android 焦點獲取流程（轉）

結論：在ViewGroup中我們可以重新實現addFocusables，已統一處理判斷View是否可以獲取到焦點.

android一般都是手機或者平板，一般都是點擊的時候獲取焦點，當我們添加遙控或手柄支持焦點移動時，這個時候焦點的查找就比較明顯了，那么Android的焦點是怎么查找的呢。

我們從handleImeFinishedEvent（ViewRootImpl.java）開始了解焦點的查找流程，handleImeFinishedEven是由dispatchImeFinishedEvent觸發，dispatchImeFinishedEvent又是由InputMethodManager觸發來的,

handleImeFinishedEvent中跟焦點相關的代碼：

 

if (direction != 0) {
                View focused = mView.findFocus();//當前擁有焦點的控件
                if (focused != null) {
                    View v = focused.focusSearch(direction);//根據direction查找下一個應該獲取焦點的控件
                    if (v != null && v != focused) {
                        // do the math the get the interesting rect
                        // of previous focused into the coord system of
                        // newly focused view
                        focused.getFocusedRect(mTempRect);
                        if (mView instanceof ViewGroup) {
                            ((ViewGroup) mView).offsetDescendantRectToMyCoords(
                                    focused, mTempRect);
                            ((ViewGroup) mView).offsetRectIntoDescendantCoords(
                                    v, mTempRect);
                        }
                        if (v.requestFocus(direction, mTempRect)) {//請求焦點網
                            playSoundEffect(SoundEffectConstants
                                    .getContantForFocusDirection(direction));
                            finishInputEvent(q, true);
                            return;
                        }
                    }

                    // Give the focused view a last chance to handle the dpad key.
                    if (mView.dispatchUnhandledMove(focused, direction)) {//以前的控件 焦點改變事件
                        finishInputEvent(q, true);
                        return;
                    }
                }

這里先獲取當前焦點控件，然后根據direction獲取下一個最佳的控件，獲取控件后調用他的requestFocus,並給前面的焦點控件一個機會處理失去焦點事件，看一下focusSearch

 

public View focusSearch(int direction) {
    if (mParent != null) {//父控件不為空,調用它的focusSearch
        return mParent.focusSearch(this, direction);
    } else {
        return null;
    }
}

一直調用parent的focusSearch，最終到

 

public View focusSearch(View focused, int direction) {
    if (isRootNamespace()) {//已經是Root層 (installDecor  mDecor.setIsRootNamespace(true);)
        // root namespace means we should consider ourselves the top of the
        // tree for focus searching; otherwise we could be focus searching
        // into other tabs.  see LocalActivityManager and TabHost for more info
        return FocusFinder.getInstance().findNextFocus(this, focused, direction);//查找下一個可獲得焦點的控件
    } else if (mParent != null) {//繼續調用父控件的focusSearch
        return mParent.focusSearch(focused, direction);
    }
    return null;
}

如果已經是根控件，調用FocusFinder的findNextFocus，最終調用它的findNextFocus

 

private View findNextFocus(ViewGroup root, View focused, Rect focusedRect, int direction) {
    View next = null;
    if (focused != null) {
        next = findNextUserSpecifiedFocus(root, focused, direction);//是xml里通過android:nextFocusUp="..."等或者代碼特別指定的焦點順序
    }
    if (next != null) {//已經找到
        return next;
    }
    ArrayList<View> focusables = mTempList;//mTempList
    try {
        focusables.clear();
        root.addFocusables(focusables, direction);//獲取所有可以獲取焦點的控件
        if (!focusables.isEmpty()) {
            next = findNextFocus(root, focused, focusedRect, direction, focusables);//查找下一個焦點控件
        }
    } finally {
        focusables.clear();
    }
    return next;
}

先看一下該控件是否已經設置過它的焦點移動事件，indNextUserSpecifiedFocus就是干這個事的，此方法先去判斷特定Id值是否存在,若存在則查詢出Id對應的view.其實這些Id就是xml里通過android:nextFocusUp="..."等或者代碼特別指定的焦點順序.所以在此過程先判斷，若存在,說明下個焦點已經找到,直接返回.，未找到，則調用findNextFocus繼續查找

 

private View findNextFocus(ViewGroup root, View focused, Rect focusedRect,
        int direction, ArrayList<View> focusables) {
    if (focused != null) {
        if (focusedRect == null) {
            focusedRect = mFocusedRect;//焦點控件大小
        }
        // 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:
                    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,//根據方向查找
                    focusedRect, direction);
        default:
            throw new IllegalArgumentException("Unknown direction: " + direction);
    }
}

調用findNextFocusInAbsoluteDirection查找下一個焦點控件

 

View findNextFocusInAbsoluteDirection(ArrayList<View> focusables, ViewGroup root, View focused,
        Rect focusedRect, int direction) {//獲得焦點控件的位置矩陣.然后通過比較得到下一個焦點的控件
    // initialize the best candidate to something impossible
    // (so the first plausible view will become the best choice)
    mBestCandidateRect.set(focusedRect);//設置mBestCandidateRect
    switch(direction) {
        case View.FOCUS_LEFT:
            mBestCandidateRect.offset(focusedRect.width() + 1, 0);
            break;
        case View.FOCUS_RIGHT:
            mBestCandidateRect.offset(-(focusedRect.width() + 1), 0);
            break;
        case View.FOCUS_UP:
            mBestCandidateRect.offset(0, focusedRect.height() + 1);
            break;
        case View.FOCUS_DOWN:
            mBestCandidateRect.offset(0, -(focusedRect.height() + 1));
    }

    View closest = null;

    int numFocusables = focusables.size();
    for (int i = 0; i < numFocusables; i++) {//查找最佳的焦點控件
        View focusable = focusables.get(i);

        // only interested in other non-root views
        if (focusable == focused || focusable == root) continue;

        // get focus bounds of other view in same coordinate system
        focusable.getFocusedRect(mOtherRect);//獲取當其擁有焦點時的區域大小
        root.offsetDescendantRectToMyCoords(focusable, mOtherRect);

        if (isBetterCandidate(direction, focusedRect, mOtherRect, mBestCandidateRect)) {//比較和Best哪個更好
            mBestCandidateRect.set(mOtherRect);
            closest = focusable;//更合適
        }
    }
    return closest;//返回
}

根據焦點控件的區域去查找一個合適的，具體查找，比較那個合適比較復雜，暫時還沒看懂。

關於移動的時候有時候沒有焦點，我們可以查看在findNextFocus(ViewGroup root, View focused, Rect focusedRect, int direction)這個函數中，root.addFocusables(focusables, direction);