Android 如何绘制视图

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当 Activity 获取焦点时，Android 框架会请求 Activity 绘制其布局。Android 框架负责处理绘制过程，但 Activity 必须提供其布局层次结构的根节点。

Android 框架会绘制布局的根节点，并测量和绘制布局树。它通过遍历树来绘制，并渲染与无效区域相交的每个 View。每个 ViewGroup 都负责使用 draw() 方法请求绘制其每个子项，而每个 View 则负责绘制自身。由于树是按先序遍历的，因此框架会在其子项之前（换句话说，在其子项“后面”）绘制父项，并按同级在树中出现的顺序绘制它们。

Android 框架通过两阶段过程绘制布局：测量阶段和布局阶段。框架在 measure(int, int) 中执行测量阶段，并对 View 树进行自上而下的遍历。在递归过程中，每个 View 都会将尺寸规范向下传递。在测量阶段结束时，每个 View 都存储其测量值。框架在 layout(int, int, int, int) 中执行第二阶段，也是自上而下的过程。在此阶段，每个父项负责使用测量阶段计算的尺寸来定位其所有子项。

布局过程的这两个阶段在以下部分中详细描述。

启动测量阶段

当 View 对象的 measure() 方法返回时，会设置其 getMeasuredWidth() 和 getMeasuredHeight() 值，以及所有 View 对象的后代的值。View 对象的测量宽度和测量高度值必须遵守其父项 View 施加的约束。这有助于确保在测量阶段结束时，所有父项都接受其所有子项的测量结果。

父项 View 可能会对其子项多次调用 measure()。例如，父项可能会先使用未指定的尺寸测量一次子项，以确定它们的首选尺寸。如果子项不受约束的尺寸之和过大或过小，父项可能会再次调用 measure()，并使用约束子项尺寸的值。

测量阶段使用两个类来传递尺寸。 ViewGroup.LayoutParams 类是 View 对象传达其首选尺寸和位置的方式。基本 ViewGroup.LayoutParams 类描述了 View 的首选宽度和高度。对于每个维度，它可以指定以下之一：

  • 确切的尺寸。
  • MATCH_PARENT，表示 View 的首选尺寸是其父项的尺寸减去内边距。
  • WRAP_CONTENT，表示 View 的首选尺寸刚好足以包含其内容，再加上内边距。

对于不同的 ViewGroup 子类，都有对应的 ViewGroup.LayoutParams 子类。例如，RelativeLayout 有自己的 ViewGroup.LayoutParams 子类，其中包含水平和垂直居中子项 View 对象的功能。

MeasureSpec 对象用于将要求从父项向下传递到子项。一个 MeasureSpec 可以处于以下三种模式之一：

  • UNSPECIFIED：父项使用此模式来确定子项 View 的目标尺寸。例如，一个 LinearLayout 可能会对其子项调用 measure()，其中高度设置为 UNSPECIFIED，宽度设置为 EXACTLY 240，以了解在给定 240 像素宽度的情况下，子项 View 希望有多高。
  • EXACTLY：父项使用此模式来强制指定子项的确切尺寸。子项必须使用此尺寸，并保证其所有后代都适合此尺寸。
  • AT MOST：父项使用此模式来强制指定子项的最大尺寸。子项必须保证其自身及其所有后代都适合此尺寸。

启动布局阶段

要启动布局，请调用 requestLayout()。当 View 认为自己不再适合其边界时，通常会对自己调用此方法。

实现自定义测量和布局逻辑

如果要实现自定义测量或布局逻辑，请重写实现该逻辑的方法：onMeasure(int, int) 和 onLayout(boolean, int, int, int, int)。这些方法分别由 measure(int, int) 和 layout(int, int) 调用。不要尝试重写 measure(int, int) 或 layout(int, int) 方法，因为这两个方法都是 final 的，因此无法重写。

以下示例展示了如何在 `SplitLayout` 类（来自 WindowManager 示例应用）中实现此功能。如果 SplitLayout 有两个或更多子视图，并且显示屏有折叠，则它会将这两个子视图放置在折叠的两侧。以下示例展示了重写测量和布局的用例，但在生产环境中，如果需要此行为，请使用 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;
      }
   }
}