读源码：PopupWindow

读源码是为了了解并学习它的实现机制，并更好的运用它，如果在读源码之前已经知道它的怎么运用，这将会更容易理解源码。所以在这读源码开头我推荐阅读一下一位大神写的相关博文，浅显易懂，条理清晰：
PopUpWindow使用详解(一)——基本使用
PopUpWindow使用详解(二)——进阶及答疑
PopupWindow这个类用来实现一个弹出框，可以使用任意布局的View作为其内容，这个弹出框是一个浮动的容器，悬浮在当前activity之上的.
PopupWindow可以说是一种view，但它不同于一般的view，它不继承自view类或其子类，而是直接继承自Object ，是基于WindowManager出生的。
PopupWindow类中有一个接口，两个内部类，从内部类从这里切入，方便理解与PopupWindow密切相关的实例，从而更容易理顺类的思路。

一，内部的接口：OnDismissListener

当弹出框被dismissed（解雇，驳回，消失）调用这个接口.其内结构非常简单：

public interface OnDismissListener {public void onDismiss();}

看到这个接口，相信都很有熟悉感，因为Dialog中也有一个一样的接口。Dialog通过setOnDismissListener(OnDismissListener onDismissListener)在点击对话框按钮消失后执行设置的行为（如dismissDialog(int id) ），PopupWindow的这个接口和对话框的功能相同，在PopupWindow同样可以找到对接口OnDismissListener进行包装的和对话框的类一样的setOnDismissListener(OnDismissListener onDismissListener)方法：

 public void setOnDismissListener(OnDismissListener onDismissListener) {mOnDismissListener = onDismissListener;}

用法也一样。

二，弹出框的根view：PopupDecorView

如果理解什么是DecorView，看到这个类名就会知道它的作用。DecorView是窗口界面所有视图的根，关于它推荐阅读Android中将布局文件/View添加至窗口过程分析 —- 从setContentView()谈起
PopupDecorView从字面意思猜测可能就是弹出框的根视图了。就是下图弹出框带来的阴影。

图片来自：PopUpWindow使用详解(一)——基本使用

PopupDecorView继承自FrameLayout，其内部七个方法可以分为两部分：事件分发和进出动画。

三，弹出框的背景view：PopupBackgroundView

这个内部类定义了弹出框的背景视图：

private class PopupBackgroundView extends FrameLayout {public PopupBackgroundView(Context context) {super(context);}@Overrideprotected int[] onCreateDrawableState(int extraSpace) {if (mAboveAnchor) {final int[] drawableState = super.onCreateDrawableState(extraSpace + 1);View.mergeDrawableStates(drawableState, ABOVE_ANCHOR_STATE_SET);return drawableState;} else {return super.onCreateDrawableState(extraSpace);}}}

之所以贴出这段代码，是推荐学习这种Drawable式的自定义方式，可以参考学习Android Drawable 那些不为人知的高效用法

四，PopupWindow

先定义了三个与输入法相关的常量：INPUT_METHOD_FROM_FOCUSABLE，INPUT_METHOD_NEEDED，INPUT_METHOD_NOT_NEEDED。这些常量表示弹出框与输入法弹出框的关系。接着声明一些成员变量并且或初始化值。
值得注意的是PopupWindow有九个构造器，这些构造器可以分为两组，代表分别为：

public PopupWindow(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
//1，获取contextmContext = context;
//2，获取WindowManagermWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);//3，加载属性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);// Preserve default behavior from Gingerbread. If the animation is// undefined or explicitly specifies the Gingerbread animation style,// use a sentinel value.if (a.hasValueOrEmpty(R.styleable.PopupWindow_popupAnimationStyle)) {final int animStyle = a.getResourceId(R.styleable.PopupWindow_popupAnimationStyle, 0);if (animStyle == R.style.Animation_PopupWindow) {mAnimationStyle = ANIMATION_STYLE_DEFAULT;} else {mAnimationStyle = animStyle;}} else {mAnimationStyle = ANIMATION_STYLE_DEFAULT;}final Transition enterTransition = getTransition(a.getResourceId(R.styleable.PopupWindow_popupEnterTransition, 0));final Transition exitTransition;if (a.hasValueOrEmpty(R.styleable.PopupWindow_popupExitTransition)) {exitTransition = getTransition(a.getResourceId(R.styleable.PopupWindow_popupExitTransition, 0));} else {exitTransition = enterTransition == null ? null : enterTransition.clone();}
//4，a.recycle();
//5，读取资源进行相关设置setEnterTransition(enterTransition);//设置动画setExitTransition(exitTransition);setBackgroundDrawable(bg);//设置背景}

 public PopupWindow(View contentView, int width, int height, boolean focusable) {if (contentView != null) {
//1，获取ContextmContext = contentView.getContext();
//2，获取WindowManagermWindowManager = (WindowManager) mContext.getSystemService(Context.WINDOW_SERVICE);}
//3，构造器参数setContentView(contentView);setWidth(width);setHeight(height);setFocusable(focusable);}

