做Android开发的人都用过Selector,可以方便的实现View在不同状态下的背景。不过，相信大部分开发者遇到过和我一样的问题，本文会从源码角度，解释这些问题。

        首先，这里简单描述一下，我遇到的问题：

界面上有个全屏的LinearLayout A，A中有一个TextView B和Button C，其中，A的clickable=true，并设置了pressed时，背景色为灰色，B设置了pressed时，背景色为蓝色

当手指触摸C下方的空白区域时，看到了这样的效果：

        在这里看到，在没有触摸B的情况下，B的pressed = true，而C的pressed = false。 C的状态暂且不讨论，按照Android消息传递的原则，因为touch的point不在B内部，所以，touch消息应该不会交给B处理，那为什么B的pressed = true？

         下面开始一步一步分析（本文分析的Android源码为4.2.2）。

Pressed状态的设定

        从View.onTouchEvent函数看起（frameworks/base/core/java/android/view/View.java):

/**
 * Implement this method to handle touch screen motion events.
 *
 * @param event The motion event.
 * @return True if the event was handled, false otherwise.
 */
public boolean onTouchEvent(MotionEvent event) {
    ......
    if (((viewFlags & CLICKABLE) == CLICKABLE || //这里是为什么设置A的clickable为true的原因，否则，press A的时候，没有界面元素处理touch event，最终会由Activity的onTouchEvent函数处理
            (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_UP:
                ......
                break;

            case MotionEvent.ACTION_DOWN:
                mHasPerformedLongPress = false;
                ......
                // Walk up the hierarchy to determine if we're inside a scrolling container.
                boolean isInScrollingContainer = isInScrollingContainer();//A已经是顶层元素了，没有ScrollView之类的控件存在，所以，isInScrollingContainer = false

                // For views inside a scrolling container, delay the pressed feedback for
                // a short period in case this is a scroll.
                if (isInScrollingContainer) {
                    mPrivateFlags |= PFLAG_PREPRESSED;
                    if (mPendingCheckForTap == null) {
                        mPendingCheckForTap = new CheckForTap();
                    }
                    postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
                } else {
                    // Not inside a scrolling container, so show the feedback right away
                    setPressed(true);//A设置pressed = true
                    checkForLongClick(0);
                }
                break;

            case MotionEvent.ACTION_CANCEL:
                ......
                break;

            case MotionEvent.ACTION_MOVE:
                ......
                break;
        }
        return true;
    }

    return false;
}

       

        从上面的代码我们知道，当手指触摸A的时候，A的pressed被设置为true。

Pressed状态的传递

        接着，我们看看setPressed函数的实现：

/**
 * Sets the pressed state for this view.
 *
 * @param pressed Pass true to set the View's internal state to "pressed", or false to reverts
 *                the View's internal state from a previously set "pressed" state.
 * @see #isClickable()
 * @see #setClickable(boolean)
 */
public void setPressed(boolean pressed) {
    final boolean needsRefresh = pressed != ((mPrivateFlags & PFLAG_PRESSED) == PFLAG_PRESSED);

    if (pressed) {
        mPrivateFlags |= PFLAG_PRESSED;
    } else {
        mPrivateFlags &= ~PFLAG_PRESSED;
    }

    if (needsRefresh) {
        refreshDrawableState();//切换背景图片
    }
    dispatchSetPressed(pressed);
}

        setPressed函数内部调用了dispatchSetPressed函数，这个让人很在意（frameworks/base/core/java/android/view/ViewGroup.java）：

@Override
protected void dispatchSetPressed(boolean pressed) {
    final View[] children = mChildren;
    final int count = mChildrenCount;
    for (int i = 0; i < count; i++) {
        final View child = children[i];
        // Children that are clickable on their own should not
        // show a pressed state when their parent view does.
        // Clearing a pressed state always propagates.
        if (!pressed || (!child.isClickable() && !child.isLongClickable())) {
            child.setPressed(pressed);
        }
    }
}

        原来，dispatchSetPressed函数会把pressed状态传递给所有clickable=false并且longclickable=false的子元素。

        到这里，前面的现象就可以解释了，因为C是button类，clickable=true，而B的clickable=false。所以，当A被触摸时，B的pressed=true，而C的pressed=false。那么，可以回答下面几个小问题了：

1. 如果不希望A的pressed=true时，B的pressed = true，该如何修改？
   1. 设置B的clickable=true
2. 如果希望A的pressed = true时，C的pressed = true，那又该如何修改？
   1. 设置C的clickable = false. 但是，这里可能又存在问题了，设置C的clickable=false，会导致button按钮的onclicklistener无法工作，这是个严重的副作用。那么可以不修改clickable，而设置为true。

       那么duplicateParentState做了些什么呢？View.setDuplicateParentStateEnabled:

public void setDuplicateParentStateEnabled(boolean enabled) {
    setFlags(enabled ? DUPLICATE_PARENT_STATE : 0, DUPLICATE_PARENT_STATE);
}

      再看看View.onCreateDrawableState()

