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);
    }

具体可以自己看代码，上面的类名都添加了超链接到github