ViewTreeObserver | API reference

class ViewTreeObserver

A view tree observer is used to register listeners that can be notified of global changes in the view tree. Such global events include, but are not limited to, layout of the whole tree, beginning of the drawing pass, touch mode change.... A ViewTreeObserver should never be instantiated by applications as it is provided by the views hierarchy. Refer to android.view.View#getViewTreeObserver() for more information.

Summary

Nested classes
abstract	`OnDrawListener` Interface definition for a callback to be invoked when the view tree is about to be drawn.
abstract	`OnGlobalFocusChangeListener` Interface definition for a callback to be invoked when the focus state within the view tree changes.
abstract	`OnGlobalLayoutListener` Interface definition for a callback to be invoked when the global layout state or the visibility of views within the view tree changes.
abstract	`OnPreDrawListener` Interface definition for a callback to be invoked when the view tree is about to be drawn.
abstract	`OnScrollChangedListener` Interface definition for a callback to be invoked when something in the view tree has been scrolled.
abstract	`OnTouchModeChangeListener` Interface definition for a callback to be invoked when the touch mode changes.
abstract	`OnWindowAttachListener` Interface definition for a callback to be invoked when the view hierarchy is attached to and detached from its window.
abstract	`OnWindowFocusChangeListener` Interface definition for a callback to be invoked when the view hierarchy's window focus state changes.
abstract	`OnWindowVisibilityChangeListener` Interface definition for a callback to be invoked when the view hierarchy's window visibility changes.

Public methods
Unit	`addOnDrawListener(listener: ViewTreeObserver.OnDrawListener!)` Register a callback to be invoked when the view tree is about to be drawn.
Unit	`addOnGlobalFocusChangeListener(listener: ViewTreeObserver.OnGlobalFocusChangeListener!)`
Unit	`addOnGlobalLayoutListener(listener: ViewTreeObserver.OnGlobalLayoutListener!)` Register a callback to be invoked when the global layout state or the visibility of views within the view tree changes
Unit	`addOnPreDrawListener(listener: ViewTreeObserver.OnPreDrawListener!)` Register a callback to be invoked when the view tree is about to be drawn
Unit	`addOnScrollChangedListener(listener: ViewTreeObserver.OnScrollChangedListener!)` Register a callback to be invoked when a view has been scrolled.
Unit	`addOnSystemGestureExclusionRectsChangedListener(listener: Consumer<MutableList<Rect!>!>)` Add a listener to be notified when the tree's transformed gesture exclusion rects change.
Unit	`addOnTouchModeChangeListener(listener: ViewTreeObserver.OnTouchModeChangeListener!)` Register a callback to be invoked when the invoked when the touch mode changes.
Unit	`addOnWindowAttachListener(listener: ViewTreeObserver.OnWindowAttachListener!)` Register a callback to be invoked when the view hierarchy is attached to a window.
Unit	`addOnWindowFocusChangeListener(listener: ViewTreeObserver.OnWindowFocusChangeListener!)` Register a callback to be invoked when the window focus state within the view tree changes.
Unit	`addOnWindowVisibilityChangeListener(listener: ViewTreeObserver.OnWindowVisibilityChangeListener)` Register a callback to be invoked when the window visibility changes.
Unit	`dispatchOnDraw()` Notifies registered listeners that the drawing pass is about to start.
Unit	`dispatchOnGlobalLayout()` Notifies registered listeners that a global layout happened.
Boolean	`dispatchOnPreDraw()` Notifies registered listeners that the drawing pass is about to start.
Unit	`dispatchOnScrollChanged()` Notifies registered listeners that something has scrolled.
Boolean	`isAlive()` Indicates whether this ViewTreeObserver is alive.
Unit	`registerFrameCommitCallback(callback: Runnable)` Adds a frame commit callback.
Unit	`removeGlobalOnLayoutListener(victim: ViewTreeObserver.OnGlobalLayoutListener!)` Remove a previously installed global layout callback
Unit	`removeOnDrawListener(victim: ViewTreeObserver.OnDrawListener!)` Remove a previously installed pre-draw callback.
Unit	`removeOnGlobalFocusChangeListener(victim: ViewTreeObserver.OnGlobalFocusChangeListener!)` Remove a previously installed focus change callback.
Unit	`removeOnGlobalLayoutListener(victim: ViewTreeObserver.OnGlobalLayoutListener!)` Remove a previously installed global layout callback
Unit	`removeOnPreDrawListener(victim: ViewTreeObserver.OnPreDrawListener!)` Remove a previously installed pre-draw callback
Unit	`removeOnScrollChangedListener(victim: ViewTreeObserver.OnScrollChangedListener!)` Remove a previously installed scroll-changed callback
Unit	`removeOnSystemGestureExclusionRectsChangedListener(listener: Consumer<MutableList<Rect!>!>)` Unsubscribe the given listener from gesture exclusion rect changes.
Unit	`removeOnTouchModeChangeListener(victim: ViewTreeObserver.OnTouchModeChangeListener!)` Remove a previously installed touch mode change callback
Unit	`removeOnWindowAttachListener(victim: ViewTreeObserver.OnWindowAttachListener!)` Remove a previously installed window attach callback.
Unit	`removeOnWindowFocusChangeListener(victim: ViewTreeObserver.OnWindowFocusChangeListener!)` Remove a previously installed window focus change callback.
Unit	`removeOnWindowVisibilityChangeListener(victim: ViewTreeObserver.OnWindowVisibilityChangeListener)` Remove a previously installed window visibility callback.
Boolean	`unregisterFrameCommitCallback(callback: Runnable)` Attempts to remove the given callback from the list of pending frame complete callbacks.

