android开发必遇问题，最有可能忘记两者之间的区别的问题之一

如下是google搜索出来的结果

记忆方法

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关键字 should position ， 主动

android:layout_gravity

关键字 should be placed， 被动

源码左证

如下是linearlayout的onlayout函数中可以看到两个参数的使用情况

void layoutvertical(int left, int top, int right, int bottom) {

final int paddingleft = mpaddingleft;

int childtop;

int childleft;

// where right end of child should go

final int width = right - left;

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.relative_horizontal_gravity_mask;

// 使用mgravity来计算第一个子view的top

switch (majorgravity) {

case gravity.bottom:

// mtotallength contains the padding already

childtop = mpaddingtop + bottom - top - mtotallength;

break;

// mtotallength contains the padding already

case gravity.center_vertical:

childtop = mpaddingtop + (bottom - top - mtotallength) / 2;

break;

case gravity.top:

default:

childtop = mpaddingtop;

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();

//在排子view时才使用到子view的layoutparams中的gravity

final linearlayout.layoutparams lp =

(linearlayout.layoutparams) child.getlayoutparams();

int gravity = lp.gravity;

if (gravity < 0) {

gravity = minorgravity;

}

相关补充

viewgroup是个抽象类，子类继承它时需要override onlayout方法 linearlayout、relativelayout等viewgroup子类就分别实现了自己的排版算法(override onlayout方法) viewgroup子类们在排版过程中使用到gravity与layout_gravity等参数来排版子view(内容)，值得注意的是mgravity不是viewgroup的成员变量，另外各子类的排版策略是不一样的，所以gravity不是必须存在的与使用的，如framelayout

如下是framelayout的onlayout方法的代码

protected void onlayout(boolean changed, int left, int top, int right, int bottom) {

layoutchildren(left, top, right, bottom, false /* no force left gravity */);

}

void layoutchildren(int left, int top, int right, int bottom,

boolean forceleftgravity) {

final int count = getchildcount();

final int parentleft = getpaddingleftwithforeground();

final int parentright = right - left - getpaddingrightwithforeground();

final int parenttop = getpaddingtopwithforeground();

final int parentbottom = bottom - top - getpaddingbottomwithforeground();

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

final view child = getchildat(i);

if (child.getvisibility() != gone) {

final layoutparams lp = (layoutparams) child.getlayoutparams();

final int width = child.getmeasuredwidth();

final int height = child.getmeasuredheight();

int childleft;

int childtop;

int gravity = lp.gravity;

if (gravity == -1) {

gravity = default_child_gravity;

}

final int layoutdirection = getlayoutdirection();

final int absolutegravity = gravity.getabsolutegravity(gravity, layoutdirection);

final int verticalgravity = gravity & gravity.vertical_gravity_mask;

switch (absolutegravity & gravity.horizontal_gravity_mask) {

case gravity.center_horizontal:

childleft = parentleft + (parentright - parentleft - width) / 2 +

lp.leftmargin - lp.rightmargin;

break;

case gravity.right:

if (!forceleftgravity) {

childleft = parentright - width - lp.rightmargin;

break;

}

case gravity.left:

default:

childleft = parentleft + lp.leftmargin;

}

switch (verticalgravity) {

case gravity.top:

childtop = parenttop + lp.topmargin;

break;

case gravity.center_vertical:

childtop = parenttop + (parentbottom - parenttop - height) / 2 +

lp.topmargin - lp.bottommargin;

break;

case gravity.bottom:

childtop = parentbottom - height - lp.bottommargin;

break;

default:

childtop = parenttop + lp.topmargin;

}

child.layout(childleft, childtop, childleft + width, childtop + height);

}

}

}

喎?>