剛接手PopupWindow的時候，我們都可能覺得很簡單，因為它確實很簡單，不過運氣不好的可能就會踩到一個坑：
點擊PopupWindow最外層布局以及點擊返回鍵PopupWindow不會消失
新手在遇到這個問題的時候可能會折騰半天，最后通過強大的網(wǎng)絡(luò)找到一個解決方案，那就是跟PopupWindow設(shè)置一個背景
popupWindow.setBackgroundDrawable(drawable),這個drawable隨便一個什么類型的都可以，只要不為空。
Demo地址：SmartPopupWindow.rar
下面從源碼（我看的是android-22）上看看到底發(fā)生了什么事情導致返回鍵不能消失彈出框：
先看看彈出框顯示的時候代碼showAsDropDown，里面有個preparePopup方法。
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); }再看preparePopup方法
/** * <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; }上面可以看到mBackground不為空的時候，會PopupViewContainer作為mContentView的Parent，下面看看PopupViewContainer到底干了什么
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) { // 這個方法里面實現(xiàn)了返回鍵處理邏輯，會調(diào)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) { // 這個方法里面實現(xiàn)點擊消失邏輯 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); } } }看到上面紅色部分的標注可以看出，這個內(nèi)部類里面封裝了處理返回鍵退出和點擊外部退出的邏輯，但是這個類對象的構(gòu)造過程中（preparePopup方法中）卻有個mBackground != null的條件才會創(chuàng)建
而mBackground對象在setBackgroundDrawable方法中被賦值，看到這里應該就明白一切了。
/** * 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 }setBackgroundDrawable方法除了被外部調(diào)用，構(gòu)造方法中也會調(diào)用，默認是從系統(tǒng)資源中取的
/** * <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); }有些版本沒有，android6.0版本preparePopup如下：
/** * 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; }這里實現(xiàn)返回鍵監(jiān)聽的代碼是mDecorView = createDecorView(mBackgroundView)，這個并沒有受到那個mBackground變量的控制，所以這個版本應該沒有我們所描述的問題，感興趣的可以自己去嘗試一下
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