layout_gravity不起作用的原因分析

使用LinearLayout布局，其中的子View设置android:layout_gravity="bottom"属性后不起作用，原因是：

当LinearLayout设置android:orientation="vertical" 时， 只有水平方向的left，right，center_horizontal设置起作用，垂直方向的设置不起作用。

同样的：

当LinearLayout设置android:orientation="horizontal" 时， 只有垂直方向的top，bottom，center_vertical设置才起作用，水平方向的设置不起作用。

从View的绘制过程分析原因：

参考了View工作原理（四）view的layout过程/article/7965398.html

onlayout()方法：

[java] view
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@Override

protected void onLayout(boolean changed, int l, int t, int r, int b) {

if (mOrientation == VERTICAL) {

layoutVertical();

} else {

layoutHorizontal();

}

}

LinearLayout有两种布局方法，垂直的和水平的，下面看垂直的布局源码

layoutVertical()方法源码：

[java] view
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void layoutVertical() {

final int paddingLeft = mPaddingLeft;

int childTop = mPaddingTop;

int childLeft;

// Where right end of child should go

final int width = mRight - mLeft;

int childRight = width - mPaddingRight;

// Space available for child

int childSpace = width - paddingLeft - mPaddingRight;

final int count = getVirtualChildCount();

final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;

final int minorGravity = mGravity & Gravity.HORIZONTAL_GRAVITY_MASK;

if (majorGravity != Gravity.TOP) {

switch (majorGravity) {

case Gravity.BOTTOM:

// mTotalLength contains the padding already, we add the top

// padding to compensate

childTop = mBottom - mTop + mPaddingTop - mTotalLength;

break;

case Gravity.CENTER_VERTICAL:

childTop += ((mBottom - mTop) - mTotalLength) / 2;

break;

}

}

for (int i = 0; i < count; i++) {

final View child = getVirtualChildAt(i);

if (child == null) {

childTop += measureNullChild(i);

} else if (child.getVisibility() != GONE) {

final int childWidth = child.getMeasuredWidth();

final int childHeight = child.getMeasuredHeight();

final LinearLayout.LayoutParams lp =

(LinearLayout.LayoutParams) child.getLayoutParams();

int gravity = lp.gravity;

if (gravity < 0) {

gravity = minorGravity;

}

switch (gravity & Gravity.HORIZONTAL_GRAVITY_MASK) {

case Gravity.LEFT:

childLeft = paddingLeft + lp.leftMargin;

break;

case Gravity.CENTER_HORIZONTAL:

childLeft = paddingLeft + ((childSpace - childWidth) / 2)

+ lp.leftMargin - lp.rightMargin;

break;

case Gravity.RIGHT:

childLeft = childRight - childWidth - lp.rightMargin;

break;

default:

childLeft = paddingLeft;

break;

}

childTop += lp.topMargin;

setChildFrame(child, childLeft, childTop + getLocationOffset(child),

childWidth, childHeight);

childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

i += getChildrenSkipCount(child, i);

}

}

}

for循环是绘制子布局，switch语句是根据gravity的值确定水平方向的起始位置，三种值分别为：LEFT,CENTER_HORIZONTAL和RIGHT，由此可以看出只有水平方向的设置起作用。