當 Activity 收到焦點時，Android 架構會要求 Activity 繪製自身的版面配置。這個架構會處理繪圖程序，但 Activity 必須提供其版面配置階層的根節點。
Android 架構繪製版面配置的根節點後，會測量及繪製版面配置樹狀結構。繪製過程中，系統會追蹤樹狀結構並算繪每個與無效區域相交的 View。每個 ViewGroup 會負責要求繪製其各個子項 (使用 draw() 方法)。每個 View 會負責繪圖本身。由於樹狀結構預先進行周遊，因此架構會先繪製父項，再複製其子項 (亦即父項在子項「後方」繪製)，同層級則按照其在樹狀結構中出現的順序繪製。
Android 架構會以兩道程序繪製版面配置：測量過程和版面配置過程。該架構會在 measure(int, int) 中執行測量程序，並對 View 樹狀結構由上而下執行周遊。各個 View 會在遞迴期間將維度規格推送到樹狀結構中。測量過程結束時，每個 View 都會儲存自身的測量結果。架構會在 layout(int, int, int, int) 中執行第二個程序，同樣是由上而下。在這個過程中，每個父項都會負責使用測量過程中計算的大小來指定所有子項。
我們會在以下各節中詳細說明版面配置程序的這兩個過程。
啟動測量過程
當 View 物件的 measure() 方法回傳時，請設定其 getMeasuredWidth() 和 getMeasuredHeight() 值，以及 View 物件所有子系的值。View 物件的測量寬度和高度值必須符合 View 物件父項所設的限制。此做法可確保在測量過程結束時，所有父項都能接受子項全部的測量結果。
父項 View 可能會對其子項多次呼叫 measure()。舉例來說，父項可以用未指定的維度測量每個子項一次，藉此判斷所需的大小。如果子項的未限制大小總和過大或過小，父項可能會透過限制子項大小所用的值，再次呼叫 measure()。
測量過程會透過兩個類別來傳送維度。View 物件會透過 ViewGroup.LayoutParams 類別傳送自身偏好的大小和位置。基本 ViewGroup.LayoutParams 類別則說明 View 偏好的寬度和高度。對於每個維度，此過程可以指定下列任一項目：
- 確切維度。
MATCH_PARENT，表示View的偏好大小是其父項大小減去邊框間距。WRAP_CONTENT，表示View的偏好大小恰巧可容納其內容以及邊框間距。
ViewGroup 的不同子類別也有適用的 ViewGroup.LayoutParams 子類別。舉例來說，RelativeLayout 有專屬的 ViewGroup.LayoutParams 子類別，能夠水平和垂直將子項 View 物件置中。
MeasureSpec 物件的用途是在樹狀結構中將要求從父項向下推到子項。MeasureSpec 可為以下三種模式之一：
UNSPECIFIED：父項會使用這個模式來決定子項View的目標維度。舉例來說，LinearLayout可能會在其子項 (高度和寬度分別設為UNSPECIFIED和EXACTLY) 上呼叫measure()，藉此確定寬度 240 像素的子項View所需的高度。EXACTLY：父項會使用此模式強制規定子項的確切大小。子項必須使用這個大小，並保證其下所有子系都符合該大小規定。AT MOST：父項會用這個模式強制規定子項的大小上限。子項必須保證自身及其下所有子系都符合該大小規定。
啟動版面配置過程
如要啟動版面配置，請呼叫 requestLayout()。當 View 認為自身不再超出邊界時，就會自行呼叫這個方法。
導入自訂的測量和版面配置邏輯
如要導入自訂的測量或版面配置邏輯，請覆寫實作邏輯的方法：onMeasure(int, int) 和 onLayout(boolean, int, int, int, int)。這些方法分別是由 measure(int, int) 和 layout(int, int, int, int) 呼叫。請勿嘗試覆寫 measure(int, int) 或 layout(int, int) 方法，這兩種方法都是 final，因此不能覆寫。
以下範例說明如何在 WindowManager 範例應用程式的 `SplitLayout` 類別中執行這項操作。如果 SplitLayout 有兩個以上的子檢視畫面，且螢幕已摺疊，那麼兩個檢視畫面將位於摺疊處兩側。以下範例顯示覆寫測量結果和版面配置的用途，但如果您希望正式版執行這個操作，請使用 SlidingPaneLayout。
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
List splitPositions = 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;
}
}
}