android官方API之ConstraintLayout

来源：https://developer.android.com/reference/android/support/constraint/ConstraintLayout

ConstraintLayout

public class ConstraintLayout 
extends ViewGroup 

java.lang.Object
↳	android.view.View
	↳	android.view.ViewGroup
		↳	android.support.constraint.ConstraintLayout

Known direct subclasses MotionLayout

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A ConstraintLayout is a ViewGroup which allows you to position and size widgets in a flexible way.

Note: ConstraintLayout is available as a support library that you can use on Android systems starting with API level 9 (Gingerbread). As such, we are planning on enriching its API and capabilities over time. This documentation will reflect those changes.

There are currently various types of constraints that you can use:

• Relative positioning
• Margins
• Centering positioning
• Circular positioning
• Visibility behavior
• Dimension constraints
• Chains
• Virtual Helpers objects
• Optimizer

Note that you cannot have a circular dependency in constraints.

Also see ConstraintLayout.LayoutParams for layout attributes

Developer Guide

Relative positioning

Relative positioning is one of the basic building block of creating layouts in ConstraintLayout. Those constraints allow you to position a given widget relative to another one. You can constrain a widget on the horizontal and vertical axis:

• Horizontal Axis: left, right, start and end sides
• Vertical Axis: top, bottom sides and text baseline

The general concept is to constrain a given side of a widget to another side of any other widget.

For example, in order to position button B to the right of button A (Fig. 1): 

 
Fig. 1 - Relative Positioning Example

you would need to do:

<Button android:id="@+id/buttonA" ... /> <Button android:id="@+id/buttonB" ... app:layout_constraintLeft_toRightOf="@+id/buttonA" /> This tells the system that we want the left side of button B to be constrained to the right side of button A. Such a position constraint means that the system will try to have both sides share the same location. 

 
Fig. 2 - Relative Positioning Constraints

Here is the list of available constraints (Fig. 2):

• layout_constraintLeft_toLeftOf
• layout_constraintLeft_toRightOf
• layout_constraintRight_toLeftOf
• layout_constraintRight_toRightOf
• layout_constraintTop_toTopOf
• layout_constraintTop_toBottomOf
• layout_constraintBottom_toTopOf
• layout_constraintBottom_toBottomOf
• layout_constraintBaseline_toBaselineOf
• layout_constraintStart_toEndOf
• layout_constraintStart_toStartOf
• layout_constraintEnd_toStartOf
• layout_constraintEnd_toEndOf

They all take a reference id to another widget, or the parent (which will reference the parent container, i.e. the ConstraintLayout):

<Button android:id="@+id/buttonB" ... app:layout_constraintLeft_toLeftOf="parent" />

Margins

 
Fig. 3 - Relative Positioning Margins

If side margins are set, they will be applied to the corresponding constraints (if they exist) (Fig. 3), enforcing the margin as a space between the target and the source side. The usual layout margin attributes can be used to this effect:

• android:layout_marginStart
• android:layout_marginEnd
• android:layout_marginLeft
• android:layout_marginTop
• android:layout_marginRight
• android:layout_marginBottom

Note that a margin can only be positive or equals to zero, and takes a Dimension.

Margins when connected to a GONE widget

When a position constraint target's visibility is View.GONE, you can also indicate a different margin value to be used using the following attributes:

• layout_goneMarginStart
• layout_goneMarginEnd
• layout_goneMarginLeft
• layout_goneMarginTop
• layout_goneMarginRight
• layout_goneMarginBottom

Centering positioning and bias

A useful aspect of ConstraintLayout is in how it deals with "impossible" constrains. For example, if we have something like:

<android.support.constraint.ConstraintLayout ...> <Button android:id="@+id/button" ... app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintRight_toRightOf="parent/> </>

Unless the ConstraintLayout happens to have the exact same size as the Button, both constraints cannot be satisfied at the same time (both sides cannot be where we want them to be).

 
Fig. 4 - Centering Positioning

What happens in this case is that the constraints act like opposite forces pulling the widget apart equally (Fig. 4); such that the widget will end up being centered in the parent container. This will apply similarly for vertical constraints.

