whether the view’s constraints need updating.

- setNeedsUpdateConstraints

Controls whether the view’s constraints need updating.

- updateConstraints

Updates constraints for the view.

- updateConstraintsIfNeeded

Updates the constraints for the receiving view and its subviews.

Debugging Auto Layout

See Auto Layout Guide for more details on debugging constraint-based layout.

- constraintsAffectingLayoutForAxis:

Returns the constraints impacting the layout of the view for a given axis.

hasAmbiguousLayout

A Boolean value that determines whether the constraints impacting the layout of the view incompletely specify the location of the view.

- exerciseAmbiguityInLayout

Randomly changes the frame of a view with an ambiguous layout between the different valid values.

Managing the User Interface Direction

semanticContentAttribute

A semantic description of the view’s contents, used to determine whether the view should be flipped when switching between left-to-right and right-to-left layouts.

effectiveUserInterfaceLayoutDirection

The user interface layout direction appropriate for arranging the immediate content of the view.

+ userInterfaceLayoutDirectionForSemanticContentAttribute:

Returns the user interface direction for the given semantic content attribute.

+ userInterfaceLayoutDirectionForSemanticContentAttribute:relativeToLayoutDirection:

Returns the layout direction implied by the specified semantic content attribute, relative to the specified layout direction.

Configuring Content Margins

layoutMargins

The default spacing to use when laying out content in the view.

preservesSuperviewLayoutMargins

A Boolean value indicating whether the current view also respects the margins of its superview.

- layoutMarginsDidChange

Notifies the view that the layout margins changed.

Drawing and Updating the View

- drawRect:

Draws the receiver’s image within the passed-in rectangle.

- setNeedsDisplay

Marks the receiver’s entire bounds rectangle as needing to be redrawn.

- setNeedsDisplayInRect:

Marks the specified rectangle of the receiver as needing to be redrawn.

contentScaleFactor

The scale factor applied to the view.

- tintColorDidChange

Called by the system when the tintColor property changes.

Formatting Printed View Content

- viewPrintFormatter

Returns a print formatter for the receiving view.

- drawRect:forViewPrintFormatter:

Implemented to draw the view’s content for printing.

Managing Gesture Recognizers

- addGestureRecognizer:

Attaches a gesture recognizer to the view.

- removeGestureRecognizer:

Detaches a gesture recognizer from the receiving view.

gestureRecognizers

The gesture-recognizer objects currently attached to the view.

- gestureRecognizerShouldBegin:

Asks the view if the gesture recognizer should be allowed to continue tracking touch events.

Animating Views with Block Objects

+ animateWithDuration:delay:options:animations:completion:

Animate changes to one or more views using the specified duration, delay, options, and completion handler.

+ animateWithDuration:animations:completion:

Animate changes to one or more views using the specified duration and completion handler.

+ animateWithDuration:animations:

Animate changes to one or more views using the specified duration.

+ transitionWithView:duration:options:animations:completion:

Creates a transition animation for the specified container view.

+ transitionFromView:toView:duration:options:completion:

Creates a transition animation between the specified views using the given parameters.

+ animateKeyframesWithDuration:delay:options:animations:completion:

Creates an animation block object that can be used to set up keyframe-based animations for the current view.

+ addKeyframeWithRelativeStartTime:relativeDuration:animations:

Specifies the timing and animation values for a single frame of a keyframe animation.

+ performSystemAnimation:onViews:options:animations:completion:

Performs a specified system-provided animation on one or more views, along with optional parallel animations that you define.

+ animateWithDuration:delay:usingSpringWithDamping:initialSpringVelocity:options:animations:completion:

Performs a view animation using a timing curve corresponding to the motion of a physical spring.

+ performWithoutAnimation:

Disables a view transition animation.

Animating Views

Use of the methods in this section is discouraged in iOS 4 and later. Use the block-based animation methods instead.

+ beginAnimations:context:

Marks the beginning of a begin/commit animation block.

+ commitAnimations

Marks the end of a begin/commit animation block and schedules the animations for execution.

+ setAnimationStartDate:

Sets the start time for the current animation block.

+ setAnimationsEnabled:

Sets whether animations are enabled.

+ setAnimationDelegate:

Sets the delegate for any animation messages.

+ setAnimationWillStartSelector:

Sets the message to send to the animation delegate when the animation starts.

