今天做一个UI突然发现一个奇怪的问题：

第一个textView我确实设置了layout_gravity = bottom，并且其父布局LinearLayout也是horizontal方向。但是1dp的textView并没有居于LinearLayout的底部。

开始分析

一、将TextView先换成View看下效果:

可以看到1dp的view居于LinearLayout的底部了。

二、将LinearLayout换成FrameLayout看下效果：

可以看到1dp的TextView居于LinearLayout的底部了。

三、源码分析

通过前2步的测试，那么可以确定的是，TextView是具有文字内容的控件，我猜测是baseline这个属性影响的。但是textView影响的是LinearLayout，所以我直接去看LinearLayout的源码：

 /**
     * Position the children during a layout pass if the orientation of this
     * LinearLayout is set to {@link #HORIZONTAL}.
     *
     * @see #getOrientation()
     * @see #setOrientation(int)
     * @see #onLayout(boolean, int, int, int, int)
     * @param left
     * @param top
     * @param right
     * @param bottom
     */
    void layoutHorizontal(int left, int top, int right, int bottom) {
        final boolean isLayoutRtl = isLayoutRtl();
        final int paddingTop = mPaddingTop;

        int childTop;
        int childLeft;

        // Where bottom of child should go
        final int height = bottom - top;
        int childBottom = height - mPaddingBottom;

        // Space available for child
        int childSpace = height - paddingTop - mPaddingBottom;

        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;

        final boolean baselineAligned = mBaselineAligned;

        final int[] maxAscent = mMaxAscent;
        final int[] maxDescent = mMaxDescent;

        final int layoutDirection = getLayoutDirection();
        switch (Gravity.getAbsoluteGravity(majorGravity, layoutDirection)) {
            case Gravity.RIGHT:
                // mTotalLength contains the padding already
                childLeft = mPaddingLeft + right - left - mTotalLength;
                break;

            case Gravity.CENTER_HORIZONTAL:
                // mTotalLength contains the padding already
                childLeft = mPaddingLeft + (right - left - mTotalLength) / 2;
                break;

            case Gravity.LEFT:
            default:
                childLeft = mPaddingLeft;
                break;
        }

        int start = 0;
        int dir = 1;
        //In case of RTL, start drawing from the last child.
        if (isLayoutRtl) {
            start = count - 1;
            dir = -1;
        }

        for (int i = 0; i < count; i++) {
            final int childIndex = start + dir * i;
            final View child = getVirtualChildAt(childIndex);
            if (child == null) {
                childLeft += measureNullChild(childIndex);
            } else if (child.getVisibility() != GONE) {
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();
                int childBaseline = -1;

                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();

                if (baselineAligned && lp.height != LayoutParams.MATCH_PARENT) {
                    childBaseline = child.getBaseline();
                }

                int gravity = lp.gravity;
                if (gravity < 0) {
                    gravity = minorGravity;
                }

                switch (gravity & Gravity.VERTICAL_GRAVITY_MASK) {
                    case Gravity.TOP:
                        childTop = paddingTop + lp.topMargin;
                        if (childBaseline != -1) {
                            childTop += maxAscent[INDEX_TOP] - childBaseline;
                        }
                        break;

                    case Gravity.CENTER_VERTICAL:
                        // Removed support for baseline alignment when layout_gravity or
                        // gravity == center_vertical. See bug #1038483.
                        // Keep the code around if we need to re-enable this feature
                        // if (childBaseline != -1) {
                        //     // Align baselines vertically only if the child is smaller than us
                        //     if (childSpace - childHeight > 0) {
                        //         childTop = paddingTop + (childSpace / 2) - childBaseline;
                        //     } else {
                        //         childTop = paddingTop + (childSpace - childHeight) / 2;
                        //     }
                        // } else {
                        childTop = paddingTop + ((childSpace - childHeight) / 2)
                                + lp.topMargin - lp.bottomMargin;
                        break;

                    case Gravity.BOTTOM:
                        childTop = childBottom - childHeight - lp.bottomMargin;
                        if (childBaseline != -1) {
                            int descent = child.getMeasuredHeight() - childBaseline;
                            childTop -= (maxDescent[INDEX_BOTTOM] - descent);
                        }
                        break;
                    default:
                        childTop = paddingTop;
                        break;
                }

                if (hasDividerBeforeChildAt(childIndex)) {
                    childLeft += mDividerWidth;
                }

                childLeft += lp.leftMargin;
                setChildFrame(child, childLeft + getLocationOffset(child), childTop,
                        childWidth, childHeight);
                childLeft += childWidth + lp.rightMargin +
                        getNextLocationOffset(child);

                i += getChildrenSkipCount(child, childIndex);
            }
        }
    }

通过源码可以定位到下面这个case:

 case Gravity.BOTTOM:
        childTop = childBottom - childHeight - lp.bottomMargin;
        if (childBaseline != -1) {
            int descent = child.getMeasuredHeight() - childBaseline;
            childTop -= (maxDescent[INDEX_BOTTOM] - descent);
        }
    break;

确定了确实是baseline影响的。
那么LinearLayout 有2个属性 baselineAligned和baselineAlignedChildIndex
如果LinearLayout的orientation为horizontal的话，其中的文字默认是文字对齐的，baselineAligned = true
baselineAlignedChildIndex是指其中第几个元素需要baseline对其。
这里再举一个列子：

此时baselineAligned默认为true，两个textView的文字是底部对其的。
当我把baselineAligned设置为false时：

此时整个TextView控件就居于底部了。