The View class represents the basic UI building block. A view occupies a rectangular area on the screen and is responsible for drawing and event handling. View is the base class for widgets, used to create interactive graphical user interfaces.

Using Views

All of the views in a window are arranged in a single tree. You can add views either from code or by specifying a tree of views in one or more XML layout files. There are many specialized subclasses of views that act as controls or are capable of displaying text, images, or other content.

Once you have created a tree of views, there are typically a few types of common operations you may wish to perform:

• Set properties: for example setting the text of a TextView. The available properties and the methods that set them will vary among the different subclasses of views. Note that properties that are known at build time can be set in the XML layout files.
• Set focus: The framework will handled moving focus in response to user input. To force focus to a specific view, call requestFocus().
• Set up listeners: Views allow clients to set listeners that will be notified when something interesting happens to the view. For example, all views will let you set a listener to be notified when the view gains or loses focus. You can register such a listener using setOnFocusChangeListener(View.OnFocusChangeListener). Other view subclasses offer more specialized listeners. For example, a Button exposes a listener to notify clients when the button is clicked.
• Set visibility: You can hide or show views using setVisibility(int).

Note: The Android framework is responsible for measuring, laying out and drawing views. You should not call methods that perform these actions on views yourself unless you are actually implementing a ViewGroup.

Implementing a Custom View

To implement a custom view, you will usually begin by providing overrides for some of the standard methods that the framework calls on all views. You do not need to override all of these methods. In fact, you can start by just overriding onDraw(android.graphics.Canvas).

Category	Methods	Description
Creation	Constructors	There is a form of the constructor that are called when the view is created from code and a form that is called when the view is inflated from a layout file. The second form should parse and apply any attributes defined in the layout file.
	onFinishInflate()	Called after a view and all of its children has been inflated from XML.
Layout	onMeasure(int, int)	Called to determine the size requirements for this view and all of its children.
	onLayout(boolean, int, int, int, int)	Called when this view should assign a size and position to all of its children.
	onSizeChanged(int, int, int, int)	Called when the size of this view has changed.
Drawing	onDraw(Canvas)	Called when the view should render its content.
Event processing	onKeyDown(int, KeyEvent)	Called when a new key event occurs.
	onKeyUp(int, KeyEvent)	Called when a key up event occurs.
	onTrackballEvent(MotionEvent)	Called when a trackball motion event occurs.
	onTouchEvent(MotionEvent)	Called when a touch screen motion event occurs.
Focus	onFocusChanged(boolean, int, Rect)	Called when the view gains or loses focus.
	onWindowFocusChanged(boolean)	Called when the window containing the view gains or loses focus.
Attaching	onAttachedToWindow()	Called when the view is attached to a window.
	onDetachedFromWindow()	Called when the view is detached from its window.
	onWindowVisibilityChanged(int)	Called when the visibility of the window containing the view has changed.

IDs

• Define a Button in the layout file and assign it a unique ID.

   <Button id="@+id/my_button"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:text="@string/my_button_text"/>
   

• From the onCreate method of an Activity, find the Button

        Button myButton = (Button) findViewById(R.id.my_button);
   

View IDs need not be unique throughout the tree, but it is good practice to ensure that they are at least unique within the part of the tree you are searching.

Position

The geometry of a view is that of a rectangle. A view has a location, expressed as a pair of left and top coordinates, and two dimensions, expressed as a width and a height. The unit for location and dimensions is the pixel.

It is possible to retrieve the location of a view by invoking the methods getLeft() and getTop(). The former returns the left, or X, coordinate of the rectangle representing the view. The latter returns the top, or Y, coordinate of the rectangle representing the view. These methods both return the location of the view relative to its parent. For instance, when getLeft() returns 20, that means the view is located 20 pixels to the right of the left edge of its direct parent.

In addition, several convenience methods are offered to avoid unnecessary computations, namely getRight() and getBottom(). These methods return the coordinates of the right and bottom edges of the rectangle representing the view. For instance, calling getRight() is similar to the following computation: getLeft() + getWidth() (see Size for more information about the width.)

Size, padding and margins

The size of a view is expressed with a width and a height. A view actually possess two pairs of width and height values.

The first pair is known as measured width and measured height. These dimensions define how big a view wants to be within its parent (see Layout for more details.) The measured dimensions can be obtained by calling getMeasuredWidth() and getMeasuredHeight().

The second pair is simply known as width and height, or sometimes drawing width and drawing height. These dimensions define the actual size of the view on screen, at drawing time and after layout. These values may, but do not have to, be different from the measured width and height. The width and height can be obtained by calling getWidth() and getHeight().

To measure its dimensions, a view takes into account its padding. The padding is expressed in pixels for the left, top, right and bottom parts of the view. Padding can be used to offset the content of the view by a specific amount of pixels. For instance, a left padding of 2 will push the view's content by 2 pixels to the right of the left edge. Padding can be set using the setPadding(int, int, int, int) method and queried by calling getPaddingLeft(), getPaddingTop(), getPaddingRight() and getPaddingBottom().

Even though a view can define a padding, it does not provide any support for margins. However, view groups provide such a support. Refer to ViewGroup and ViewGroup.MarginLayoutParams for further information.

Layout

Layout is a two pass process: a measure pass and a layout pass. The measuring pass is implemented in measure(int, int) and is a top-down traversal of the view tree. Each view pushes dimension specifications down the tree during the recursion. At the end of the measure pass, every view has stored its measurements. The second pass happens in layout(int, int, int, int) and is also top-down. During this pass each parent is responsible for positioning all of its children using the sizes computed in the measure pass.

