在最近寫的項目中遇到由focus引起的問題，例如：
在兩個嵌套的RecyclerView中，外層滑動停止后，由于內層的RecyclerView獲取了焦點，導致外層又自動滑動了一些距離；
需要監(jiān)聽EditText的焦點變化來控制界面的顯示于隱藏，在調用EditText的cleanFocus()方法時，回調接口會調用兩次，并且EditText的焦點依然還在。
對遇到的這兩個問題的解決方法都一樣，就是在外層，或根布局中添加focusInTouchModel:true,focus:true兩個屬性，優(yōu)先獲取到焦點就可以了。
布局文件中和focus有關的屬性
//是否可獲取焦點
focusable:true|false
//觸摸模式下是否可獲取焦點
focusableInTouchMode:true|false
descendantFocusability:blocksDescendants|beforeDescendants|afterDescendants
這里解釋一下descendantFocusability的三個屬性值，分別代表的意思：
- blocksDescendants：ViewGroup攔截，不讓子 view獲取焦點。
- beforeDescendants：ViewGroup優(yōu)先嘗試（嘗試的意思是，根據(jù)View或ViewGroup當前狀態(tài)來判斷是否能得到焦點，如是否可見，是否可獲取焦點等等，在View的requestFocus方法的注釋中提到，下同）獲取焦點，若ViewGroup沒拿到焦點，再遍歷子 view（包括所有直接子 view和間接子 view），讓子 view嘗試獲取焦點。
3.afterDescendants：先遍歷子 view，讓子 view嘗試獲取焦點，若所有子 view（包括所有直接子 view和間接子 view）都沒拿到焦點，才讓ViewGroup嘗試獲取焦點。
下面是ViewGroup的與descendantFocusability有關的源碼
@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " ViewGroup.requestFocus direction="
+ direction);
}
//獲取當前viewgroup的descendantFocusability屬性值
int descendantFocusability = getDescendantFocusability();
switch (descendantFocusability) {
case FOCUS_BLOCK_DESCENDANTS:
//直接調用超類也就是View的方法獲取焦點，忽略子 view
return super.requestFocus(direction, previouslyFocusedRect);
case FOCUS_BEFORE_DESCENDANTS: {
//當前ViewGroup先嘗試獲取焦點，返回值表示是否拿到來焦點，若沒有，則讓子 view嘗試獲取
final boolean took = super.requestFocus(direction, previouslyFocusedRect);
return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
}
case FOCUS_AFTER_DESCENDANTS: {
//與上面剛好相反
final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
return took ? took : super.requestFocus(direction, previouslyFocusedRect);
}
default:
throw new IllegalStateException("descendant focusability must be "
+ "one of FOCUS_BEFORE_DESCENDANTS, FOCUS_AFTER_DESCENDANTS, FOCUS_BLOCK_DESCENDANTS "
+ "but is " + descendantFocusability);
}
}
requestFocus() 方法的執(zhí)行流程
上面看的是ViewGroup的requestFocus()方法，而ViewGroup的這個方法只是對ViewGroup和子 view之間獲取焦點的順序做的一個處理，要拿到焦點，最終還是要通過調用View的requestFocus()方法來拿到焦點。所以接下來主要看的是View的方法。
public final boolean requestFocus() {
return requestFocus(View.FOCUS_DOWN);
}
public final boolean requestFocus(int direction) {
return requestFocus(direction, null);
}
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
return requestFocusNoSearch(direction, previouslyFocusedRect);
}
private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) {
// need to be focusable,判斷是否可獲取焦點或是否可見
if ((mViewFlags & FOCUSABLE_MASK) != FOCUSABLE ||
(mViewFlags & VISIBILITY_MASK) != VISIBLE) {
return false;
}
// need to be focusable in touch mode if in touch mode，判斷在觸摸模式下是否可獲取焦點
if (isInTouchMode() &&
(FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
return false;
}
// need to not have any parents blocking us，判斷是否有父view攔截
if (hasAncestorThatBlocksDescendantFocus()) {
return false;
}
handleFocusGainInternal(direction, previouslyFocusedRect);
return true;
}
可以看到，在外部調用requestFocus()這個無參方法后，經(jīng)過層層傳遞后，最終是調用requestFocusNoSearch(int direction, Rect previouslyFocusedRect)方法，這幾個方法的返回值表示是否拿到了焦點，在里面可以看到有3個if判斷語句，這就是我們上面提到的“嘗試”的意思了，即需要根據(jù)當前狀態(tài)判斷，通過判斷，最后直接返回了true，表示拿到了焦點，而在返回之前，還執(zhí)行了一個很重要的方法handleFocusGainInternal(direction, previouslyFocusedRect)，來看看里面都干了些什么：
void handleFocusGainInternal(@FocusRealDirection int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " requestFocus()");
}
// 若當前已拿到焦點，則方法結束，所以一個已獲焦點的view，在執(zhí)行requestFocus()后，它的onFocusChanged方法是不會回調的
if ((mPrivateFlags & PFLAG_FOCUSED) == 0) {
//標記已拿到焦點
mPrivateFlags |= PFLAG_FOCUSED;
//上個拿到焦點的view
View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null;
if (mParent != null) {
//遞歸調用，重置ViewGroup的mFocused的值，重置oldFocus的mPrivateFlags標記位
mParent.requestChildFocus(this, this);
}
if (mAttachInfo != null) {
mAttachInfo.mTreeObserver.dispatchOnGlobalFocusChange(oldFocus, this);
}
onFocusChanged(true, direction, previouslyFocusedRect);
refreshDrawableState();
}
}
ViewGroup的requestChildFocus(this, this)方法
@Override
public void requestChildFocus(View child, View focused) {
if (DBG) {
System.out.println(this + " requestChildFocus()");
}
if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {
return;
}
// Unfocus us, if necessary
super.unFocus(focused);
// We had a previous notion of who had focus. Clear it.
