PopupWindow 点击外部和返回键无法消失背后的真相（setBackgroundDrawable(Drawable background)）

 public void showAsDropDown(View anchor, int xoff, int yoff, int gravity) {
        if (isShowing() || mContentView == null) {
            return;
        }

        registerForScrollChanged(anchor, xoff, yoff, gravity);

        mIsShowing = true;
        mIsDropdown = true;

        WindowManager.LayoutParams p = createPopupLayout(anchor.getWindowToken());
        preparePopup(p);

        updateAboveAnchor(findDropDownPosition(anchor, p, xoff, yoff, gravity));

        if (mHeightMode < 0) p.height = mLastHeight = mHeightMode;
        if (mWidthMode < 0) p.width = mLastWidth = mWidthMode;

        p.windowAnimations = computeAnimationResource();

        invokePopup(p);
  }

    /**
     * <p>Prepare the popup by embedding in into a new ViewGroup if the
     * background drawable is not null. If embedding is required, the layout
     * parameters' height is modified to take into account the background's
     * padding.</p>
     *
     * @param p the layout parameters of the popup's content view
     */
    private void preparePopup(WindowManager.LayoutParams p) {
        if (mContentView == null || mContext == null || mWindowManager == null) {
            throw new IllegalStateException("You must specify a valid content view by "
                    + "calling setContentView() before attempting to show the popup.");
        }

        if (mBackground != null) {
            final ViewGroup.LayoutParams layoutParams = mContentView.getLayoutParams();
            int height = ViewGroup.LayoutParams.MATCH_PARENT;
            if (layoutParams != null &&
                    layoutParams.height == ViewGroup.LayoutParams.WRAP_CONTENT) {
                height = ViewGroup.LayoutParams.WRAP_CONTENT;
            }

            // when a background is available, we embed the content view
            // within another view that owns the background drawable
            PopupViewContainer popupViewContainer = new PopupViewContainer(mContext);
            PopupViewContainer.LayoutParams listParams = new PopupViewContainer.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT, height
            );
            popupViewContainer.setBackground(mBackground);
            popupViewContainer.addView(mContentView, listParams);

            mPopupView = popupViewContainer;
        } else {
            mPopupView = mContentView;
        }

        mPopupView.setElevation(mElevation);
        mPopupViewInitialLayoutDirectionInherited =
                (mPopupView.getRawLayoutDirection() == View.LAYOUT_DIRECTION_INHERIT);
        mPopupWidth = p.width;
        mPopupHeight = p.height;
    }

    private class PopupViewContainer extends FrameLayout {
        private static final String TAG = "PopupWindow.PopupViewContainer";

        public PopupViewContainer(Context context) {
            super(context);
        }

        @Override
        protected int[] onCreateDrawableState(int extraSpace) {
            if (mAboveAnchor) {
                // 1 more needed for the above anchor state
                final int[] drawableState = super.onCreateDrawableState(extraSpace + 1);
                View.mergeDrawableStates(drawableState, ABOVE_ANCHOR_STATE_SET);
                return drawableState;
            } else {
                return super.onCreateDrawableState(extraSpace);
            }
        }

        @Override
        public boolean dispatchKeyEvent(KeyEvent event) {　　// 这个方法里面实现了返回键处理逻辑，会调用dismiss
            if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
                if (getKeyDispatcherState() == null) {
                    return super.dispatchKeyEvent(event);
                }

                if (event.getAction() == KeyEvent.ACTION_DOWN
                        && event.getRepeatCount() == 0) {
                    KeyEvent.DispatcherState state = getKeyDispatcherState();
                    if (state != null) {
                        state.startTracking(event, this);
                    }
                    return true;
                } else if (event.getAction() == KeyEvent.ACTION_UP) {
                    KeyEvent.DispatcherState state = getKeyDispatcherState();
                    if (state != null && state.isTracking(event) && !event.isCanceled()) {
                        dismiss();
                        return true;
                    }
                }
                return super.dispatchKeyEvent(event);
            } else {
                return super.dispatchKeyEvent(event);
            }
        }

