The Android framework asks an
Activity
to draw its layout when the
Activity
receives focus. The Android framework handles the procedure for drawing, but the
Activity
must provide the root node of its layout hierarchy.
The Android framework draws the root node of the layout and measures and draws the layout tree. It
draws by walking the tree and rendering each
View
that intersects the invalid region.
Each ViewGroup
is responsible for
requesting that each of its children be drawn, using the
draw()
method, and each View
is responsible for drawing itself. Because the tree is traversed
pre-order, the framework draws parents before—in other words, behind—their
children, and it draws siblings in the order they appear in the tree.
The Android framework draws the layout in a two-pass process: a measure pass and a layout pass. The
framework performs the measure pass in
measure(int, int)
and
performs 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
stores its measurements. The framework performs the second pass 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.
The two passes of the layout process are described in more detail in the following sections.
Initiate a measure pass
When a View
object's
measure()
method
returns, set its
getMeasuredWidth()
and
getMeasuredHeight()
values, along with those for all of the View
object's descendants. A View
object's measured width and measured height values must respect the constraints imposed by the
View
object's parents. This helps ensure that at the end of the measure pass, all parents
accept all of their children's measurements.
A parent View
might call measure()
more than once on its children. For
example, the parent might measure the children once with unspecified dimensions to determine their
preferred sizes. If the sum of the children's unconstrained sizes is too big or too small, the parent
might call measure()
again with values that constrain the children's sizes.
The measure pass uses two classes to communicate dimensions. The
ViewGroup.LayoutParams
class is how View
objects communicate their preferred sizes and positions. The base
ViewGroup.LayoutParams
class describes the preferred width and height of the
View
. For each dimension, it can specify one of the following:
- An exact dimension.
MATCH_PARENT
, meaning the preferred size for theView
is the size of its parent, minus padding.WRAP_CONTENT
, meaning the preferred size for theView
is just big enough to enclose its content, plus padding.
There are subclasses of ViewGroup.LayoutParams
for different subclasses of
ViewGroup
. For example,
RelativeLayout
has its own
subclass of ViewGroup.LayoutParams
that includes the ability to center child
View
objects horizontally and vertically.
MeasureSpec
objects are
used to push requirements down the tree from parent to child. A MeasureSpec
can be in
one of three modes:
UNSPECIFIED
: the parent uses this to determine the target dimension of a childView
. For example, aLinearLayout
might callmeasure()
on its child with the height set toUNSPECIFIED
and a width ofEXACTLY
240 to find out how tall the childView
wants to be, given a width of 240 pixels.EXACTLY
: the parent uses this to impose an exact size on the child. The child must use this size and guarantee that all of its descendants fit within this size.AT MOST
: the parent uses this to impose a maximum size on the child. The child must guarantee that it and all of its descendants fit within this size.
Initiate a layout pass
To initiate a layout, call
requestLayout()
. This
method is typically called by a View
on itself when it believes it can no longer fit
within its bounds.
Implement a custom measurement and layout logic
If you want to implement a custom measurement or layout logic, override the methods where the logic
is implemented:
onMeasure(int, int)
and
onLayout(boolean, int, int, int, int)
.
These methods are called by measure(int, int)
and
layout(int, int, int, int)
, respectively. Don't try to override the
measure(int, int)
or layout(int, int)
methods—both of these methods
are final
, so they can't be overridden.
The following example shows how to do this in the
`SplitLayout`
class from the
WindowManager
sample app. If the SplitLayout
has two or more child views, and the display has a fold,
then it positions the two child views on either side of the fold. The following example shows a use
case for overriding the measurement and layout, but for production, use
SlidingPaneLayout
if you want this behavior.
