一、TouchMode
Android支持多种交互方式：D-pad、滚动球、触摸屏等等。这些交互方式可以分为两类：TouchMode和非TouchMode。TouchMode对应触摸屏；非TouchMode对应实体键盘。两种模式中对Focus的需求也不同，例如点击button，在非TouchMode中需要先focus到button，再确认触发button的点击事件；在TouchMode中不需要先focus这个button，触摸按键直接触发button的点击事件。两种模式的对应的Event和Event分发模式也不同，TouchMode产生TouchEvent，对于[TouchEvent分发机制]，非TouchMode产生KeyEvent，对应分发机制较为简单，直接分发给当前获取了焦点的View。
判断是否是TouchMode
boolean isInTouchMode=mView.isInTouchMode();
设置TouchMode
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null) {
viewRoot.ensureTouchMode(false); //设置为非TouchMode
viewRoot.ensureTouchMode(true); //设置为TouchMode
}
二、focus相关属性和方法
1. focusable／focusableInTouchModel
含义
focusable：view属性，表示view是否可以获取到焦点
focusableInTouchModel： view属性，表示在TouchMode时view是否可以获取到焦点。
使用
focusableInTouchModel和focusable属性并不是单独设置的，当调用setFocusable()和setFocusableInTouchMode()设置时，会根据属性值同时修改focusableInTouchModel和focusable。
| 方法 | focusable | focusableInTouchMode |
|---|---|---|
| setFocusable(true) | 设置为true | 不影响 |
| setFocusable(false) | 设置为false | 设置为false |
| setFocusableInTouchMode(true) | 设置为true | 设置为true |
| setFocusableInTouchMode(false) | 不影响 | 设置为false |
影响
对于非TouchMode，只有focusable等于true的view可以获取到焦点，获取到焦点才可以在之后获取到KeyEvent事件。
对于TouchMode，focusableInTouchModel等于true的view接受到TouchEvent后，会先尝试获取焦点。即点击控件时，不会立即执行相应的点击逻辑，而是先显示焦点（即控件被选中），再点击才执行逻辑。
原理
public boolean onTouchEvent(MotionEvent event) {
...
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
//检查是否需要获取焦点并尝试获取焦点，获取焦点成功后设置focusTaken=true
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
removeLongPressCallback();
//当focusTaken=true时不触发performClick，即不触发点击事件
if (!focusTaken) {
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
...
}
2. descendantFocusability
含义
descendantFocusability：ViewGroup属性，定义了当ViewGroup需要获取焦点时执行的策略，默认值为afterDescendants。
- beforeDescendants：viewgroup会优先其子类控件而获取到焦点
- afterDescendants：viewgroup只有当其子类控件不需要获取焦点时才获取焦点
- blocksDescendants：viewgroup子类控件不获得焦点。
实现原理
ViewGroup中重写了requestFocus()方法，根据descendantFocusability执行不同的策略
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
if (DBG) {
System.out.println(this + " ViewGroup.requestFocus direction="
+ direction);
}
int descendantFocusability = getDescendantFocusability();
switch (descendantFocusability) {
case FOCUS_BLOCK_DESCENDANTS:
//当FOCUS_BLOCK_DESCENDANTS时，仅ViewGroup自身尝试获取焦点
return super.requestFocus(direction, previouslyFocusedRect);
case FOCUS_BEFORE_DESCENDANTS: {
//当FOCUS_BEFORE_DESCENDANTS时，先ViewGroup自身尝试获取焦点，获取焦点失败后遍历子view尝试获取焦点
final boolean took = super.requestFocus(direction, previouslyFocusedRect);
return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
}
case FOCUS_AFTER_DESCENDANTS: {
//当FOCUS_AFTER_DESCENDANTS时，先遍历子view尝试获取焦点，当没有子view获取焦点时ViewGroup自身尝试获取焦点
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);
}
}
3. 获取焦点
尝试获取焦点，获取条件:
- TouchMode：focusable=true, focusableInTouchMode=true,
- 非TouchMode：focusable=true
private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) {
//检查focusable属性
if ((mViewFlags & FOCUSABLE_MASK) != FOCUSABLE ||
(mViewFlags & VISIBILITY_MASK) != VISIBLE) {
return false;
}
//检查TouchMode和focuableInTouchMode属性
if (isInTouchMode() &&
(FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
return false;
}
//检查是否有ViewParent设置了blocksDescendants
if (hasAncestorThatBlocksDescendantFocus()) {
return false;
}
//获取焦点
handleFocusGainInternal(direction, previouslyFocusedRect);
return true;
}
4. isFocused／hasFocus
- hasFocus：Returns true if this view has focus itself, or is the ancestor of the view that has focus.
