闲来无事把popupwindow的源码看了一遍，能力有限只看懂了一部分。下面我们来看看源码：
<span style="font-size:14px;">private int mWindowLayoutType = WindowManager.LayoutParams.TYPE_APPLICATION_PANEL;</span>
分析：从这里可以看出来PopupWindow(弹出窗口)实际上是一个子窗口，它是一个独立的类（并不继承于Window）。
<span style="font-size:14px;">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);
}
</span>
分析：这个一看就很清楚，这是它的构造函数，里面通过context对象获取了windowManager对象和属性typedArray和一些动画style，a.recycle是释放
typedArray。里面还有很多构造函数，主要是对它进行一些参数的初始化，比如设置contentView、宽、高、焦点等。详细请看源码。
接下来主要介绍的是我们经常要用到的显示popupwindow位置的几个方法。因为严格来说这个PopuWindow就是用来在指定位置显示一个View的。主要方法有下面：
public void showAsDropDown(View anchor)
showAsDropDown(View anchor, int xoff, int yoff)
showAtLocation(View parent, int gravity, int x, int y)
下面来看看他们的源码：
public void showAsDropDown(View anchor) {
showAsDropDown(anchor, 0, 0);
}
public void showAsDropDown(View anchor, int xoff, int yoff) {
showAsDropDown(anchor, xoff, yoff, DEFAULT_ANCHORED_GRAVITY);
}<pre name="code" class="java">public void showAtLocation(View parent, int gravity, int x, int y) {
showAtLocation(parent.getWindowToken(), gravity, x, y);
}
第一个调用的是第二个方法，所以我们直接分析第二个方法：
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);
}
<pre name="code" class="java">public void showAtLocation(IBinder token, int gravity, int x, int y) {
if (isShowing() || mContentView == null) {
return;
}
unregisterForScrollChanged();
mIsShowing = true;
mIsDropdown = false;
WindowManager.LayoutParams p = createPopupLayout(token);
p.windowAnimations = computeAnimationResource();
preparePopup(p);
if (gravity == Gravity.NO_GRAVITY) {
gravity = Gravity.TOP | Gravity.START;
}
p.gravity = gravity;
p.x = x;
p.y = y;
if (mHeightMode < 0) p.height = mLastHeight = mHeightMode;
if (mWidthMode < 0) p.width = mLastWidth = mWidthMode;
invokePopup(p);
}
分析：
先解释下参数：
anchor:就是PopupWindow要显示位置的参照物
xoff: PopupWindow相对于anchor的左下角x轴方向的偏移大小
yoff: PopupWindow相对于anchor的左下角y轴方向的偏移大小
我觉得这两个方法的区别就是showAtLocation能够设置popupwindow显示在参照物view的哪个方位然后设置偏移值，而showAsDropDown则是默认弹出窗显示在参照物view的左下角，然后设置偏移值。代码具体分析：
刚开始isShowing()判断当前PopupWindow是否显示，或者mContentView是否为空，是的话就不用去继续执行下面的代码了。然后unregisterForScrollChanged()，为了滑动改变注册，就是有可能anchor有滑动，或者ContentView过大，有的地方放不开。
private void unregisterForScrollChanged() {
WeakReference<View> anchorRef = mAnchor;
View anchor = null;
if (anchorRef != null) {
anchor = anchorRef.get();
}
if (anchor != null) {
ViewTreeObserver vto = anchor.getViewTreeObserver();
vto.removeOnScrollChangedListener(mOnScrollChangedListener);
}
mAnchor = null;
}
就是注册一下滑动的全局的监听，注册之前先注销一下之前的注册，防止ViewTreeObserver失效。
在这里使用了一个弱引用对anchor，防止anchor这个类已经无用了，anchor仍然无法回收内存。
接下来为PopupWindow设置显示参数，调用了createPopupLayout(anchor.getWindowToken())
看下这个方法：
private WindowManager.LayoutParams createPopupLayout(IBinder token) {
// generates the layout parameters for the drop down
// we want a fixed size view located at the bottom left of the anchor
WindowManager.LayoutParams p = new WindowManager.LayoutParams();<span style="color:#ff0000;">//<span style="font-family: Simsun; line-height: 24px;">创建一个WindowManager.LayoutParams的实例</span></span>
// these gravity settings put the view at the top left corner of the
// screen. The view is then positioned to the appropriate location
// by setting the x and y offsets to match the anchor's bottom
// left corner
p.gravity = Gravity.START | Gravity.TOP;<span style="color:#ff0000;">//接下来为实例p设置了一系列的参数</span>
p.width = mLastWidth = mWidth;
p.height = mLastHeight = mHeight;
if (mBackground != null) {
p.format = mBackground.getOpacity();
} else {
p.format = PixelFormat.TRANSLUCENT;
}
p.flags = computeFlags(p.flags);
p.type = mWindowLayoutType;
p.token = token;
p.softInputMode = mSoftInputMode;
p.setTitle("PopupWindow:" + Integer.toHexString(hashCode()));
return p;
}
接下来分析方法：invokePopup(p)，它才是真正的把PopupWindow显示在特定的位置上。其实就使用Activity中的WindowManager对象对mPopupView进行了添加显示。
<span style="font-size:14px;">private void invokePopup(WindowManager.LayoutParams p) {
if (mContext != null) {
p.packageName = mContext.getPackageName();
}
mPopupView.setFitsSystemWindows(mLayoutInsetDecor);
setLayoutDirectionFromAnchor();
mWindowManager.addView(mPopupView, p);//<span style="color: rgb(255, 0, 0); font-family: Arial; line-height: 26px;">调用addView完成添加</span>
}</span>