+ setAnimationDidStopSelector:

Sets the message to send to the animation delegate when animation stops.

+ setAnimationDuration:

Sets the duration (measured in seconds) of the animations in an animation block.

+ setAnimationDelay:

Sets the amount of time (in seconds) to wait before animating property changes within an animation block.

+ setAnimationCurve:

Sets the curve to use when animating property changes within an animation block.

+ setAnimationRepeatCount:

Sets the number of times animations within an animation block repeat.

+ setAnimationRepeatAutoreverses:

Sets whether the animations within an animation block automatically reverse themselves.

+ setAnimationBeginsFromCurrentState:

Sets whether the animation should begin playing from the current state.

+ setAnimationTransition:forView:cache:

Sets a transition to apply to a view during an animation block.

areAnimationsEnabled

Returns a Boolean value indicating whether animations are enabled.

Using Motion Effects

- addMotionEffect:

Begins applying a motion effect to the view.

motionEffects

The array of motion effects for the view.

- removeMotionEffect:

Stops applying a motion effect to the view.

Preserving and Restoring State

restorationIdentifier

The identifier that determines whether the view supports state restoration.

- encodeRestorableStateWithCoder:

Encodes state-related information for the view.

- decodeRestorableStateWithCoder:

Decodes and restores state-related information for the view.

Capturing a View Snapshot

- snapshotViewAfterScreenUpdates:

Returns a snapshot view based on the contents of the current view.

- resizableSnapshotViewFromRect:afterScreenUpdates:withCapInsets:

Returns a snapshot view based on the specified contents of the current view, with stretchable insets.

- drawViewHierarchyInRect:afterScreenUpdates:

Renders a snapshot of the complete view hierarchy as visible onscreen into the current context.

Identifying the View at Runtime

tag

An integer that you can use to identify view objects in your application.

- viewWithTag:

Returns the view whose tag matches the specified value.

Converting Between View Coordinate Systems

- convertPoint:toView:

Converts a point from the receiver’s coordinate system to that of the specified view.

- convertPoint:fromView:

Converts a point from the coordinate system of a given view to that of the receiver.

- convertRect:toView:

Converts a rectangle from the receiver’s coordinate system to that of another view.

- convertRect:fromView:

Converts a rectangle from the coordinate system of another view to that of the receiver.

Hit Testing in a View

- hitTest:withEvent:

Returns the farthest descendant of the receiver in the view hierarchy (including itself) that contains a specified point.

- pointInside:withEvent:

Returns a Boolean value indicating whether the receiver contains the specified point.

Ending a View Editing Session

- endEditing:

Causes the view (or one of its embedded text fields) to resign the first responder status.

Observing View-Related Changes

- didAddSubview:

Tells the view that a subview was added.

- willRemoveSubview:

Tells the view that a subview is about to be removed.

- willMoveToSuperview:

Tells the view that its superview is about to change to the specified superview.

- didMoveToSuperview

Tells the view that its superview changed.

- willMoveToWindow:

Tells the view that its window object is about to change.

- didMoveToWindow

Tells the view that its window object changed.

Observing Focus

canBecomeFocused

A Boolean value that indicates whether the view is currently capable of being focused.

inheritedAnimationDuration

Returns the inherited duration of the current animation.

focused

A Boolean value that indicates whether the item is currently focused.

Constants

UIViewAnimationOptions

Options for animating views using block objects.

UIViewAnimationCurve

Specifies the supported animation curves.

UIViewContentMode

Options to specify how a view adjusts its content when its size changes.

UILayoutConstraintAxis

Keys that specify a horizontal or vertical layout constraint between objects.

UIViewTintAdjustmentMode

The tint adjustment mode for the view.

UISystemAnimation

Option to remove the views from the hierarchy when animation is complete.

Fitting Size

View fitting options used in the systemLayoutSizeFittingSize: method.

UIView Intrinsic Metric Constant

The option to indicate that a view has no intrinsic metric for a given numeric property.

UIViewAutoresizing

Options for automatic view resizing.

UIViewAnimationTransition

Animation transition options for use in an animation block object.

UISemanticContentAttribute

A semantic description of the view’s contents, used to determine whether the view should be flipped when switching between left-to-right and right-to-left layouts.

Initializers