/**
    * Generate the new {@link android.graphics.drawable.Drawable} state for
    * this view. This is called by the view
    * system when the cached Drawable state is determined to be invalid.  To
    * retrieve the current state, you should use {@link #getDrawableState}.
    *
    * @param extraSpace if non-zero, this is the number of extra entries you
    *                   would like in the returned array in which you can place your own
    *                   states.
    * @return Returns an array holding the current {@link Drawable} state of
    * the view.
    * @see #mergeDrawableStates(int[], int[])
    */
   protected int[] onCreateDrawableState(int extraSpace) {
       if ((mViewFlags & DUPLICATE_PARENT_STATE) == DUPLICATE_PARENT_STATE &&
               mParent instanceof View) {
           return ((View) mParent).onCreateDrawableState(extraSpace);
       }

       ......
   }

        从上面的代码，可以看到，当设置duplicateParentState为true时，View的DrawableState直接使用了其parent的。所以，他的drawable状态会一直保持与其parent同步。

题外，为什么当ListView中包含focusable为true的控件时，OnItemClickListener不会触发

        因为ListView未重载onTouchEvent事件，所以，需要看其父类的AbsListView.onTouchEvent(frameworks/base/core/java/android/widget/AbsListView):

@Override
public boolean onTouchEvent(MotionEvent ev) {
    ......
    switch (action & MotionEvent.ACTION_MASK) {
    case MotionEvent.ACTION_DOWN: {
        ......
        break;
    }

    case MotionEvent.ACTION_MOVE: {
        ......
        break;
    }

    case MotionEvent.ACTION_UP: {
        switch (mTouchMode) {
        case TOUCH_MODE_DOWN:
        case TOUCH_MODE_TAP:
        case TOUCH_MODE_DONE_WAITING:
            final int motionPosition = mMotionPosition;
            final View child = getChildAt(motionPosition - mFirstPosition);

            final float x = ev.getX();
            final boolean inList = x > mListPadding.left && x < getWidth() - mListPadding.right;

            if (child != null && !child.hasFocusable() && inList) {
                if (mTouchMode != TOUCH_MODE_DOWN) {
                    child.setPressed(false);
                }

                if (mPerformClick == null) {
                    mPerformClick = new PerformClick();
                }

                ......
                performClick.run();
                ......
            }
            ......
        case TOUCH_MODE_SCROLL:
            ......
            break;

        case TOUCH_MODE_OVERSCROLL:
        ......
        break;
    }
    ......
    case MotionEvent.ACTION_CANCEL: {
        ......
        break;
    }

    case MotionEvent.ACTION_POINTER_UP: {
        ......
        break;
    }

    case MotionEvent.ACTION_POINTER_DOWN: {
        ......
        break;
    }
    }

    return true;
}

         仅在child.hasFocusable()=false时， PerformClick对象才会执行ViewGroup.hasFocusable:

/**
 * {@inheritDoc}
 */
@Override
public boolean hasFocusable() {
    if ((mViewFlags & VISIBILITY_MASK) != VISIBLE) {
        return false;
    }

    if (isFocusable()) {
        return true;
    }

    final int descendantFocusability = getDescendantFocusability();
    if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
        final int count = mChildrenCount;
        final View[] children = mChildren;

        for (int i = 0; i < count; i++) {
            final View child = children[i];
            if (child.hasFocusable()) {
                return true;
            }
        }
    }

    return false;
}

        仅在所有的clild的hasFocusable为false时，ListView才会执行performClick（AbsListView.PerformClick）：

private class PerformClick extends WindowRunnnable implements Runnable {
    int mClickMotionPosition;

    public void run() {
        // The data has changed since we posted this action in the event queue,
        // bail out before bad things happen
        if (mDataChanged) return;

        final ListAdapter adapter = mAdapter;
        final int motionPosition = mClickMotionPosition;
        if (adapter != null && mItemCount > 0 &&
                motionPosition != INVALID_POSITION &&
                motionPosition < adapter.getCount() && sameWindow()) {
            final View view = getChildAt(motionPosition - mFirstPosition);
            // If there is no view, something bad happened (the view scrolled off the
            // screen, etc.) and we should cancel the click
            if (view != null) {
                performItemClick(view, motionPosition, adapter.getItemId(motionPosition));
            }
        }
    }
}

        而PerformClick会调用performItemClick(AdsListView.performItemClick)：

@Override
   public boolean performItemClick(View view, int position, long id) {
       boolean handled = false;
       boolean dispatchItemClick = true;

       ......

       if (dispatchItemClick) {
           handled |= super.performItemClick(view, position, id);
       }

       return handled;
   }

       AdapterView.preformItemClick:

/**
 * Call the OnItemClickListener, if it is defined.
 *
 * @param view The view within the AdapterView that was clicked.
 * @param position The position of the view in the adapter.
 * @param id The row id of the item that was clicked.
 * @return True if there was an assigned OnItemClickListener that was
 *         called, false otherwise is returned.
 */
public boolean performItemClick(View view, int position, long id) {
    if (mOnItemClickListener != null) {
        playSoundEffect(SoundEffectConstants.CLICK);
        if (view != null) {
            view.sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
        }
        mOnItemClickListener.onItemClick(this, view, position, id);
        return true;
    }

    return false;
}

        所以，如果ListView item中包含focusable为true的控件（例如：button， radiobutton），会导致ItemClickListener失效。解决方案两个：

设置特定的控件focusable = false

不使用onItemClickListener，而直接在Item上设置onClickListener监听点击事件。