前言

PopupWindow 和 Dialog 对于移动端来说都太平常了。 是最基础的控件， 所以对其的了解还是非常有必要的。

基本使用

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class MyPopupWindow(context: Context) : PopupWindow(context) {
    init {
        contentView = View(context)
    }
}

MyPopWindow(this).showAsDropDown(view)

不做太多的代码， 上面的代码足矣我们分析其流程。 所以我们对其进行分析下。

源码分析

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public PopupWindow(Context context) {
    this(context, null);
}

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);

    // 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();
    }

    a.recycle();

    setEnterTransition(enterTransition);
    setExitTransition(exitTransition);
    setBackgroundDrawable(bg);
}

和 Dialog 中的一样， PopupWindow 也是持有 WMS。 那设置布局等也会跟 Dialog 一样吗？ 我们接着看下 show 相关的代码。

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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);
}

public void showAsDropDown(View anchor, int xoff, int yoff, int gravity) {
    if (isShowing() || !hasContentView()) {
        return;
    }

    TransitionManager.endTransitions(mDecorView);

    attachToAnchor(anchor, xoff, yoff, gravity);

    mIsShowing = true;
    mIsDropdown = true;

    final WindowManager.LayoutParams p =
            createPopupLayoutParams(anchor.getApplicationWindowToken());
    preparePopup(p);

    final boolean aboveAnchor = findDropDownPosition(anchor, p, xoff, yoff,
            p.width, p.height, gravity, mAllowScrollingAnchorParent);
    updateAboveAnchor(aboveAnchor);
    p.accessibilityIdOfAnchor = (anchor != null) ? anchor.getAccessibilityViewId() : -1;

    invokePopup(p);
}

其中 attachToAnchor 是对相对的 view 做一些记录跟踪及保存。 中间会设置一些监听， 比如相对于的 view 发生了改变, 那么当前 popupWindow 也会跟着发生改变。 来我们来看下其实现。

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protected void attachToAnchor(View anchor, int xoff, int yoff, int gravity) {
    detachFromAnchor();

    final ViewTreeObserver vto = anchor.getViewTreeObserver();
    if (vto != null) {
        vto.addOnScrollChangedListener(mOnScrollChangedListener);
    }
    anchor.addOnAttachStateChangeListener(mOnAnchorDetachedListener);

    final View anchorRoot = anchor.getRootView();
    anchorRoot.addOnAttachStateChangeListener(mOnAnchorRootDetachedListener);
    anchorRoot.addOnLayoutChangeListener(mOnLayoutChangeListener);

    mAnchor = new WeakReference<>(anchor);
    mAnchorRoot = new WeakReference<>(anchorRoot);
    mIsAnchorRootAttached = anchorRoot.isAttachedToWindow();
    mParentRootView = mAnchorRoot;

    mAnchorXoff = xoff;
    mAnchorYoff = yoff;
    mAnchoredGravity = gravity;
}

看着很清晰了， 先移出之前的 anchor, 然后在进行设置保存记录。
我们记着往下看。

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private void preparePopup(WindowManager.LayoutParams p) {
    if (mBackground != null) {
        mBackgroundView = createBackgroundView(mContentView);
        mBackgroundView.setBackground(mBackground);
    } else {
        mBackgroundView = mContentView;
    }

    mDecorView = createDecorView(mBackgroundView);
    mDecorView.setIsRootNamespace(true);
}

看看我们看到了什么， 看到 DecorView， 还记得这个名词吧， 在 Activity 和 Dialog 是在 PhoneWindow 里创建的， 而 PopWindow 是在自身内部创建的。 

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private PopupDecorView createDecorView(View contentView) {
    final ViewGroup.LayoutParams layoutParams = mContentView.getLayoutParams();
    final int height;
    if (layoutParams != null && layoutParams.height == WRAP_CONTENT) {
        height = WRAP_CONTENT;
    } else {
        height = MATCH_PARENT;
    }

    final PopupDecorView decorView = new PopupDecorView(mContext);
    decorView.addView(contentView, MATCH_PARENT, height);
    decorView.setClipChildren(false);
    decorView.setClipToPadding(false);

    return decorView;
}

我们从 createDecorView 可以发现一个事情哈， 就是我们设置的布局不再是像 Activity 和 Dialog 一样， 添加到 id 为 content 中了， 而是直接 addView 添加了我们设置的布局。
我们可以看到 mDecorView 是 PopupDecorView。 