Bias

The default when encountering such opposite constraints is to center the widget; but you can tweak the positioning to favor one side over another using the bias attributes:

• layout_constraintHorizontal_bias
• layout_constraintVertical_bias

 
Fig. 5 - Centering Positioning with Bias

For example the following will make the left side with a 30% bias instead of the default 50%, such that the left side will be shorter, with the widget leaning more toward the left side (Fig. 5):

<android.support.constraint.ConstraintLayout ...> <Button android:id="@+id/button" ... app:layout_constraintHorizontal_bias="0.3" app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintRight_toRightOf="parent/> </> Using bias, you can craft User Interfaces that will better adapt to screen sizes changes.

Circular positioning (Added in 1.1)

You can constrain a widget center relative to another widget center, at an angle and a distance. This allows you to position a widget on a circle (see Fig. 6). The following attributes can be used:

• layout_constraintCircle : references another widget id
• layout_constraintCircleRadius : the distance to the other widget center
• layout_constraintCircleAngle : which angle the widget should be at (in degrees, from 0 to 360)

  
Fig. 6 - Circular Positioning

 

<Button android:id="@+id/buttonA" ... /> <Button android:id="@+id/buttonB" ... app:layout_constraintCircle="@+id/buttonA" app:layout_constraintCircleRadius="100dp" app:layout_constraintCircleAngle="45" />

Visibility behavior

ConstraintLayout has a specific handling of widgets being marked as View.GONE.

GONE widgets, as usual, are not going to be displayed and are not part of the layout itself (i.e. their actual dimensions will not be changed if marked as GONE).

But in terms of the layout computations, GONE widgets are still part of it, with an important distinction:

• For the layout pass, their dimension will be considered as zero (basically, they will be resolved to a point)
• If they have constraints to other widgets they will still be respected, but any margins will be as if equals to zero

 
Fig. 7 - Visibility Behavior

This specific behavior allows to build layouts where you can temporarily mark widgets as being GONE, without breaking the layout (Fig. 7), which can be particularly useful when doing simple layout animations.

Note: The margin used will be the margin that B had defined when connecting to A (see Fig. 7 for an example). In some cases, this might not be the margin you want (e.g. A had a 100dp margin to the side of its container, B only a 16dp to A, marking A as gone, B will have a margin of 16dp to the container). For this reason, you can specify an alternate margin value to be used when the connection is to a widget being marked as gone (see the section above about the gone margin attributes).

Dimensions constraints

Minimum dimensions on ConstraintLayout

You can define minimum and maximum sizes for the ConstraintLayout itself:

• android:minWidth set the minimum width for the layout
• android:minHeight set the minimum height for the layout
• android:maxWidth set the maximum width for the layout
• android:maxHeight set the maximum height for the layout

Those minimum and maximum dimensions will be used by ConstraintLayout when its dimensions are set to WRAP_CONTENT.

Widgets dimension constraints

The dimension of the widgets can be specified by setting the android:layout_width and android:layout_heightattributes in 3 different ways:

• Using a specific dimension (either a literal value such as 123dp or a Dimension reference)
• Using WRAP_CONTENT, which will ask the widget to compute its own size
• Using 0dp, which is the equivalent of "MATCH_CONSTRAINT"

 
Fig. 8 - Dimension Constraints

The first two works in a similar fashion as other layouts. The last one will resize the widget in such a way as matching the constraints that are set (see Fig. 8, (a) is wrap_content, (b) is 0dp). If margins are set, they will be taken in account in the computation (Fig. 8, (c) with 0dp).

Important: MATCH_PARENT is not recommended for widgets contained in a ConstraintLayout. Similar behavior can be defined by using MATCH_CONSTRAINT with the corresponding left/right or top/bottom constraints being set to "parent".