Kotlin
/** * An example of split-layout for two views, separated by a display * feature that goes across the window. When both start and end views are * added, it checks whether there are display features that separate the area * in two—such as a fold or hinge—and places them side-by-side or * top-bottom. */ class SplitLayout : FrameLayout { private var windowLayoutInfo: WindowLayoutInfo? = null private var startViewId = 0 private var endViewId = 0 private var lastWidthMeasureSpec: Int = 0 private var lastHeightMeasureSpec: Int = 0 ... fun updateWindowLayout(windowLayoutInfo: WindowLayoutInfo) { this.windowLayoutInfo = windowLayoutInfo requestLayout() } override fun onLayout(changed: Boolean, left: Int, top: Int, right: Int, bottom: Int) { val startView = findStartView() val endView = findEndView() val splitPositions = splitViewPositions(startView, endView) if (startView != null && endView != null && splitPositions != null) { val startPosition = splitPositions[0] val startWidthSpec = MeasureSpec.makeMeasureSpec(startPosition.width(), EXACTLY) val startHeightSpec = MeasureSpec.makeMeasureSpec(startPosition.height(), EXACTLY) startView.measure(startWidthSpec, startHeightSpec) startView.layout( startPosition.left, startPosition.top, startPosition.right, startPosition.bottom ) val endPosition = splitPositions[1] val endWidthSpec = MeasureSpec.makeMeasureSpec(endPosition.width(), EXACTLY) val endHeightSpec = MeasureSpec.makeMeasureSpec(endPosition.height(), EXACTLY) endView.measure(endWidthSpec, endHeightSpec) endView.layout( endPosition.left, endPosition.top, endPosition.right, endPosition.bottom ) } else { super.onLayout(changed, left, top, right, bottom) } } /** * Gets the position of the split for this view. * @return A rect that defines of split, or {@code null} if there is no split. */ private fun splitViewPositions(startView: View?, endView: View?): Array? { if (windowLayoutInfo == null || startView == null || endView == null) { return null } // Calculate the area for view's content with padding. val paddedWidth = width - paddingLeft - paddingRight val paddedHeight = height - paddingTop - paddingBottom windowLayoutInfo?.displayFeatures ?.firstOrNull { feature -> isValidFoldFeature(feature) } ?.let { feature -> getFeaturePositionInViewRect(feature, this)?.let { if (feature.bounds.left == 0) { // Horizontal layout. val topRect = Rect( paddingLeft, paddingTop, paddingLeft + paddedWidth, it.top ) val bottomRect = Rect( paddingLeft, it.bottom, paddingLeft + paddedWidth, paddingTop + paddedHeight ) if (measureAndCheckMinSize(topRect, startView) && measureAndCheckMinSize(bottomRect, endView) ) { return arrayOf(topRect, bottomRect) } } else if (feature.bounds.top == 0) { // Vertical layout. val leftRect = Rect( paddingLeft, paddingTop, it.left, paddingTop + paddedHeight ) val rightRect = Rect( it.right, paddingTop, paddingLeft + paddedWidth, paddingTop + paddedHeight ) if (measureAndCheckMinSize(leftRect, startView) && measureAndCheckMinSize(rightRect, endView) ) { return arrayOf(leftRect, rightRect) } } } } // You previously tried to fit the children and measure them. Since they // don't fit, measure again to update the stored values. measure(lastWidthMeasureSpec, lastHeightMeasureSpec) return null } override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) { super.onMeasure(widthMeasureSpec, heightMeasureSpec) lastWidthMeasureSpec = widthMeasureSpec lastHeightMeasureSpec = heightMeasureSpec } /** * Measures a child view and sees if it fits in the provided rect. * This method calls [View.measure] on the child view, which updates its * stored values for measured width and height. If the view ends up with * different values, measure again. */ private fun measureAndCheckMinSize(rect: Rect, childView: View): Boolean { val widthSpec = MeasureSpec.makeMeasureSpec(rect.width(), AT_MOST) val heightSpec = MeasureSpec.makeMeasureSpec(rect.height(), AT_MOST) childView.measure(widthSpec, heightSpec) return childView.measuredWidthAndState and MEASURED_STATE_TOO_SMALL == 0 && childView.measuredHeightAndState and MEASURED_STATE_TOO_SMALL == 0 } private fun isValidFoldFeature(displayFeature: DisplayFeature) = (displayFeature as? FoldingFeature)?.let { feature -> getFeaturePositionInViewRect(feature, this) != null } ?: false }
Java