        @Override
        public boolean dispatchTouchEvent(MotionEvent ev) {
            if (mTouchInterceptor != null && mTouchInterceptor.onTouch(this, ev)) {
                return true;
            }
            return super.dispatchTouchEvent(ev);
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) { // 这个方法里面实现点击消失逻辑
            final int x = (int) event.getX();
            final int y = (int) event.getY();
            
            if ((event.getAction() == MotionEvent.ACTION_DOWN)
                    && ((x < 0) || (x >= getWidth()) || (y < 0) || (y >= getHeight()))) {
                dismiss();
                return true;
            } else if (event.getAction() == MotionEvent.ACTION_OUTSIDE) {
                dismiss();
                return true;
            } else {
                return super.onTouchEvent(event);
            }
        }

        @Override
        public void sendAccessibilityEvent(int eventType) {
            // clinets are interested in the content not the container, make it event source
            if (mContentView != null) {
                mContentView.sendAccessibilityEvent(eventType);
            } else {
                super.sendAccessibilityEvent(eventType);
            }
        }
    }

   /**
     * Specifies the background drawable for this popup window. The background
     * can be set to {@code null}.
     *
     * @param background the popup's background
     * @see #getBackground()
     * @attr ref android.R.styleable#PopupWindow_popupBackground
     */
    public void setBackgroundDrawable(Drawable background) {
        mBackground = background;
        // 省略其他的
    }

    /**
     * <p>Create a new, empty, non focusable popup window of dimension (0,0).</p>
     * 
     * <p>The popup does not provide a background.</p>
     */
    public PopupWindow(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        mContext = context;
        mWindowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);

        final TypedArray a = context.obtainStyledAttributes(
                attrs, R.styleable.PopupWindow, defStyleAttr, defStyleRes);
        final Drawable bg = a.getDrawable(R.styleable.PopupWindow_popupBackground);
        mElevation = a.getDimension(R.styleable.PopupWindow_popupElevation, 0);
        mOverlapAnchor = a.getBoolean(R.styleable.PopupWindow_overlapAnchor, false);

        final int animStyle = a.getResourceId(R.styleable.PopupWindow_popupAnimationStyle, -1);
        mAnimationStyle = animStyle == R.style.Animation_PopupWindow ? -1 : animStyle;

        a.recycle();

        setBackgroundDrawable(bg);
    }

    /**
     * Prepare the popup by embedding it into a new ViewGroup if the background
     * drawable is not null. If embedding is required, the layout parameters'
     * height is modified to take into account the background's padding.
     *
     * @param p the layout parameters of the popup's content view
     */
    private void preparePopup(WindowManager.LayoutParams p) {
        if (mContentView == null || mContext == null || mWindowManager == null) {
            throw new IllegalStateException("You must specify a valid content view by "
                    + "calling setContentView() before attempting to show the popup.");
        }

        // The old decor view may be transitioning out. Make sure it finishes
        // and cleans up before we try to create another one.
        if (mDecorView != null) {
            mDecorView.cancelTransitions();
        }

        // When a background is available, we embed the content view within
        // another view that owns the background drawable.
        if (mBackground != null) {
            mBackgroundView = createBackgroundView(mContentView);
            mBackgroundView.setBackground(mBackground);
        } else {
            mBackgroundView = mContentView;
        }

        mDecorView = createDecorView(mBackgroundView);

        // The background owner should be elevated so that it casts a shadow.
        mBackgroundView.setElevation(mElevation);

        // We may wrap that in another view, so we'll need to manually specify
        // the surface insets.
        final int surfaceInset = (int) Math.ceil(mBackgroundView.getZ() * 2);
        p.surfaceInsets.set(surfaceInset, surfaceInset, surfaceInset, surfaceInset);
        p.hasManualSurfaceInsets = true;

        mPopupViewInitialLayoutDirectionInherited =
                (mContentView.getRawLayoutDirection() == View.LAYOUT_DIRECTION_INHERIT);
        mPopupWidth = p.width;
        mPopupHeight = p.height;
    }