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private class PopupDecorView extends FrameLayout {

    public boolean dispatchKeyEvent(KeyEvent event) {
    }
    
    public boolean dispatchTouchEvent(MotionEvent ev) {
    }
    
    public boolean onTouchEvent(MotionEvent event) {
    }
    
    // ......
}

我们可以看到 PopupDecorView 是一个 FrameLayout， 其内部对事件进行了处理或者分发。
ok， 当这里， 我们还没有看到 View 添加到 WMS 进行管理显示呢， 我们接着往下看。
我们先看下 findDropDownPosition 方法。 这个方法在 invokePopup 之前。

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protected boolean findDropDownPosition(View anchor, WindowManager.LayoutParams outParams,
        int xOffset, int yOffset, int width, int height, int gravity, boolean allowScroll) {
    final int anchorHeight = anchor.getHeight();
    final int anchorWidth = anchor.getWidth();
    if (mOverlapAnchor) {
        yOffset -= anchorHeight;
    }

    // Initially, align to the bottom-left corner of the anchor plus offsets.
    final int[] appScreenLocation = mTmpAppLocation;
    final View appRootView = getAppRootView(anchor);
    appRootView.getLocationOnScreen(appScreenLocation);

    final int[] screenLocation = mTmpScreenLocation;
    anchor.getLocationOnScreen(screenLocation);

    final int[] drawingLocation = mTmpDrawingLocation;
    drawingLocation[0] = screenLocation[0] - appScreenLocation[0];
    drawingLocation[1] = screenLocation[1] - appScreenLocation[1];
    outParams.x = drawingLocation[0] + xOffset;
    outParams.y = drawingLocation[1] + anchorHeight + yOffset;

    final Rect displayFrame = new Rect();
    appRootView.getWindowVisibleDisplayFrame(displayFrame);
    if (width == MATCH_PARENT) {
        width = displayFrame.right - displayFrame.left;
    }
    if (height == MATCH_PARENT) {
        height = displayFrame.bottom - displayFrame.top;
    }

    // Let the window manager know to align the top to y.
    outParams.gravity = computeGravity();
    outParams.width = width;
    outParams.height = height;

    // If we need to adjust for gravity RIGHT, align to the bottom-right
    // corner of the anchor (still accounting for offsets).
    final int hgrav = Gravity.getAbsoluteGravity(gravity, anchor.getLayoutDirection())
            & Gravity.HORIZONTAL_GRAVITY_MASK;
    if (hgrav == Gravity.RIGHT) {
        outParams.x -= width - anchorWidth;
    }

    // First, attempt to fit the popup vertically without resizing.
    final boolean fitsVertical = tryFitVertical(outParams, yOffset, height,
            anchorHeight, drawingLocation[1], screenLocation[1], displayFrame.top,
            displayFrame.bottom, false);

    // Next, attempt to fit the popup horizontally without resizing.
    final boolean fitsHorizontal = tryFitHorizontal(outParams, xOffset, width,
            anchorWidth, drawingLocation[0], screenLocation[0], displayFrame.left,
            displayFrame.right, false);

    // If the popup still doesn't fit, attempt to scroll the parent.
    if (!fitsVertical || !fitsHorizontal) {
        final int scrollX = anchor.getScrollX();
        final int scrollY = anchor.getScrollY();
        final Rect r = new Rect(scrollX, scrollY, scrollX + width + xOffset,
                scrollY + height + anchorHeight + yOffset);
        if (allowScroll && anchor.requestRectangleOnScreen(r, true)) {
            // Reset for the new anchor position.
            anchor.getLocationOnScreen(screenLocation);
            drawingLocation[0] = screenLocation[0] - appScreenLocation[0];
            drawingLocation[1] = screenLocation[1] - appScreenLocation[1];
            outParams.x = drawingLocation[0] + xOffset;
            outParams.y = drawingLocation[1] + anchorHeight + yOffset;

