Android:都是Layout的BaselineAligned惹的祸

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此问题来自一个网友的提问http://ask.csdn.net/questions/206909#answer_140060

看下面的布局文件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    tools:context=".MainActivity" >    <TextView        android:layout_width="150dp"        android:layout_height="60dp"        android:background="#8f8f8f"        android:text="第一个" />    <TextView        android:layout_width="100dp"        android:layout_height="60dp"        android:background="#8f00"        android:gravity="center"        android:text="第二个"        android:textSize="20dp" /></LinearLayout>

这个布局非常简单，LinearLayout里面嵌套了两个TextView组件，他们的高度一样，宽度不一样，期望结果是，两个TextView顶端是对齐的，高度一样，大小，文字对齐方式可以任意设置。这个布局实际渲染出来的结构是这样

完全错位了，只要你设置不同的字号，Gravity，都会引起位置错乱，究其原因，就是LinearLayout的BaselineAligned属性惹的祸，看一下LinearLayout的默认属性

很多时候，我们都被那个绿色框框给迷惑了，对于boolean属性，这个状态不一定是false，它只代表与系统设定的默认值一致，那么系统对这个变量是怎样设置的呢，看看LinearLayout源码（位置：\sdks\sources\android-19\android\widget）

    /**     * Whether the children of this layout are baseline aligned.  Only applicable     * if {@link #mOrientation} is horizontal.     */    @ViewDebug.ExportedProperty(category = "layout")    private boolean mBaselineAligned = true;

看到了吧，默认值就是true，而且告诉我们只对水平布局有效。那么这个属性对布局产生了怎样的影响呢，继续看LinearLayout布局时都干了啥：

    @Override    protected void onLayout(boolean changed, int l, int t, int r, int b) {        if (mOrientation == VERTICAL) {            layoutVertical(l, t, r, b);        } else {            layoutHorizontal(l, t, r, b);        }    }

横向布局时，会调用layoutHorizontal，下面摘取一段代码

            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);            }

我们看到在else...if代码段里，出现了baselineAligned的身影：

if (baselineAligned && lp.height != LayoutParams.MATCH_PARENT)

如果是基线对齐，并且布局参数的高度设定的不是match_parent，那么就要获取chile的BaseLine，这里也就是TextView的BaseLine的值。经过一些列的计算，最后通过

                setChildFrame(child, childLeft + getLocationOffset(child), childTop,                        childWidth, childHeight);

对child进行OnLayout布局

    private void setChildFrame(View child, int left, int top, int width, int height) {                child.layout(left, top, left + width, top + height);    }

再来看看TextView的getBaseline是咋样的

    @Override    public int getBaseline() {        if (mLayout == null) {            return super.getBaseline();        }        int voffset = 0;        if ((mGravity & Gravity.VERTICAL_GRAVITY_MASK) != Gravity.TOP) {            voffset = getVerticalOffset(true);        }        if (isLayoutModeOptical(mParent)) {            voffset -= getOpticalInsets().top;        }        return getExtendedPaddingTop() + voffset + mLayout.getLineBaseline(0);    }

如果不是Gravity.TOP，就要通过getVerticalOffset计算voffset值了

    int getVerticalOffset(boolean forceNormal) {        int voffset = 0;        final int gravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;        Layout l = mLayout;        if (!forceNormal && mText.length() == 0 && mHintLayout != null) {            l = mHintLayout;        }        if (gravity != Gravity.TOP) {            int boxht = getBoxHeight(l);            int textht = l.getHeight();            if (textht < boxht) {                if (gravity == Gravity.BOTTOM)                    voffset = boxht - textht;                else // (gravity == Gravity.CENTER_VERTICAL)                    voffset = (boxht - textht) >> 1;            }        }        return voffset;    }

这个方法会计算一个垂直方向的偏移值，就是这个偏移值，直接影响了Layout布局中的childTop值，导致布局混乱，基线对齐，就是多个TextView的文字是保持对齐的，才不管你top和bottom。

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