Reduce the number of measure calls for children of Flexbox (#514)

This PR reduces the number of measure calls for the children of the
Flexbox.
If any items in a FlexLine don't have the flexGrow or flexShrink
attributes set, the children in the FlexLine don't have to be measured
in the FlexboxHelper#determineMainSize method.

Similarly, if the AlignItems attribute for the Flexbox is not equal to
AlignItems.STRETCH nor any items in a FlexLine don't have the
AlignSelf.STRETCH attributes, the items in the FlexLine don't have to be
measured in the FlexboxHelpers#stretchViews method.

Also this PR changes the default values of FlexboxLayout as follows:
- mAlignItems: STRETCH -> FLEX_START
- mAlignContent: STRETCH -> FLEX_START

flexbox/src/androidTest/java/com/google/android/flexbox/FlexboxHelperTest.kt

@@ -427,4 +427,43 @@ class FlexboxHelperTest {
         assertThat(flexboxHelper.extractHigherInt(combined), `is`(higher))
         assertThat(flexboxHelper.extractLowerInt(combined), `is`(lower))
     }
+
+    @Test
+    fun testFlexLine_anyItemsHaveFlexGrow() {
+        val activity = activityRule.activity
+        val lp1 = FlexboxLayout.LayoutParams(100, 100).apply {
+            flexGrow = 1.0f
+        }
+        val view1 = View(activity)
+        view1.layoutParams = lp1
+        val lp2 = FlexboxLayout.LayoutParams(100, 200)
+        val view2 = View(activity)
+        view2.layoutParams = lp2
+        val lp3 = FlexboxLayout.LayoutParams(100, 300)
+        val view3 = View(activity)
+        view3.layoutParams = lp3
+        val lp4 = FlexboxLayout.LayoutParams(100, 400).apply {
+            flexGrow = 2.0f
+        }
+        val view4 = View(activity)
+        view4.layoutParams = lp4
+        flexContainer.apply {
+            addView(view1)
+            addView(view2)
+            addView(view3)
+            addView(view4)
+            flexDirection = FlexDirection.COLUMN
+            flexWrap = FlexWrap.WRAP
+            alignContent = AlignContent.STRETCH
+        }
+        val widthMeasureSpec = View.MeasureSpec.makeMeasureSpec(1000, View.MeasureSpec.EXACTLY)
+        val heightMeasureSpec = View.MeasureSpec.makeMeasureSpec(500, View.MeasureSpec.EXACTLY)
+        val result = FlexboxHelper.FlexLinesResult()
+        flexboxHelper.calculateVerticalFlexLines(result, widthMeasureSpec, heightMeasureSpec)
+        flexContainer.flexLines = result.mFlexLines
+        assertThat(flexContainer.flexLines.size, `is`(3))
+        assertThat(flexContainer.flexLines[0].mAnyItemsHaveFlexGrow, `is`(true))
+        assertThat(flexContainer.flexLines[1].mAnyItemsHaveFlexGrow, `is`(false))
+        assertThat(flexContainer.flexLines[2].mAnyItemsHaveFlexGrow, `is`(true))
+    }
 }

flexbox/src/androidTest/java/com/google/android/flexbox/test/FlexboxAndroidTest.kt

@@ -833,6 +833,7 @@ class FlexboxAndroidTest {
                     override fun apply(flexboxLayout: FlexboxLayout) {
                         flexboxLayout.justifyContent = JustifyContent.FLEX_END
                         flexboxLayout.flexDirection = FlexDirection.COLUMN
+                        flexboxLayout.alignItems = AlignItems.STRETCH
                     }
                 })
 

flexbox/src/androidTest/res/layout/activity_stretch_test.xml

@@ -20,7 +20,8 @@
     android:layout_height="360dp"
     app:flexDirection="row"
     app:flexWrap="wrap"
-    app:alignItems="stretch">
+    app:alignItems="stretch"
+    app:alignContent="stretch" >
 
     <TextView
         android:id="@+id/text1"

flexbox/src/main/java/com/google/android/flexbox/FlexItem.java

@@ -35,6 +35,9 @@ interface FlexItem extends Parcelable {
     /** The default value for the flex shrink attribute */
     float FLEX_SHRINK_DEFAULT = 1f;
 
+    /** The value representing the flex shrink attribute is not set  */
+    float FLEX_SHRINK_NOT_SET = 0f;
+
     /** The default value for the flex basis percent attribute */
     float FLEX_BASIS_PERCENT_DEFAULT = -1f;
 

flexbox/src/main/java/com/google/android/flexbox/FlexLine.java

@@ -65,7 +65,7 @@ public class FlexLine {
 
     /**
      * The largest value of the individual child's baseline (obtained by View#getBaseline()
-     * if the {@link FlexboxLayout#mAlignItems} value is not {@link AlignItems#BASELINE}
+     * if the {@link FlexContainer#getAlignItems()} value is not {@link AlignItems#BASELINE}
      * or the flex direction is vertical, this value is not used.
      * If the alignment direction is from the bottom to top,
      * (e.g. flexWrap == WRAP_REVERSE and flexDirection == ROW)
@@ -90,6 +90,18 @@ public class FlexLine {
 
     int mLastIndex;
 