            // Preserve the gravity adjustment.
            if (hgrav == Gravity.RIGHT) {
                outParams.x -= width - anchorWidth;
            }
        }

        // Try to fit the popup again and allowing resizing.
        tryFitVertical(outParams, yOffset, height, anchorHeight, drawingLocation[1],
                screenLocation[1], displayFrame.top, displayFrame.bottom, mClipToScreen);
        tryFitHorizontal(outParams, xOffset, width, anchorWidth, drawingLocation[0],
                screenLocation[0], displayFrame.left, displayFrame.right, mClipToScreen);
    }

    // Return whether the popup's top edge is above the anchor's top edge.
    return outParams.y < drawingLocation[1];
}

可以看到该方法根据传入的偏移等数据计算和更新了当前 popupWindow 的一些 x、 y 等数据。

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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);
    }
}

终于看到 WMS 添加 view 了。WMS 添加 view 的操作跟 Activity 和 Dialog 的一样， 这里不再多说了。 你可能会发现， 跟 Activity 和 Dialog 中间差了一个 window 层， 在具体点就是 popupWIndow 没有 PhoneWindow。
show 方法看过了， 我们看下 dismiss 方法。

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public void dismiss() {
    if (!isShowing() || isTransitioningToDismiss()) {
        return;
    }

    final PopupDecorView decorView = mDecorView;
    final View contentView = mContentView;

    final ViewGroup contentHolder;
    final ViewParent contentParent = contentView.getParent();
    if (contentParent instanceof ViewGroup) {
        contentHolder = ((ViewGroup) contentParent);
    } else {
        contentHolder = null;
    }

    // Ensure any ongoing or pending transitions are canceled.
    decorView.cancelTransitions();

    mIsShowing = false;
    mIsTransitioningToDismiss = true;

    // This method may be called as part of window detachment, in which
    // case the anchor view (and its root) will still return true from
    // isAttachedToWindow() during execution of this method; however, we
    // can expect the OnAttachStateChangeListener to have been called prior
    // to executing this method, so we can rely on that instead.
    final Transition exitTransition = mExitTransition;
    if (exitTransition != null && decorView.isLaidOut()
            && (mIsAnchorRootAttached || mAnchorRoot == null)) {
        // The decor view is non-interactive and non-IME-focusable during exit transitions.
        final LayoutParams p = (LayoutParams) decorView.getLayoutParams();
        p.flags |= LayoutParams.FLAG_NOT_TOUCHABLE;
        p.flags |= LayoutParams.FLAG_NOT_FOCUSABLE;
        p.flags &= ~LayoutParams.FLAG_ALT_FOCUSABLE_IM;
        mWindowManager.updateViewLayout(decorView, p);

        final View anchorRoot = mAnchorRoot != null ? mAnchorRoot.get() : null;
        final Rect epicenter = getTransitionEpicenter();

        // Once we start dismissing the decor view, all state (including
        // the anchor root) needs to be moved to the decor view since we
        // may open another popup while it's busy exiting.
        decorView.startExitTransition(exitTransition, anchorRoot, epicenter,
                new TransitionListenerAdapter() {
                    @Override
                    public void onTransitionEnd(Transition transition) {
                        dismissImmediate(decorView, contentHolder, contentView);
                    }
                });
    } else {
        dismissImmediate(decorView, contentHolder, contentView);
    }

    // Clears the anchor view.
    detachFromAnchor();

    if (mOnDismissListener != null) {
        mOnDismissListener.onDismiss();
    }
}

private void dismissImmediate(View decorView, ViewGroup contentHolder, View contentView) {
    // If this method gets called and the decor view doesn't have a parent,
    // then it was either never added or was already removed. That should
    // never happen, but it's worth checking to avoid potential crashes.
    if (decorView.getParent() != null) {
        mWindowManager.removeViewImmediate(decorView);
    }

    if (contentHolder != null) {
        contentHolder.removeView(contentView);
    }

    // This needs to stay until after all transitions have ended since we
    // need the reference to cancel transitions in preparePopup().
    mDecorView = null;
    mBackgroundView = null;
    mIsTransitioningToDismiss = false;
}

可以看到基本都是类似的， 调用 WMS 的 removeViewImmediate 方法。