WRAP_CONTENT : enforcing constraints (Added in 1.1)

If a dimension is set to WRAP_CONTENT, in versions before 1.1 they will be treated as a literal dimension -- meaning, constraints will not limit the resulting dimension. While in general this is enough (and faster), in some situations, you might want to use WRAP_CONTENT, yet keep enforcing constraints to limit the resulting dimension. In that case, you can add one of the corresponding attribute:

• app:layout_constrainedWidth=”true|false”
• app:layout_constrainedHeight=”true|false”

MATCH_CONSTRAINT dimensions (Added in 1.1)

When a dimension is set to MATCH_CONSTRAINT, the default behavior is to have the resulting size take all the available space. Several additional modifiers are available:

• layout_constraintWidth_min and layout_constraintHeight_min : will set the minimum size for this dimension
• layout_constraintWidth_max and layout_constraintHeight_max : will set the maximum size for this dimension
• layout_constraintWidth_percent and layout_constraintHeight_percent : will set the size of this dimension as a percentage of the parent

Min and Max

The value indicated for min and max can be either a dimension in Dp, or "wrap", which will use the same value as what WRAP_CONTENT would do.

Percent dimension

To use percent, you need to set the following:

• The dimension should be set to MATCH_CONSTRAINT (0dp)
• The default should be set to percent app:layout_constraintWidth_default="percent" or app:layout_constraintHeight_default="percent" 
  (Note: this is necessary in 1.1-beta1 and 1.1-beta2, but will not be needed in following versions if the percent attribute is defined)
• Then set the layout_constraintWidth_percent or layout_constraintHeight_percent attributes to a value between 0 and 1

Ratio

You can also define one dimension of a widget as a ratio of the other one. In order to do that, you need to have at least one constrained dimension be set to 0dp (i.e., MATCH_CONSTRAINT), and set the attribute layout_constraintDimensionRatio to a given ratio. For example:

<Button android:layout_width="wrap_content" android:layout_height="0dp" app:layout_constraintDimensionRatio="1:1" /> will set the height of the button to be the same as its width.

The ratio can be expressed either as:

• a float value, representing a ratio between width and height
• a ratio in the form "width:height"

You can also use ratio if both dimensions are set to MATCH_CONSTRAINT (0dp). In this case the system sets the largest dimensions the satisfies all constraints and maintains the aspect ratio specified. To constrain one specific side based on the dimensions of another, you can pre append W," or H, to constrain the width or height respectively. For example, If one dimension is constrained by two targets (e.g. width is 0dp and centered on parent) you can indicate which side should be constrained, by adding the letter W (for constraining the width) or H (for constraining the height) in front of the ratio, separated by a comma:

<Button android:layout_width="0dp" android:layout_height="0dp" app:layout_constraintDimensionRatio="H,16:9" app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintTop_toTopOf="parent"/> will set the height of the button following a 16:9 ratio, while the width of the button will match the constraints to parent.

Chains

Chains provide group-like behavior in a single axis (horizontally or vertically). The other axis can be constrained independently.

Creating a chain

A set of widgets are considered a chain if they are linked together via a bi-directional connection (see Fig. 9, showing a minimal chain, with two widgets).

 
Fig. 9 - Chain

Chain heads

Chains are controlled by attributes set on the first element of the chain (the "head" of the chain):

 
Fig. 10 - Chain Head

The head is the left-most widget for horizontal chains, and the top-most widget for vertical chains.

Margins in chains

If margins are specified on connections, they will be taken in account. In the case of spread chains, margins will be deducted from the allocated space.

Chain Style

When setting the attribute layout_constraintHorizontal_chainStyle or layout_constraintVertical_chainStyle on the first element of a chain, the behavior of the chain will change according to the specified style (default is CHAIN_SPREAD).

• CHAIN_SPREAD -- the elements will be spread out (default style)
• Weighted chain -- in CHAIN_SPREAD mode, if some widgets are set to MATCH_CONSTRAINT, they will split the available space
• CHAIN_SPREAD_INSIDE -- similar, but the endpoints of the chain will not be spread out
• CHAIN_PACKED -- the elements of the chain will be packed together. The horizontal or vertical bias attribute of the child will then affect the positioning of the packed elements

 
Fig. 11 - Chains Styles

Weighted chains

The default behavior of a chain is to spread the elements equally in the available space. If one or more elements are using MATCH_CONSTRAINT, they will use the available empty space (equally divided among themselves). The attribute layout_constraintHorizontal_weight and layout_constraintVertical_weight will control how the space will be distributed among the elements using MATCH_CONSTRAINT. For exemple, on a chain containing two elements using MATCH_CONSTRAINT, with the first element using a weight of 2 and the second a weight of 1, the space occupied by the first element will be twice that of the second element.

