在最近写的项目中遇到由focus引起的问题，例如：
在两个嵌套的RecyclerView中，外层滑动停止后，由于内层的RecyclerView获取了焦点，导致外层又自动滑动了一些距离；
需要监听EditText的焦点变化来控制界面的显示于隐藏，在调用EditText的cleanFocus()方法时，回调接口会调用两次，并且EditText的焦点依然还在。
对遇到的这两个问题的解决方法都一样，就是在外层，或根布局中添加focusInTouchModel:true,focus:true两个属性，优先获取到焦点就可以了。
布局文件中和focus有关的属性
//是否可获取焦点
focusable:true|false
//触摸模式下是否可获取焦点
focusableInTouchMode:true|false
descendantFocusability:blocksDescendants|beforeDescendants|afterDescendants
这里解释一下descendantFocusability的三个属性值，分别代表的意思：
- blocksDescendants：ViewGroup拦截，不让子 view获取焦点。
- beforeDescendants：ViewGroup优先尝试（尝试的意思是，根据View或ViewGroup当前状态来判断是否能得到焦点，如是否可见，是否可获取焦点等等，在View的requestFocus方法的注释中提到，下同）获取焦点，若ViewGroup没拿到焦点，再遍历子 view（包括所有直接子 view和间接子 view），让子 view尝试获取焦点。
3.afterDescendants：先遍历子 view，让子 view尝试获取焦点，若所有子 view（包括所有直接子 view和间接子 view）都没拿到焦点，才让ViewGroup尝试获取焦点。
下面是ViewGroup的与descendantFocusability有关的源码
@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " ViewGroup.requestFocus direction="
+ direction);
}
//获取当前viewgroup的descendantFocusability属性值
int descendantFocusability = getDescendantFocusability();
switch (descendantFocusability) {
case FOCUS_BLOCK_DESCENDANTS:
//直接调用超类也就是View的方法获取焦点，忽略子 view
return super.requestFocus(direction, previouslyFocusedRect);
case FOCUS_BEFORE_DESCENDANTS: {
//当前ViewGroup先尝试获取焦点，返回值表示是否拿到来焦点，若没有，则让子 view尝试获取
final boolean took = super.requestFocus(direction, previouslyFocusedRect);
return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
}
case FOCUS_AFTER_DESCENDANTS: {
//与上面刚好相反
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() 方法的执行流程
上面看的是ViewGroup的requestFocus()方法，而ViewGroup的这个方法只是对ViewGroup和子 view之间获取焦点的顺序做的一个处理，要拿到焦点，最终还是要通过调用View的requestFocus()方法来拿到焦点。所以接下来主要看的是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,判断是否可获取焦点或是否可见
if ((mViewFlags & FOCUSABLE_MASK) != FOCUSABLE ||
(mViewFlags & VISIBILITY_MASK) != VISIBLE) {
return false;
}
// need to be focusable in touch mode if in touch mode，判断在触摸模式下是否可获取焦点
if (isInTouchMode() &&
(FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
return false;
}
// need to not have any parents blocking us，判断是否有父view拦截
if (hasAncestorThatBlocksDescendantFocus()) {
return false;
}
handleFocusGainInternal(direction, previouslyFocusedRect);
return true;
}
可以看到，在外部调用requestFocus()这个无参方法后，经过层层传递后，最终是调用requestFocusNoSearch(int direction, Rect previouslyFocusedRect)方法，这几个方法的返回值表示是否拿到了焦点，在里面可以看到有3个if判断语句，这就是我们上面提到的“尝试”的意思了，即需要根据当前状态判断，通过判断，最后直接返回了true，表示拿到了焦点，而在返回之前，还执行了一个很重要的方法handleFocusGainInternal(direction, previouslyFocusedRect)，来看看里面都干了些什么：
void handleFocusGainInternal(@FocusRealDirection int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " requestFocus()");
}
// 若当前已拿到焦点，则方法结束，所以一个已获焦点的view，在执行requestFocus()后，它的onFocusChanged方法是不会回调的
if ((mPrivateFlags & PFLAG_FOCUSED) == 0) {
//标记已拿到焦点
mPrivateFlags |= PFLAG_FOCUSED;
//上个拿到焦点的view
View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null;
if (mParent != null) {
//递归调用，重置ViewGroup的mFocused的值，重置oldFocus的mPrivateFlags标记位
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变量值
mFocused = child;
}
if (mParent != null) {
mParent.requestChildFocus(this, focused);
}
}
然后通过View的unFocus()重置标记位
void unFocus(View focused) {
if (DBG) {
System.out.println(this + " unFocus()");
}
//注意看两个参数都是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()主要流程是判断view当前状态是否能拿到焦点，若能，则清除原来获取焦点的view的标记位,然后返回结果。这里说明一下：ViewGroup的mFocused属性，它表示当前ViewGroup的一个直接子 view获取，而真正拿到焦点的view则是mFocused或mFocused的子 view。
再来看看CleanFocus()方法
public void clearFocus() {
if (DBG) {
System.out.println(this + " clearFocus()");
}
//注意参数
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) {
//若当前没有拿到焦点，则直接返回
if ((mPrivateFlags & PFLAG_FOCUSED) != 0) {
mPrivateFlags &= ~PFLAG_FOCUSED;
if (propagate && mParent != null) {
//将ViewGroup的mFocused置空
mParent.clearChildFocus(this);
}
onFocusChanged(false, 0, null);
refreshDrawableState();
//rootViewRequestFocus()让根视图重新设置focus，就是之前说的requestFocus方法，按一定顺序重新给焦点的过程
if (propagate && (!refocus || !rootViewRequestFocus())) {
notifyGlobalFocusCleared(this);
}
}
}
看了上面的代码可以解释上面遇到的第二个问题了，当EditText是第一个能获得焦点的view时，执行cleanFocus()方法，在重置焦点标记位时会调用一次onFocusChanged回调方法，在之后的requestFocus流程中，又得到了焦点，所以经历了两次回调，所以在父 view或根视图加上focusable和focusableInTouchMode为true时，就解决了这个问题。
关于focus的内容说到这里就完了，相信大家看完这个，对focus应该会有了解了，有不对的地方，欢迎大家指出，互相学习。虽然遇到focus的问题比较少，而且解决起来也很简单，但还是研究了一下源码，主要就是学习看源码的方法，就是debug，跟着一步一步的走，搞清代码执行的流程。
