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Android 如何绘制视图

Android 框架会在 Activity 获得焦点时请求 Activity 绘制其布局。Android 框架会处理绘制流程，但该 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。它会通过遍历布局树并渲染与无效区域相交的每个 View 进行绘制。每个 ViewGroup 负责请求绘制其每个子级（使用 draw() 方法），而每个 View 负责绘制其本身。由于布局树已经过系统预先遍历，因此该框架会在子级之前（即后方）绘制父级，而其同级会按照它们在布局树中出现的顺序进行绘制。

注意：该框架不会绘制有效区域之外的 View 对象，也不会负责为您绘制 View 背景。您可以通过调用 invalidate() 来强制绘制 View。

Android 框架绘制布局包含两个遍历流程：一个测量遍历和一个布局遍历。该框架会在 measure(int, int) 中执行测量遍历，并执行 View 树的自上而下遍历。在递归过程中，每个 View 都会将维度规范下推到布局树。测量遍历结束时，每个 View 都会存储其测量值。该框架会在 layout(int, int, int, int) 中执行第二次遍历，也是自上而下遍历。在此次遍历中，每个父级负责使用测量遍历中计算的尺寸来定位其所有的子级。

以下各部分更详细地介绍了布局流程的两个遍历。

启动测量遍历

当返回 View 对象的 measure() 方法时，请设置其 getMeasuredWidth() 和 getMeasuredHeight() 值，以及 View 对象的所有后代的值。View 对象的测量宽度值和测量高度值必须遵守 View 对象的父级所施加的限制。这有助于保证在测量遍历结束时，所有父级都会接受其子级的所有测量值。

父级 View 可以对其子级多次调用 measure()。例如，父级可以使用未指定的维度测量子级一次，以确定它们的首选尺寸。如果子级的不受限尺寸的总和过大或过小，则父级可以使用限制子级尺寸的值再次调用 measure()。

测量遍历使用两个类来传达维度。View 对象使用 ViewGroup.LayoutParams 来传达其首选尺寸和位置。基本 ViewGroup.LayoutParams 类描述了 View 的首选宽度和高度。针对每个维度，它可以指定以下某一项：

一个确切的尺寸。
MATCH_PARENT，此参数意味着 View 的首选尺寸是其父级的尺寸（负填充）。
WRAP_CONTENT，此参数意味着 View 的首选尺寸恰好足以容纳其内容（正填充）。

有适用于 ViewGroup 的不同子类的 ViewGroup.LayoutParams 子类。例如，RelativeLayout 有自己的 ViewGroup.LayoutParams 子类，其中包括可使子级 View 对象水平居中和垂直居中的功能。

MeasureSpec 对象用于在树中将要求从父级下推到子级。MeasureSpec 可以是下述三种模式之一：

UNSPECIFIED：父级使用此模式来确定子级 View 的目标维度。例如，LinearLayout 可对高度设置为 UNSPECIFIED 且宽度设置为 EXACTLY 240 的子级调用 measure()，以确定给定宽度为 240 像素的子级 View 所需的高度。
EXACTLY：父级使用此模式来强制子级使用某个确切尺寸。子级必须使用此尺寸，并保证其所有的后代都能放入此尺寸。
AT MOST：父级使用此模式来强制规定子级的最大尺寸。子级必须保证它及其所有的后代都能放入此尺寸。

启动布局遍历

如需启动布局，请调用 requestLayout()。当 View 认为自己无法再放入当前范围时，通常会调用此方法。

实现自定义测量和布局逻辑

如果您要实现自定义测量或布局逻辑，请替换实现该逻辑的方法：onMeasure(int, int) 和 onLayout(boolean, int, int, int, int)。这些方法分别由 measure(int, int) 和 layout(int, int, int, int) 调用。请勿尝试替换 measure(int, int) 或 layout(int, int) 方法 - 这两种方法都是 final，因此无法被替换。

以下示例展示了如何在 WindowManager 示例应用的 `SplitLayout` 类中执行此操作。如果 SplitLayout 有两个或更多子视图，并且显示屏有折叠边，那么它会将两个子视图放置在折叠边的任一侧。以下示例展示了用于替换测量和布局的用例，但对于生产环境，请使用 SlidingPaneLayout（如果您希望出现此行为）。

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;
      }
   }
}