Margins and chains (in 1.1)

When using margins on elements in a chain, the margins are additive.

For example, on a horizontal chain, if one element defines a right margin of 10dp and the next element defines a left margin of 5dp, the resulting margin between those two elements is 15dp.

An item plus its margins are considered together when calculating leftover space used by chains to position items. The leftover space does not contain the margins.

Virtual Helper objects

In addition to the intrinsic capabilities detailed previously, you can also use special helper objects in ConstraintLayout to help you with your layout. Currently, the Guideline object allows you to create Horizontal and Vertical guidelines which are positioned relative to the ConstraintLayout container. Widgets can then be positioned by constraining them to such guidelines. In 1.1, Barrier and Group were added too.

Optimizer (in 1.1)

In 1.1 we exposed the constraints optimizer. You can decide which optimizations are applied by adding the tag app:layout_optimizationLevel to the ConstraintLayout element.

• none : no optimizations are applied
• standard : Default. Optimize direct and barrier constraints only
• direct : optimize direct constraints
• barrier : optimize barrier constraints
• chain : optimize chain constraints (experimental)
• dimensions : optimize dimensions measures (experimental), reducing the number of measures of match constraints elements

This attribute is a mask, so you can decide to turn on or off specific optimizations by listing the ones you want. For example: app:layout_optimizationLevel="direct|barrier|chain"

See also:

• Guideline

Summary

Nested classes
class	ConstraintLayout.LayoutParams This class contains the different attributes specifying how a view want to be laid out inside a ConstraintLayout.

Inherited constants
From class android.view.ViewGroup
From class android.view.View

Fields
protectedConstraintLayoutStates	mConstraintLayoutSpec

Inherited fields
From class android.view.View

Public constructors
ConstraintLayout(Context context)
ConstraintLayout(Context context, AttributeSet attrs)
ConstraintLayout(Context context, AttributeSet attrs, int defStyleAttr)

Public methods
int	getMaxHeight() The maximum height of this view.
int	getMaxWidth()
int	getMinHeight() The minimum height of this view.
int	getMinWidth() The minimum width of this view.
int	getOptimizationLevel() Return the current optimization level for the layout resolution
void	loadLayoutDescription(int layoutDescription)
void	requestLayout()
void	setConstraintSet(ConstraintSet set) Sets a ConstraintSet object to manage constraints.
void	setMaxHeight(int value) Set the max height for this view
void	setMaxWidth(int value) Set the max width for this view
void	setMinHeight(int value) Set the min height for this view
void	setMinWidth(int value) Set the min width for this view
void	setOnConstraintsChanged(ConstraintsChangedListenerconstraintsChangedListener)
void	setOptimizationLevel(int level) Set the optimization for the layout resolution.
void	setState(int id, int screenWidth, int screenHeight) Set the State of the ConstraintLayout, causing it to load a particular ConstraintSet.

Protected methods
boolean	checkLayoutParams(ViewGroup.LayoutParams p)
ConstraintLayout.LayoutParams	generateDefaultLayoutParams()
ViewGroup.LayoutParams	generateLayoutParams(ViewGroup.LayoutParams p)
void	onLayout(boolean changed, int left, int top, int right, int bottom)
void	onMeasure(int widthMeasureSpec, int heightMeasureSpec)

Inherited methods
From class android.view.ViewGroup
From class android.view.View
From class java.lang.Object
From interface android.view.ViewParent
From interface android.view.ViewManager
From interface android.graphics.drawable.Drawable.Callback
From interface android.view.KeyEvent.Callback
From interface android.view.accessibility.AccessibilityEventSource

