详解介绍android:layout_gravity 和 android:gravity 之间的区别

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

联想/形像記法喎? f/ware/vc/"="" target="_blank" class="keylink">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"这里写图片描述" src="/uploadfile/collfiles/20180606/2018060608595671.png" title="\" />

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?>