然后其他的构造器都是这两个构造器的参数依次设为了默认。这两组构造器侧重点不同，在使用的时候可以根据情景需要和使用习惯进行选择。上面的第一组构造器方法在我们平时的自定义view中比较典型，所以如果记住会在自定义view中有所帮助。并且在其中值得注意的是在TypedArray后的recycle()调用，在TypedArray后调用recycle主要是为了缓存Style中属性，重复使用。当recycle被调用后，这就说明这个对象从现在可以被重用了。TypedArray 内部持有部分数组，它们缓存在Resources类中的静态字段中，这样就不用每次使用前都需要分配内存。recycle()源码为：

 /*** Give back a previously retrieved StyledAttributes, for later re-use.*/public void recycle() {synchronized (mResources.mTmpValue) {TypedArray cached = mResources.mCachedStyledAttributes;if (cached == null || cached.mData.length < mData.length) {mXml = null;mResources.mCachedStyledAttributes = this;}}}

接下来是一些属性方法的set和get方法，然后这个类中的比较重要的逻辑方法：

public void showAtLocation(IBinder token, int gravity, int x, int y) {
//1if (isShowing() || mContentView == null) {return;}
//2TransitionManager.endTransitions(mDecorView);
//3unregisterForScrollChanged();
//4mIsShowing = true;mIsDropdown = false;
//5final WindowManager.LayoutParams p = createPopupLayoutParams(token);preparePopup(p);
//6// Only override the default if some gravity was specified.if (gravity != Gravity.NO_GRAVITY) {p.gravity = gravity;}p.x = x;p.y = y;
//7invokePopup(p);}

和

  public void showAsDropDown(View anchor, int xoff, int yoff, int gravity) {
//1 确定没出现并且view不为空if (isShowing() || mContentView == null) {return;}
//2 动画TransitionManager.endTransitions(mDecorView);
//3 滚动监听registerForScrollChanged(anchor, xoff, yoff, gravity);
//4 相关值设置mIsShowing = true;mIsDropdown = true;
//5 在此位置准备弹出框final WindowManager.LayoutParams p = createPopupLayoutParams(anchor.getWindowToken());preparePopup(p);
//6 更新，，，final boolean aboveAnchor = findDropDownPosition(anchor, p, xoff, yoff, gravity);updateAboveAnchor(aboveAnchor);
//7 唤起弹出框invokePopup(p);}

showAtLocation()弹出框是在父控件绝对位置显示，showAsDropDown()是在相对位置出现。比较这两个方法可以发现，大致流程相似，某些细节不同。
先看此第三步骤，滚动监听。在类的开头部分有一个匿名内部类，作用为滚动监听：

//匿名内部类private final OnScrollChangedListener mOnScrollChangedListener = new OnScrollChangedListener() {@Overridepublic void onScrollChanged() {final View anchor = mAnchor != null ? mAnchor.get() : null;if (anchor != null && mDecorView != null) {final WindowManager.LayoutParams p = (WindowManager.LayoutParams)mDecorView.getLayoutParams();updateAboveAnchor(findDropDownPosition(anchor, p, mAnchorXoff, mAnchorYoff,mAnchoredGravity));update(p.x, p.y, -1, -1, true);}}};

然后再类中有注册监听和取消监听的私有方法：

 private void unregisterForScrollChanged() {final WeakReference<View> anchorRef = mAnchor;final View anchor = anchorRef == null ? null : anchorRef.get();if (anchor != null) {final ViewTreeObserver vto = anchor.getViewTreeObserver();vto.removeOnScrollChangedListener(mOnScrollChangedListener);}mAnchor = null;}private void registerForScrollChanged(View anchor, int xoff, int yoff, int gravity) {unregisterForScrollChanged();mAnchor = new WeakReference<>(anchor);final ViewTreeObserver vto = anchor.getViewTreeObserver();if (vto != null) {vto.addOnScrollChangedListener(mOnScrollChangedListener);}mAnchorXoff = xoff;mAnchorYoff = yoff;mAnchoredGravity = gravity;}

