Android 如何繪製檢視畫面

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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;
      }
   }
}