- isFocused：Returns true if this view has focus
三、focus机制
1. View的focus相关属性
focusable=false, focusableInTouchMode=false
说明：TouchMode和非TouchMode都不获取焦点
举例：TextView、LinearLayoutView、RelativeLayoutView等，在TouchMode和非TouchMode中都不需要获取焦点
focusable=true, focusableInTouchMode=false
说明：非TouchMode需要获取焦点，TouchMode不需要获取焦点
举例：Button，在非TouchMode时，需要获取到焦点才能触发对应事件，在TouchMode时，不需要获取焦点就能触发对应事件
focusable=true, focusableInTouchMode=true
说明：TouchMode和非TouchMode都需要获取焦点
举例：EditText，不管在哪种模式下，都需要先获取到焦点才能进行输入
2. focus初始化焦点
当Activity创建成功后，初始化focus，遍历view并尝试设置焦点，根据descendantFocusability默认为afterDescendants，会优先遍历子view，当子view满足focus设置条件时设置为焦点view。（此时为TouchMode，focus设置条件：focusable=true, focusableInTouchMode=true）
ViewRootImpl
if (mFirst) {
// handle first focus request
if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: mView.hasFocus()="
+ mView.hasFocus());
if (mView != null) {
if (!mView.hasFocus()) {
mView.requestFocus(View.FOCUS_FORWARD);
if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: requested focused view="
+ mView.findFocus());
} else {
if (DEBUG_INPUT_RESIZE) Log.v(mTag, "First: existing focused view="
+ mView.findFocus());
}
}
}
3. focus自定义初始化焦点
<RequestFocus/>
适用于：xml布局文件，容器控件和非容器控件都可以使用，需要focusable等于true并且focusableInTouchMode等于true
<EditText
android:id="@+id/text"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<requestFocus />
</EditText>
实现原理：
LayoutInflater中处理，rInflate方法中处理<RequestFocus/>标签
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
final int depth = parser.getDepth();
int type;
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
//requestFocus标签
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
if (finishInflate) {
parent.onFinishInflate();
}
}
private void parseRequestFocus(XmlPullParser parser, View view)
throws XmlPullParserException, IOException {
//view获取标签
view.requestFocus();
consumeChildElements(parser);
}
调用时机：
setContentView()
requestFocus()
适用于：java代码
四、focus常见问题分析
事件1: ListView中自定义包含ImageButton，Button，CheckBox等控件的子Item，点击时这些子控件会将焦点获取到，所以常常当点击item时变化的是子控件，item本身的点击没有响应。
OnItemClick事件只有在ListView子view都是不可聚焦的情况下触发。如果view可以设置focusable为false，则可以直接设置为false解决；如果view要求focusable必须为true，可以为每个子控件添加onClickListener，放弃使用OnItemClickListener。
事件2: 有一个ViewGroup A, 包含一个View B, 然后 A, B的focusableInTouchMode都是true, 第一次点击B, onClickListener不会触发, 而后续点击都可以。
focusable和focusableInTouchMode为true的view，在处理Touch事件时，会优先获取焦点，只有在已获取到焦点时才去触发onClick事件。
事件3: EditText自动获取焦点。
在焦点初始化中，focusable和focusableInTouchMode为true的view会获取焦点。可以通过指定初始化焦点、设置优先级更高的的可聚焦view解决。
