前言
现在基本上都是触摸屏手机，导致很多开发者对焦点并没有深刻认识，但仍然存在一些非触屏手机，还有Android TV平台都与焦点息息相关，这里就总结下自己看过多篇文章和自己的实践经验。先看两张焦点图。
Android非触屏手机
Android TV
焦点基础及源码分析通过两篇来带大家深入认识。第一篇从最基本的View，ViewGroup获取焦点说起。
术语
- 父类 extends后的类叫做父类
- 父View View树中的父级View称作父View
- 拥有焦点 表示该View获取到了焦点，focused状态为true
- 包含焦点 表示该View或者其子View获取到了焦点，而自己focused状态有可能为false，但一定有一个子View focused为true。
View树中焦点
先看下Activity的View树结构
image
其顶层就是DecorView,子View可以是View也可以是ViewGroup,但父View一定是ViewGroup.
获取焦点的前提
- View#isFocusable返回true, 如果在触摸模式, 则View#isFocusableInTouchMode也要返回true
- View及父View必须处于可见
- 父View不能有FOCUS_BLOCK_DESCENDANTS属性
View
请求焦点流程
调用View#requestFocus方法
View.java
public final boolean requestFocus() {
return requestFocus(View.FOCUS_DOWN);
}
private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) {
// need to be focusable
// 1. 判断View是否可获取焦点和是否可见
if ((mViewFlags & FOCUSABLE) != FOCUSABLE
|| (mViewFlags & VISIBILITY_MASK) != VISIBLE) {
return false;
}
// 2. 如果为触摸模式下，则FOCUSABLE_IN_TOUCH_MODE必须为true才可以去获得焦点
// need to be focusable in touch mode if in touch mode
if (isInTouchMode() &&
(FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
return false;
}
// 3. 判断所有父View是否有BLOCK子类焦点的属性，如果没有才可以去获取焦点
// need to not have any parents blocking us
if (hasAncestorThatBlocksDescendantFocus()) {
return false;
}
// 4 下文分析
handleFocusGainInternal(direction, previouslyFocusedRect);
return true;
}
注释1,2,3就是上文提到的获取焦点的3个前提。
接下来分析注释4
void handleFocusGainInternal(@FocusRealDirection int direction, Rect previouslyFocusedRect) {
// 1. 置FOCUSED标记位
if ((mPrivateFlags & PFLAG_FOCUSED) == 0) {
mPrivateFlags |= PFLAG_FOCUSED;
// 调用RootView
View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null;
if (mParent != null) {
// 2. 告知父View自己已聚焦，下文详细分析
mParent.requestChildFocus(this, this);
updateFocusedInCluster(oldFocus, direction);
}
if (mAttachInfo != null) {
// 3. 通知OnGlobalFocusChangeListener
mAttachInfo.mTreeObserver.dispatchOnGlobalFocusChange(oldFocus, this);
}
// 4. 触发OnFocusChangeListener回调，只能受到自己View焦点状态的改变
onFocusChanged(true, direction, previouslyFocusedRect);
...
}
}
继续分析注释2的requestChildFocus，该方法作用通知父View自己已获取焦点，然后父View去更新自己管辖范围(直接子View)内焦点状态，将状态继续通知给其父View
ViewGroup.java
@Override
public void requestChildFocus(View child, View focused) {
if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {
return;
}
// 1. 清除自己的焦点，如果有的话
// Unfocus us, if necessary
super.unFocus(focused);
// We had a previous notion of who had focus. Clear it.
// 2. mFocused用指向自己直接子View拥有或者包含焦点的View，如果存在则让其失焦
if (mFocused != child) {
if (mFocused != null) {
mFocused.unFocus(focused);
}
// 3. mFocused重新指向新的请求焦点的View，
mFocused = child;
}
if (mParent != null) {
// 4. 继续通知父View自己已有或者已拥有焦点
mParent.requestChildFocus(this, focused);
}
}
这里解释下mFocused是什么，直接看下图
mFocused标识了自己的直接子View的焦点状态。
mParent.requestChildFocus方法一级一级向上调用通知自己焦点状态直到顶层View，对于Activity，顶层View就是DecorView，然后继续调用到所有View的管理类ViewRootImpl的requestChildFocus的方法，其进行整个界面重绘。
ViewRoolImpl.java
@Override
public void requestChildFocus(View child, View focused) {
checkThread();
// 会执行performTraversals
scheduleTraversals();
}
// 界面重绘
private void performTraversals() {
...
performMeasure(); // 回调onMeasue()
...
performLayout(); // 回调onLayout()
...
performDraw(); // 回调ondraw()
...
}
清除焦点
View#clearFocus主动放弃焦点
View.java
public void clearFocus() {
clearFocusInternal(null, true, true);
}
void clearFocusInternal(View focused, boolean propagate, boolean refocus) {
// 1. 改变焦点标志位
if ((mPrivateFlags & PFLAG_FOCUSED) != 0) {
mPrivateFlags &= ~PFLAG_FOCUSED;
if (propagate && mParent != null) {
// 2. 通知父view当前View放弃焦点,和requestChildFocus相反
mParent.clearChildFocus(this);
}
// 3. 触发OnFocusChangeListener回调
onFocusChanged(false, 0, null);
refreshDrawableState();
// 4. rootViewRequestFocus()调用当前View树的根View(rootView)的requestFocus方法
if (propagate && (!refocus || !rootViewRequestFocus())) {
notifyGlobalFocusCleared(this);
}
}
}
ViewGroup
请求焦点
也是调用requestFocus()方法
ViewGroup.java
@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
int descendantFocusability = getDescendantFocusability();
switch (descendantFocusability) {
// 1. 如果BLOCK子View焦点，则直接走当前View请求焦点的流程
case FOCUS_BLOCK_DESCENDANTS:
return super.requestFocus(direction, previouslyFocusedRect);
case FOCUS_BEFORE_DESCENDANTS: {
// 2. 如果为BEFORE子View，则先走View请求焦点流程，如果没有得到焦点则走下个方法onRequestFocusInDescendants方法，其根据direction查找所有子View去请求焦点
final boolean took = super.requestFocus(direction, previouslyFocusedRect);
return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
}
case FOCUS_AFTER_DESCENDANTS: {
// 3. AFTER子View，和BEFORE相反，先遍历所有子View请求焦点，如果没有请求到，则走当前View请求焦点流程
final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
return took ? took : super.requestFocus(direction, previouslyFocusedRect);
}
}
}
清除焦点
ViewGroup.java
@Override
public void clearFocus() {
if (mFocused == null) {
// mFocused为null，则子View无焦点，则清除自己焦点
super.clearFocus();
} else {
// mFocused置为null，并使其清除焦点
View focused = mFocused;
mFocused = null;
focused.clearFocus();
}
}
总结
界面是由View树组成，父View通过mFocused来标识直接子View的焦点状态，当子View焦点状态改变会通知其父View，如果该ViewGroup之前有拥有焦点的View，父View会清除其焦点，并更新mFocused为新View，然后继续将自己状态通知其父View，这样一级一级更新，整个View树焦点状态保持最新，最后由ViewRootImpl更新界面
请求焦点流程
**按D-pad(遥控器上下左右就是)键盘，焦点如何移动，源码分析尽在下篇:)