When a view's measure() method returns, its getMeasuredWidth() and getMeasuredHeight() values must be set, along with those for all of that view's descendants. A view's measured width and measured height values must respect the constraints imposed by the view's parents. This guarantees that at the end of the measure pass, all parents accept all of their children's measurements. A parent view may call measure() more than once on its children. For example, the parent may measure each child once with unspecified dimensions to find out how big they want to be, then call measure() on them again with actual numbers if the sum of all the children's unconstrained sizes is too big or too small.

The measure pass uses two classes to communicate dimensions. The View.MeasureSpec class is used by views to tell their parents how they want to be measured and positioned. The base LayoutParams class just describes how big the view wants to be for both width and height. For each dimension, it can specify one of:

• an exact number
• FILL_PARENT, which means the view wants to be as big as its parent (minus padding)
• WRAP_CONTENT, which means that the view wants to be just big enough to enclose its content (plus padding).

MeasureSpecs are used to push requirements down the tree from parent to child. A MeasureSpec can be in one of three modes:

• UNSPECIFIED: This is used by a parent to determine the desired dimension of a child view. For example, a LinearLayout may call measure() on its child with the height set to UNSPECIFIED and a width of EXACTLY 240 to find out how tall the child view wants to be given a width of 240 pixels.
• EXACTLY: This is used by the parent to impose an exact size on the child. The child must use this size, and guarantee that all of its descendants will fit within this size.
• AT_MOST: This is used by the parent to impose a maximum size on the child. The child must gurantee that it and all of its descendants will fit within this size.

To intiate a layout, call requestLayout(). This method is typically called by a view on itself when it believes that is can no longer fit within its current bounds.

Drawing

Drawing is handled by walking the tree and rendering each view that intersects the the invalid region. Because the tree is traversed in-order, this means that parents will draw before (i.e., behind) their children, with siblings drawn in the order they appear in the tree.

The framework will not draw views that are not in the invalid region, and also will take care of drawing the views background for you.

To force a view to draw, call invalidate().

Event Handling and Threading

The basic cycle of a view is as follows:

1. An event comes in and is dispatched to the appropriate view. The view handles the event and notifies any listeners.
2. If in the course of processing the event, the view's bounds may need to be changed, the view will call requestLayout().
3. Similarly, if in the course of processing the event the view's appearance may need to be changed, the view will call invalidate().
4. If either requestLayout() or invalidate() were called, the framework will take care of measuring, laying out, and drawing the tree as appropriate.

Note: The entire view tree is single threaded. You must always be on the UI thread when calling any method on any view. If you are doing work on other threads and want to update the state of a view from that thread, you should use a Handler.

Focus Handling

The framework will handle routine focus movement in response to user input. This includes changing the focus as views are removed or hidden, or as new views become available. Views indicate their willingness to take focus through the isFocusable() method. To change whether a view can take focus, call setFocusable(boolean). When in touch mode (see notes below) views indicate whether they still would like focus via isFocusableInTouchMode() and can change this via setFocusableInTouchMode(boolean).

Focus movement is based on an algorithm which finds the nearest neighbor in a given direction. In rare cases, the default algorithm may not match the intended behavior of the developer. In these situations, you can provide explicit overrides by using these XML attributes in the layout file:

 nextFocusDown
 nextFocusLeft
 nextFocusRight
 nextFocusUp
 

Views indicate their preference for receiving focus over their descendants via the getFocusType() method. NORMAL_FOCUS (the default), indicates it will give itself focus before looking in descendants. WEAK_FOCUS indicates that it will try to give focus to its descendants before trying itself. This focus dispatch logic is implemented for you in requestFocus(int, android.graphics.Rect)

To get a particular view to take focus, call requestFocus().

Touch Mode

When a user is navigating a user interface via directional keys such as a D-pad, it is necessary to give focus to actionable items such as buttons so the user can see what will take input. If the device has touch capabilities, however, and the user begins interacting with the interface by touching it, it is no longer necessary to always highlight, or give focus to, a particular view. This motivates a mode for interaction named 'touch mode'.

For a touch capable device, once the user touches the screen, the device will enter touch mode. From this point onward, only views for which isFocusableInTouchMode() is true will be focusable, such as text editing widgets. Other views that are touchable, like buttons, will not take focus when touched; they will only fire the on click listeners.

Any time a user hits a directional key, such as a D-pad direction, the view device will exit touch mode, and find a view to take focus, so that the user may resume interacting with the user interface without touching the screen again.

The touch mode state is maintained across Activitys. Call inTouchMode() to see whether the device is currently in touch mode.

Scrolling

The framework provides basic support for views that wish to internally scroll their content. This includes keeping track of the X and Y scroll offset as well as mechanisms for drawing scrollbars. See scrollBy(int, int), scrollTo(int, int), and awakenScrollBars() for more details.

Tags

Unlike IDs, tags are not used to identify views. Tags are essentially an extra piece of information that can be associated with a view. They are most often used as a convenience to store data related to views in the views themselves rather than by putting them in a separate structure.

Animation

You can attach an Animation object to a view using setAnimation(Animation) or startAnimation(Animation). The animation can alter the scale, rotation, translation and alpha of a view over time. If the animation is attached to a view that has children, the animation will affect the entire subtree rooted by that node. When an animation is started, the framework will take care of redrawing the appropriate views until the animation completes.

Nested Classes

Known Direct Subclasses

Known Indirect Subclasses

XML Attributes

Constants

Fields

Public Constructors

Public Methods