創建Demo
MainActivity.java
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//初始化控件
MyLinearLayout mgLayout = (MyLinearLayout) findViewById(R.id.mylayout);
Button btn = (Button) findViewById(R.id.btn);
//添加onClick事件
mgLayout.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Log.i("chuyibo","點擊了自定義線性佈局MyLinearLayout");
}
});
//添加onClick事件
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Log.i("chuyibo","點擊了btn");
}
});
}
}activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<bo.yi.chu.chuyibo2.MyLinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/mylayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:background="#fff">
<Button
android:layout_width="150dp"
android:layout_height="50dp"
android:id="@+id/btn"
android:background="#684132"
android:text="點擊"
android:textSize="16sp"
android:textColor="#fff"/>
</bo.yi.chu.chuyibo2.MyLinearLayout>MyLinearLayout.java
public class MyLinearLayout extends LinearLayout {
public MyLinearLayout(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
}運行
點擊Button查看Log日誌
點擊空白區域查看Log日誌
查看日誌得出結論，myLayout爲btn的父類，兩者同時添加onClick監聽，點擊btn，只有btn的onClick()方法執行。點擊myLayout（空白區域）只有myLayout的onClick()方法執行，Demo中MyLinearLayout繼承LinearLayout，並重寫了onInterceptTouchEvent()。
onInterceptTouchEvent()方法中有一個返回值，Demo中的返回值爲false。把該方法的返回值改爲true，重新點擊Button和空白區域查看Log日誌。
public boolean onInterceptTouchEvent(MotionEvent ev) {
return true;//返回值改爲true
}點擊Button查看Log日誌
點擊空白區域查看Log日誌
查看日誌得出結論，當onInterceptTouchEvent()方法返回true時，無論點擊btn還是myLayout，都只執行myLayout的onClick()方法。
android中的佈局直接或者間接的繼承了ViewGroup，ViewGroup又是View的子類，但是ViewGroup重寫了dispatchTouchEvent()方法。ViewGroup中可以存放多個View，當點擊一個ViewGroup中的子View時觸摸事件是先傳遞給了ViewGroup，然後有ViewGroup的dispatchTouchEvent()分發給子View，最後再有子View的dispatchTouchEvent()來處理觸摸事件。同時ViewGroup也可以攔截事件從而使觸摸事件不傳遞給子View，onInterceptTouchEvent()方法就是事件傳遞的攔截器。
源碼分析
dispatchTouchEvent()
public boolean dispatchTouchEvent(MotionEvent ev) {
final int action = ev.getAction();
final float xf = ev.getX();
final float yf = ev.getY();
final float scrolledXFloat = xf + mScrollX;
final float scrolledYFloat = yf + mScrollY;
final Rect frame = mTempRect;
boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (action == MotionEvent.ACTION_DOWN) {
if (mMotionTarget != null) {
// this is weird, we got a pen down, but we thought it was
// already down!
// XXX: We should probably send an ACTION_UP to the current
// target.
mMotionTarget = null;
}
// If we're disallowing intercept or if we're allowing and we didn't
// intercept
if (disallowIntercept || !onInterceptTouchEvent(ev)) {
// reset this event's action (just to protect ourselves)
ev.setAction(MotionEvent.ACTION_DOWN);
// We know we want to dispatch the event down, find a child
// who can handle it, start with the front-most child.
final int scrolledXInt = (int) scrolledXFloat;
final int scrolledYInt = (int) scrolledYFloat;
final View[] children = mChildren;
final int count = mChildrenCount;
for (int i = count - 1; i >= 0; i--) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null) {
child.getHitRect(frame);
if (frame.contains(scrolledXInt, scrolledYInt)) {
// offset the event to the view's coordinate system
final float xc = scrolledXFloat - child.mLeft;
final float yc = scrolledYFloat - child.mTop;
ev.setLocation(xc, yc);
if (child.dispatchTouchEvent(ev)) {
// Event handled, we have a target now.
mMotionTarget = child;
return true;
}
// The event didn't get handled, try the next view.
// Don't reset the event's location, it's not
// necessary here.
}
}
}
}
}
boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
(action == MotionEvent.ACTION_CANCEL);
if (isUpOrCancel) {
// Note, we've already copied the previous state to our local
// variable, so this takes effect on the next event
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// The event wasn't an ACTION_DOWN, dispatch it to our target if
// we have one.
final View target = mMotionTarget;
if (target == null) {
// We don't have a target, this means we're handling the
// event as a regular view.
ev.setLocation(xf, yf);
return super.dispatchTouchEvent(ev);
}
// if have a target, see if we're allowed to and want to intercept its
// events
if (!disallowIntercept && onInterceptTouchEvent(ev)) {
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
ev.setAction(MotionEvent.ACTION_CANCEL);
ev.setLocation(xc, yc);
if (!target.dispatchTouchEvent(ev)) {
// target didn't handle ACTION_CANCEL. not much we can do
// but they should have.
}
// clear the target
mMotionTarget = null;
// Don't dispatch this event to our own view, because we already
// saw it when intercepting; we just want to give the following
// event to the normal onTouchEvent().
return true;
}
if (isUpOrCancel) {
mMotionTarget = null;
}
// finally offset the event to the target's coordinate system and
// dispatch the event.
final float xc = scrolledXFloat - (float) target.mLeft;
final float yc = scrolledYFloat - (float) target.mTop;
ev.setLocation(xc, yc);
return target.dispatchTouchEvent(ev);
}第21行是一個if判斷，disallowIntercept表示是否禁止掉攔截事件的功能，默認爲false，因此if判斷爲true或false取決於onInterceptTouchEvent()方法的返回值。
onInterceptTouchEvent()方法返回false表示不攔截事件。if條件判斷成立，在第30行對ViewGroup中的子View進行遍歷，並在第35行進行判斷當前子View是否是正在點擊的View。如果是，執行View的dispatchTouchEvent()。因爲View的dispatchTouchEvent()方法始終返回true，所以40行if條件判斷成立，執行return，後面的代碼不執行。
onInterceptTouchEvent()方法返回true表示攔截事件，if條件判斷不成立。或ViewGroup中沒有子View，以及ViewGroup中的所有子View都不是當前正在點擊的View時執行後面的語句。第66行判斷target是否爲null。target一般不會爲null，執行第70行super.dispatchTouchEvent()就是調用父類View中的dispatchTouchEvent()方法。