+    /**
+     * Set to true if any {@link FlexItem}s in this line have {@link FlexItem#getFlexGrow()}
+     * attributes set (have the value other than {@link FlexItem#FLEX_GROW_DEFAULT})
+     */
+    boolean mAnyItemsHaveFlexGrow;
+
+    /**
+     * Set to true if any {@link FlexItem}s in this line have {@link FlexItem#getFlexShrink()}
+     * attributes set (have the value other than {@link FlexItem#FLEX_SHRINK_NOT_SET})
+     */
+    boolean mAnyItemsHaveFlexShrink;
+
     /**
      * @return the size of the flex line in pixels along the main axis of the flex container.
      */

flexbox/src/main/java/com/google/android/flexbox/FlexboxHelper.java

@@ -18,6 +18,9 @@ package com.google.android.flexbox;
 
 import static com.google.android.flexbox.FlexContainer.NOT_SET;
 import static com.google.android.flexbox.FlexItem.FLEX_BASIS_PERCENT_DEFAULT;
+import static com.google.android.flexbox.FlexItem.FLEX_GROW_DEFAULT;
+import static com.google.android.flexbox.FlexItem.FLEX_SHRINK_DEFAULT;
+import static com.google.android.flexbox.FlexItem.FLEX_SHRINK_NOT_SET;
 
 import static androidx.recyclerview.widget.RecyclerView.NO_POSITION;
 
@@ -554,6 +557,9 @@ class FlexboxHelper {
                 flexLine.mItemCount++;
                 indexInFlexLine++;
             }
+            flexLine.mAnyItemsHaveFlexGrow |= flexItem.getFlexGrow() != FLEX_GROW_DEFAULT;
+            flexLine.mAnyItemsHaveFlexShrink |= flexItem.getFlexShrink() != FLEX_SHRINK_NOT_SET;
+
             if (mIndexToFlexLine != null) {
                 mIndexToFlexLine[i] = flexLines.size();
             }
@@ -971,10 +977,10 @@ class FlexboxHelper {
         List<FlexLine> flexLines = mFlexContainer.getFlexLinesInternal();
         for (int i = flexLineIndex, size = flexLines.size(); i < size; i++) {
             FlexLine flexLine = flexLines.get(i);
-            if (flexLine.mMainSize < mainSize) {
+            if (flexLine.mMainSize < mainSize && flexLine.mAnyItemsHaveFlexGrow) {
                 expandFlexItems(widthMeasureSpec, heightMeasureSpec, flexLine,
                         mainSize, paddingAlongMainAxis, false);
-            } else {
+            } else if (flexLine.mMainSize > mainSize && flexLine.mAnyItemsHaveFlexShrink) {
                 shrinkFlexItems(widthMeasureSpec, heightMeasureSpec, flexLine,
                         mainSize, paddingAlongMainAxis, false);
             }
@@ -1571,7 +1577,7 @@ class FlexboxHelper {
 
     /**
      * Expand the view if the {@link FlexContainer#getAlignItems()} attribute is set to {@link
-     * AlignItems#STRETCH} or {@link FlexboxLayout.LayoutParams#mAlignSelf} is set as
+     * AlignItems#STRETCH} or {@link FlexItem#getAlignSelf()} is set as
      * {@link AlignItems#STRETCH}.
      *
      * @param fromIndex the index from which value, stretch is calculated
@@ -1579,7 +1585,7 @@ class FlexboxHelper {
      * @see FlexContainer#setFlexDirection(int)
      * @see FlexContainer#getAlignItems()
      * @see FlexContainer#setAlignItems(int)
-     * @see FlexboxLayout.LayoutParams#mAlignSelf
+     * @see FlexItem#getAlignSelf()
      */
     void stretchViews(int fromIndex) {
         if (fromIndex >= mFlexContainer.getFlexItemCount()) {
@@ -2008,6 +2014,7 @@ class FlexboxHelper {
             return index - another.index;
         }
 
+        @NonNull
         @Override
         public String toString() {
             return "Order{" +

flexbox/src/main/java/com/google/android/flexbox/FlexboxLayout.java

@@ -102,7 +102,7 @@ public class FlexboxLayout extends ViewGroup implements FlexContainer {
 
     /**
      * The current value of the {@link AlignItems}, the default value is
-     * {@link AlignItems#STRETCH}.
+     * {@link AlignItems#FLEX_START}.
      *
      * @see AlignItems
      */
@@ -110,7 +110,7 @@ public class FlexboxLayout extends ViewGroup implements FlexContainer {
 
     /**
      * The current value of the {@link AlignContent}, the default value is
-     * {@link AlignContent#STRETCH}.
+     * {@link AlignContent#FLEX_START}.
      *
      * @see AlignContent
      */
@@ -223,8 +223,8 @@ public class FlexboxLayout extends ViewGroup implements FlexContainer {
         mFlexWrap = a.getInt(R.styleable.FlexboxLayout_flexWrap, FlexWrap.NOWRAP);
         mJustifyContent = a
                 .getInt(R.styleable.FlexboxLayout_justifyContent, JustifyContent.FLEX_START);
-        mAlignItems = a.getInt(R.styleable.FlexboxLayout_alignItems, AlignItems.STRETCH);
-        mAlignContent = a.getInt(R.styleable.FlexboxLayout_alignContent, AlignContent.STRETCH);
+        mAlignItems = a.getInt(R.styleable.FlexboxLayout_alignItems, AlignItems.FLEX_START);
+        mAlignContent = a.getInt(R.styleable.FlexboxLayout_alignContent, AlignContent.FLEX_START);
         mMaxLine = a.getInt(R.styleable.FlexboxLayout_maxLine, NOT_SET);
         Drawable drawable = a.getDrawable(R.styleable.FlexboxLayout_dividerDrawable);
         if (drawable != null) {