if (mFocused != child) {
if (mFocused != null) {
mFocused.unFocus(focused);
}
//重置mFocused變量值
mFocused = child;
}
if (mParent != null) {
mParent.requestChildFocus(this, focused);
}
}
然后通過View的unFocus()重置標記位
void unFocus(View focused) {
if (DBG) {
System.out.println(this + " unFocus()");
}
//注意看兩個參數(shù)都是false
clearFocusInternal(focused, false, false);
}
/**
* Clears focus from the view, optionally propagating the change up through
* the parent hierarchy and requesting that the root view place new focus.
*
* @param propagate whether to propagate the change up through the parent
* hierarchy
* @param refocus when propagate is true, specifies whether to request the
* root view place new focus
*/
void clearFocusInternal(View focused, boolean propagate, boolean refocus) {
if ((mPrivateFlags & PFLAG_FOCUSED) != 0) {
mPrivateFlags &= ~PFLAG_FOCUSED;
if (propagate && mParent != null) {
mParent.clearChildFocus(this);
}
onFocusChanged(false, 0, null);
refreshDrawableState();
if (propagate && (!refocus || !rootViewRequestFocus())) {
notifyGlobalFocusCleared(this);
}
}
}
requestFocus()主要流程是判斷view當前狀態(tài)是否能拿到焦點，若能，則清除原來獲取焦點的view的標記位,然后返回結果。這里說明一下：ViewGroup的mFocused屬性，它表示當前ViewGroup的一個直接子 view獲取，而真正拿到焦點的view則是mFocused或mFocused的子 view。
再來看看CleanFocus()方法
public void clearFocus() {
if (DBG) {
System.out.println(this + " clearFocus()");
}
//注意參數(shù)
clearFocusInternal(null, true, true);
}
/**
* Clears focus from the view, optionally propagating the change up through
* the parent hierarchy and requesting that the root view place new focus.
*
* @param propagate whether to propagate the change up through the parent
* hierarchy
* @param refocus when propagate is true, specifies whether to request the
* root view place new focus
*/
void clearFocusInternal(View focused, boolean propagate, boolean refocus) {
//若當前沒有拿到焦點，則直接返回
if ((mPrivateFlags & PFLAG_FOCUSED) != 0) {
mPrivateFlags &= ~PFLAG_FOCUSED;
if (propagate && mParent != null) {
//將ViewGroup的mFocused置空
mParent.clearChildFocus(this);
}
onFocusChanged(false, 0, null);
refreshDrawableState();
//rootViewRequestFocus()讓根視圖重新設置focus，就是之前說的requestFocus方法，按一定順序重新給焦點的過程
if (propagate && (!refocus || !rootViewRequestFocus())) {
notifyGlobalFocusCleared(this);
}
}
}
看了上面的代碼可以解釋上面遇到的第二個問題了，當EditText是第一個能獲得焦點的view時，執(zhí)行cleanFocus()方法，在重置焦點標記位時會調用一次onFocusChanged回調方法，在之后的requestFocus流程中，又得到了焦點，所以經(jīng)歷了兩次回調，所以在父 view或根視圖加上focusable和focusableInTouchMode為true時，就解決了這個問題。
關于focus的內容說到這里就完了，相信大家看完這個，對focus應該會有了解了，有不對的地方，歡迎大家指出，互相學習。雖然遇到focus的問題比較少，而且解決起來也很簡單，但還是研究了一下源碼，主要就是學習看源碼的方法，就是debug，跟著一步一步的走，搞清代碼執(zhí)行的流程。