第五步骤的不同之处是在不同的位置上准备弹出框，首先获取位置p,然后调用preparePopup(p)方法准备弹出框：

 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;}

最大的不同在于第六步，showAtLocation的简单，比较一目了然，相比之下showAsDropDown的就比较复杂，显示调用findDropDownPosition()，然后调用updateAboveAnchor()：

private boolean findDropDownPosition(View anchor, WindowManager.LayoutParams p, int xoff,int yoff, int gravity) {final int anchorHeight = anchor.getHeight();final int anchorWidth = anchor.getWidth();if (mOverlapAnchor) {yoff -= anchorHeight;}anchor.getLocationInWindow(mDrawingLocation);p.x = mDrawingLocation[0] + xoff;p.y = mDrawingLocation[1] + anchorHeight + yoff;final int hgrav = Gravity.getAbsoluteGravity(gravity, anchor.getLayoutDirection())& Gravity.HORIZONTAL_GRAVITY_MASK;if (hgrav == Gravity.RIGHT) {// Flip the location to align the right sides of the popup and// anchor instead of left.p.x -= mPopupWidth - anchorWidth;}boolean onTop = false;p.gravity = Gravity.LEFT | Gravity.TOP;anchor.getLocationOnScreen(mScreenLocation);final Rect displayFrame = new Rect();anchor.getWindowVisibleDisplayFrame(displayFrame);final int screenY = mScreenLocation[1] + anchorHeight + yoff;final View root = anchor.getRootView();if (screenY + mPopupHeight > displayFrame.bottom|| p.x + mPopupWidth - root.getWidth() > 0) {// If the drop down disappears at the bottom of the screen, we try// to scroll a parent scrollview or move the drop down back up on// top of the edit box.if (mAllowScrollingAnchorParent) {final int scrollX = anchor.getScrollX();final int scrollY = anchor.getScrollY();final Rect r = new Rect(scrollX, scrollY, scrollX + mPopupWidth + xoff,scrollY + mPopupHeight + anchorHeight + yoff);anchor.requestRectangleOnScreen(r, true);}// Now we re-evaluate the space available, and decide from that// whether the pop-up will go above or below the anchor.anchor.getLocationInWindow(mDrawingLocation);p.x = mDrawingLocation[0] + xoff;p.y = mDrawingLocation[1] + anchorHeight + yoff;// Preserve the gravity adjustment.if (hgrav == Gravity.RIGHT) {p.x -= mPopupWidth - anchorWidth;}// Determine whether there is more space above or below the anchor.anchor.getLocationOnScreen(mScreenLocation);onTop = (displayFrame.bottom - mScreenLocation[1] - anchorHeight - yoff) <(mScreenLocation[1] - yoff - displayFrame.top);if (onTop) {p.gravity = Gravity.LEFT | Gravity.BOTTOM;p.y = root.getHeight() - mDrawingLocation[1] + yoff;} else {p.y = mDrawingLocation[1] + anchorHeight + yoff;}}if (mClipToScreen) {final int displayFrameWidth = displayFrame.right - displayFrame.left;final int right = p.x + p.width;if (right > displayFrameWidth) {p.x -= right - displayFrameWidth;}if (p.x < displayFrame.left) {p.x = displayFrame.left;p.width = Math.min(p.width, displayFrameWidth);}if (onTop) {final int popupTop = mScreenLocation[1] + yoff - mPopupHeight;if (popupTop < 0) {p.y += popupTop;}} else {p.y = Math.max(p.y, displayFrame.top);}}p.gravity |= Gravity.DISPLAY_CLIP_VERTICAL;return onTop;}private void updateAboveAnchor(boolean aboveAnchor) {if (aboveAnchor != mAboveAnchor) {mAboveAnchor = aboveAnchor;if (mBackground != null && mBackgroundView != null) {// If the background drawable provided was a StateListDrawable// with above-anchor and below-anchor states, use those.// Otherwise, rely on refreshDrawableState to do the job.if (mAboveAnchorBackgroundDrawable != null) {if (mAboveAnchor) {mBackgroundView.setBackground(mAboveAnchorBackgroundDrawable);} else {mBackgroundView.setBackground(mBelowAnchorBackgroundDrawable);}} else {mBackgroundView.refreshDrawableState();}}}}

第七步骤，弹出弹出框

private void invokePopup(WindowManager.LayoutParams p) {if (mContext != null) {p.packageName = mContext.getPackageName();}final PopupDecorView decorView = mDecorView;decorView.setFitsSystemWindows(mLayoutInsetDecor);setLayoutDirectionFromAnchor();mWindowManager.addView(decorView, p);if (mEnterTransition != null) {decorView.requestEnterTransition(mEnterTransition);}}private void setLayoutDirectionFromAnchor() {if (mAnchor != null) {View anchor = mAnchor.get();if (anchor != null && mPopupViewInitialLayoutDirectionInherited) {mDecorView.setLayoutDirection(anchor.getLayoutDirection());}}}

showAtLocation和showAsDropDown就到此为止了，这个类中比较重要的方法还有dismiss()，update()等；还有比较典型的属性设置方法有setContentView(View contentView)，setBackgroundDrawable(Drawable background)，设置动画的方法逻辑等，在有时候自定义view时可以拿来参考。
另外，虽然PopupWindow相当于一个view，但由于它不是继承自view或其子类，所以PopupWindow类中没有onLayout(),onDraw()等方法。
这个源码读的比较粗暴，某些细节以后知识运用更熟练了再补充吧