android 4.2的新特性layoutRtl，让布局自动从右往左显示

android4.2有一个新特性 layoutRtl，当然是对于开发者而言的，主要是方便开发者去支持阿拉伯语/波斯语等阅读习惯是从右往左的。

可以在manifest的application标签添加：android:supportsRtl 取值：true/false

这样就可以打开layoutRtl这个功能。如果当前系统语言是阿拉伯语/波斯语，打开了这个功能的应用的布局就会自动变成从右往左的，当然前提是布局没有写死控件间的位置。

layoutRtl这个功能主要是在ViewGroup的子类ViewRootImpl在控制，具体怎么实现，还是需要自己去看看代码，

如果布局变成了从右往左的话，焦点的移动也会有一些变化。EditText有一个标签是用来让输入法提供类似“next”的按钮的：imeoptions=“actionNext”。你按“next”的时候会发现，本来应该往右/下移动的光标，变成往左/上移动。

View焦点（focus）的控制方向是由FocuseFinder类进行控制的。你按下“next”的事件会由TextView的onEditorAction()进行处理。

public void onEditorAction(int actionCode) {          final Editor.InputContentType ict = mEditor == null ? null : mEditor.mInputContentType;          if (ict != null) {              if (ict.onEditorActionListener != null) {                  if (ict.onEditorActionListener.onEditorAction(this,                          actionCode, null)) {                      return;                  }              }                // This is the handling for some default action.              // Note that for backwards compatibility we don't do this              // default handling if explicit ime options have not been given,              // instead turning this into the normal enter key codes that an              // app may be expecting.              if (actionCode == EditorInfo.IME_ACTION_NEXT) {                  View v = focusSearch(FOCUS_FORWARD);                  if (v != null) {                      if (!v.requestFocus(FOCUS_FORWARD)) {                          throw new IllegalStateException("focus search returned a view " +                                  "that wasn't able to take focus!");                      }                  }                  return;                } else if (actionCode == EditorInfo.IME_ACTION_PREVIOUS) {                  View v = focusSearch(FOCUS_BACKWARD);                  if (v != null) {                      if (!v.requestFocus(FOCUS_BACKWARD)) {                          throw new IllegalStateException("focus search returned a view " +                                  "that wasn't able to take focus!");                      }                  }                  return;                } else if (actionCode == EditorInfo.IME_ACTION_DONE) {                  InputMethodManager imm = InputMethodManager.peekInstance();                  if (imm != null && imm.isActive(this)) {                      imm.hideSoftInputFromWindow(getWindowToken(), 0);                  }                  return;              }          }            ViewRootImpl viewRootImpl = getViewRootImpl();          if (viewRootImpl != null) {              long eventTime = SystemClock.uptimeMillis();              viewRootImpl.dispatchKeyFromIme(                      new KeyEvent(eventTime, eventTime,                      KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_ENTER, 0, 0,                      KeyCharacterMap.VIRTUAL_KEYBOARD, 0,                      KeyEvent.FLAG_SOFT_KEYBOARD | KeyEvent.FLAG_KEEP_TOUCH_MODE                      | KeyEvent.FLAG_EDITOR_ACTION));              viewRootImpl.dispatchKeyFromIme(                      new KeyEvent(SystemClock.uptimeMillis(), eventTime,                      KeyEvent.ACTION_UP, KeyEvent.KEYCODE_ENTER, 0, 0,                      KeyCharacterMap.VIRTUAL_KEYBOARD, 0,                      KeyEvent.FLAG_SOFT_KEYBOARD | KeyEvent.FLAG_KEEP_TOUCH_MODE                      | KeyEvent.FLAG_EDITOR_ACTION));          }      }

最终会调用FocuseFinder类的findNextFocus()方法来查找下一个可以获得focus的view。

private View findNextFocus(ViewGroup root, View focused, Rect focusedRect,              int direction, ArrayList<View> focusables) {          if (focused != null) {              if (focusedRect == null) {                  focusedRect = mFocusedRect;              }              // fill in interesting rect from focused              focused.getFocusedRect(focusedRect);              root.offsetDescendantRectToMyCoords(focused, focusedRect);          } else {              if (focusedRect == null) {                  focusedRect = mFocusedRect;                  // make up a rect at top left or bottom right of root                  switch (direction) {                      case View.FOCUS_RIGHT:                      case View.FOCUS_DOWN:                          setFocusTopLeft(root, focusedRect);                          break;                      case View.FOCUS_FORWARD:  //这里root是ViewRootImpl的实例                          if (root.isLayoutRtl()) {                              setFocusBottomRight(root, focusedRect);                          } else {                              setFocusTopLeft(root, focusedRect);                          }                          break;                        case View.FOCUS_LEFT:                      case View.FOCUS_UP:                          setFocusBottomRight(root, focusedRect);                          break;                      case View.FOCUS_BACKWARD:                          if (root.isLayoutRtl()) {                              setFocusTopLeft(root, focusedRect);                          } else {                              setFocusBottomRight(root, focusedRect);                          break;                      }                  }              }          }            switch (direction) {              case View.FOCUS_FORWARD:              case View.FOCUS_BACKWARD:                  return findNextFocusInRelativeDirection(focusables, root, focused, focusedRect,                          direction);              case View.FOCUS_UP:              case View.FOCUS_DOWN:              case View.FOCUS_LEFT:              case View.FOCUS_RIGHT:                  return findNextFocusInAbsoluteDirection(focusables, root, focused,                          focusedRect, direction);              default:                  throw new IllegalArgumentException("Unknown direction: " + direction);          }      }

private void setFocusBottomRight(ViewGroup root, Rect focusedRect) {          final int rootBottom = root.getScrollY() + root.getHeight();          final int rootRight = root.getScrollX() + root.getWidth();          focusedRect.set(rootRight, rootBottom, rootRight, rootBottom);      }        private void setFocusTopLeft(ViewGroup root, Rect focusedRect) {          final int rootTop = root.getScrollY();          final int rootLeft = root.getScrollX();          focusedRect.set(rootLeft, rootTop, rootLeft, rootTop);      }