/** * An example of split-layout for two views, separated by a display feature * that goes across the window. When both start and end views are added, it checks * whether there are display features that separate the area in two—such as * fold or hinge—and places them side-by-side or top-bottom. */ public class SplitLayout extends FrameLayout { @Nullable private WindowLayoutInfo windowLayoutInfo = null; private int startViewId = 0; private int endViewId = 0; private int lastWidthMeasureSpec = 0; private int lastHeightMeasureSpec = 0; ... void updateWindowLayout(WindowLayoutInfo windowLayoutInfo) { this.windowLayoutInfo = windowLayoutInfo; requestLayout(); } @Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { @Nullable View startView = findStartView(); @Nullable View endView = findEndView(); @Nullable ListsplitPositions = splitViewPositions(startView, endView); if (startView != null && endView != null && splitPositions != null) { Rect startPosition = splitPositions.get(0); int startWidthSpec = MeasureSpec.makeMeasureSpec(startPosition.width(), EXACTLY); int startHeightSpec = MeasureSpec.makeMeasureSpec(startPosition.height(), EXACTLY); startView.measure(startWidthSpec, startHeightSpec); startView.layout( startPosition.left, startPosition.top, startPosition.right, startPosition.bottom ); Rect endPosition = splitPositions.get(1); int endWidthSpec = MeasureSpec.makeMeasureSpec(endPosition.width(), EXACTLY); int endHeightSpec = MeasureSpec.makeMeasureSpec(endPosition.height(), EXACTLY); startView.measure(endWidthSpec, endHeightSpec); startView.layout( endPosition.left, endPosition.top, endPosition.right, endPosition.bottom ); } else { super.onLayout(changed, left, top, right, bottom); } } /** * Gets the position of the split for this view. * @return A rect that defines of split, or {@code null} if there is no split. */ @Nullable private List splitViewPositions(@Nullable View startView, @Nullable View endView) { if (windowLayoutInfo == null || startView == null || endView == null) { return null; } int paddedWidth = getWidth() - getPaddingLeft() - getPaddingRight(); int paddedHeight = getHeight() - getPaddingTop() - getPaddingBottom(); List displayFeatures = windowLayoutInfo.getDisplayFeatures(); @Nullable DisplayFeature feature = displayFeatures .stream() .filter(item -> isValidFoldFeature(item) ) .findFirst() .orElse(null); if (feature != null) { Rect position = SampleToolsKt.getFeaturePositionInViewRect(feature, this, true); Rect featureBounds = feature.getBounds(); if (featureBounds.left == 0) { // Horizontal layout. Rect topRect = new Rect( getPaddingLeft(), getPaddingTop(), getPaddingLeft() + paddedWidth, position.top ); Rect bottomRect = new Rect( getPaddingLeft(), position.bottom, getPaddingLeft() + paddedWidth, getPaddingTop() + paddedHeight ); if (measureAndCheckMinSize(topRect, startView) && measureAndCheckMinSize(bottomRect, endView)) { ArrayList rects = new ArrayList (); rects.add(topRect); rects.add(bottomRect); return rects; } } else if (featureBounds.top == 0) { // Vertical layout. Rect leftRect = new Rect( getPaddingLeft(), getPaddingTop(), position.left, getPaddingTop() + paddedHeight ); Rect rightRect = new Rect( position.right, getPaddingTop(), getPaddingLeft() + paddedWidth, getPaddingTop() + paddedHeight ); if (measureAndCheckMinSize(leftRect, startView) && measureAndCheckMinSize(rightRect, endView)) { ArrayList rects = new ArrayList (); rects.add(leftRect); rects.add(rightRect); return rects; } } } // You previously tried to fit the children and measure them. Since // they don't fit, measure again to update the stored values. measure(lastWidthMeasureSpec, lastHeightMeasureSpec); return null; } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); lastWidthMeasureSpec = widthMeasureSpec; lastHeightMeasureSpec = heightMeasureSpec; } /** * Measures a child view and sees if it fits in the provided rect. * This method calls [View.measure] on the child view, which updates * its stored values for measured width and height. If the view ends up with * different values, measure again. */ private boolean measureAndCheckMinSize(Rect rect, View childView) { int widthSpec = MeasureSpec.makeMeasureSpec(rect.width(), AT_MOST); int heightSpec = MeasureSpec.makeMeasureSpec(rect.height(), AT_MOST); childView.measure(widthSpec, heightSpec); return (childView.getMeasuredWidthAndState() & MEASURED_STATE_TOO_SMALL) == 0 && (childView.getMeasuredHeightAndState() & MEASURED_STATE_TOO_SMALL) == 0; } private boolean isValidFoldFeature(DisplayFeature displayFeature) { if (displayFeature instanceof FoldingFeature) { return SampleToolsKt.getFeaturePositionInViewRect(displayFeature, this, true) != null; } else { return false; } } }