Public methods

addOnSystemGestureExclusionRectsChangedListener

fun addOnSystemGestureExclusionRectsChangedListener(listener: Consumer<MutableList<Rect!>!>): Unit

Add a listener to be notified when the tree's transformed gesture exclusion rects change. This could be the result of an animation or other layout change, or a view calling View.setSystemGestureExclusionRects(List).

Parameters
`listener`	Consumer<MutableList<Rect!>!>: listener to add. This value cannot be `null`.

dispatchOnDraw

fun dispatchOnDraw(): Unit

Notifies registered listeners that the drawing pass is about to start.

dispatchOnGlobalLayout

fun dispatchOnGlobalLayout(): Unit

Notifies registered listeners that a global layout happened. This can be called manually if you are forcing a layout on a View or a hierarchy of Views that are not attached to a Window or in the GONE state.

dispatchOnPreDraw

fun dispatchOnPreDraw(): Boolean

Notifies registered listeners that the drawing pass is about to start. If a listener returns true, then the drawing pass is canceled and rescheduled. This can be called manually if you are forcing the drawing on a View or a hierarchy of Views that are not attached to a Window or in the GONE state.

Return
`Boolean`	True if the current draw should be canceled and rescheduled, false otherwise.

dispatchOnScrollChanged

fun dispatchOnScrollChanged(): Unit

Notifies registered listeners that something has scrolled.

isAlive

fun isAlive(): Boolean

Indicates whether this ViewTreeObserver is alive. When an observer is not alive, any call to a method (except this one) will throw an exception. If an application keeps a long-lived reference to this ViewTreeObserver, it should always check for the result of this method before calling any other method.

Return
`Boolean`	True if this object is alive and be used, false otherwise.

registerFrameCommitCallback

fun registerFrameCommitCallback(callback: Runnable): Unit

Adds a frame commit callback. This callback will be invoked when the current rendering content has been rendered into a frame and submitted to the swap chain. The frame may not currently be visible on the display when this is invoked, but it has been submitted. This callback is useful in combination with PixelCopy to capture the current rendered content of the UI reliably. Note: Only works with hardware rendering. Does nothing otherwise.

Parameters
`callback`	Runnable: The callback to invoke when the frame is committed. This value cannot be `null`.

unregisterFrameCommitCallback

fun unregisterFrameCommitCallback(callback: Runnable): Boolean

Attempts to remove the given callback from the list of pending frame complete callbacks.

Parameters
`callback`	Runnable: The callback to remove. This value cannot be `null`.

Return
`Boolean`	Whether or not the callback was removed. If this returns true the callback will not be invoked. If false is returned then the callback was either never added or may already be pending execution and was unable to be removed

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