Fields

mConstraintLayoutSpec

protected ConstraintLayoutStates mConstraintLayoutSpec

Public constructors

ConstraintLayout

public ConstraintLayout (Context context)

Parameters
context	Context

ConstraintLayout

public ConstraintLayout (Context context, AttributeSet attrs)

Parameters
context	Context
attrs	AttributeSet

ConstraintLayout

public ConstraintLayout (Context context, AttributeSet attrs, int defStyleAttr)

Parameters
context	Context
attrs	AttributeSet
defStyleAttr	int

Public methods

getMaxHeight

public int getMaxHeight ()

The maximum height of this view.

Returns
int	The maximum height of this view

See also:

• setMaxHeight(int)

getMaxWidth

public int getMaxWidth ()

Returns
int

getMinHeight

public int getMinHeight ()

The minimum height of this view.

Returns
int	The minimum height of this view

See also:

• setMinHeight(int)

getMinWidth

public int getMinWidth ()

The minimum width of this view.

Returns
int	The minimum width of this view

See also:

• setMinWidth(int)

getOptimizationLevel

public int getOptimizationLevel ()

Return the current optimization level for the layout resolution

Returns
int	the current level

loadLayoutDescription

public void loadLayoutDescription (int layoutDescription)

Parameters
layoutDescription	int

requestLayout

public void requestLayout ()

setConstraintSet

public void setConstraintSet (ConstraintSet set)

Sets a ConstraintSet object to manage constraints. The ConstraintSet overrides LayoutParams of child views.

Parameters
set	ConstraintSet: Layout children using ConstraintSet

setMaxHeight

public void setMaxHeight (int value)

Set the max height for this view

setMaxWidth

public void setMaxWidth (int value)

Set the max width for this view

setMinHeight

public void setMinHeight (int value)

Set the min height for this view

setMinWidth

public void setMinWidth (int value)

Set the min width for this view

setOnConstraintsChanged

public void setOnConstraintsChanged (ConstraintsChangedListener constraintsChangedListener)

Parameters
constraintsChangedListener	ConstraintsChangedListener

setOptimizationLevel

public void setOptimizationLevel (int level)

Set the optimization for the layout resolution.

The optimization can be any of the following:

• Optimizer.OPTIMIZATION_NONE
• Optimizer.OPTIMIZATION_STANDARD
• a mask composed of specific optimizations

The mask can be composed of any combination of the following:

• Optimizer.OPTIMIZATION_DIRECT
• Optimizer.OPTIMIZATION_BARRIER
• Optimizer.OPTIMIZATION_CHAIN (experimental)
• Optimizer.OPTIMIZATION_DIMENSIONS (experimental)

Note that the current implementation of Optimizer.OPTIMIZATION_STANDARD is as a mask of DIRECT and BARRIER.

Parameters
level	int: optimization level

setState

public void setState (int id, int screenWidth, int screenHeight)

Set the State of the ConstraintLayout, causing it to load a particular ConstraintSet. For states with variants the variant with matching width and height constraintSet will be chosen

Parameters
id	int: set the state width and height
screenWidth	int: the width of the screen in pixels
screenHeight	int: the height of the screen in pixels

Protected methods

checkLayoutParams

protected boolean checkLayoutParams (ViewGroup.LayoutParams p)

Parameters
p	ViewGroup.LayoutParams

Returns
boolean

generateDefaultLayoutParams

protected ConstraintLayout.LayoutParams generateDefaultLayoutParams ()

Returns
ConstraintLayout.LayoutParams

generateLayoutParams

protected ViewGroup.LayoutParams generateLayoutParams (ViewGroup.LayoutParams p)

Parameters
p	ViewGroup.LayoutParams

Returns
ViewGroup.LayoutParams

onLayout

protected void onLayout (boolean changed, int left, int top, int right, int bottom)

Parameters
changed	boolean
left	int
top	int
right	int
bottom	int

onMeasure

protected void onMeasure (int widthMeasureSpec, int heightMeasureSpec)

Parameters
widthMeasureSpec	int
heightMeasureSpec	int