Pressed状态和clickable,duplicateParentState的关系

        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android:duplicateParentState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。