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

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