WindowInsets 在View下的的分发（一）

绪论

  Android 4.4后，可以通过将StatusBar和NavigationBar的背景设置为透明或者通过 getWindow().getDecorView().setSystemUiVisibility 的方式，使得 contentView 可以铺满整个DecorView。然而大多数情况下，我们并不希望有实质性的内容被StatusBar或者NavigationBar给覆盖掉，那么Android是如何处理这些看似额外的空间的分发呢，这就涉及到了WindowInsets了。

什么是WindowInsets？

  在Android源码的注释中解释为 window content 的一系列插入集合，final 型，不可修改，但后期可能继续扩展。其主要成员包括 mSystemWindowInsets， mWindowDecorInsets， mStableInsets。

• mSystemWindowInsets

  表示全窗口下，被StatusBar, NavigationBar, IME 或者其它系统窗口部分或者全部覆盖的区域。
- mWindowDecorInsets

   表示内容窗口下，被Android FrameWork提供的窗体，诸如ActionBar, TitleBar, ToolBar，部分或全部覆盖区域。
- mStableInsets

   表示全窗口下，被系统UI部分或者全部覆盖的区域。

如何理解WindowInsets

以 mSystemWindowInsets 为例：

private Rect mSystemWindowInsets;public int getSystemWindowInsetLeft() {    return mSystemWindowInsets.left;}public int getSystemWindowInsetTop() {    return mSystemWindowInsets.top;}public int getSystemWindowInsetRight() {    return mSystemWindowInsets.right;}public int getSystemWindowInsetBottom() {    return mSystemWindowInsets.bottom;}

这里的Rect的概念已经区别于View的Rect了，它的四个点已经不再表示围成矩形的坐标，而表示的是insets需要的左右的宽度，顶部和底部需要的高度。

消费Windowinsets

以 mSystemWindowInsets 为例：

private boolean mSystemWindowInsetsConsumed = false;public WindowInsets consumeSystemWindowInsets() {    final WindowInsets result = new WindowInsets(this);    result.mSystemWindowInsets = EMPTY_RECT;    result.mSystemWindowInsetsConsumed = true;    return result;}public WindowInsets consumeSystemWindowInsets(boolean left, boolean top,        boolean right, boolean bottom) {    if (left || top || right || bottom) {        final WindowInsets result = new WindowInsets(this);        result.mSystemWindowInsets = new Rect(                left ? 0 : mSystemWindowInsets.left,                top ? 0 : mSystemWindowInsets.top,                right ? 0 : mSystemWindowInsets.right,                bottom ? 0 : mSystemWindowInsets.bottom);        return result;    }    return this;}

从上可以看出，mSystemWindowInsets的消费分为全部消费和部分消费，如果不存在消费，则返回对象本身，如果消费了，则返回将消费部分置为0的对象copy。

//判断WindowInsets是否被消费掉public boolean isConsumed() {    return mSystemWindowInsetsConsumed && mWindowDecorInsetsConsumed && mStableInsetsConsumed;}

可见要消费掉WindowInsets，需要同时消耗掉 mSystemWindowInsets， mWindowDecorInsets， mStableInsets。

谁在消费WindowInsets

   从WindowInsets的注释中，可以发现两个函数

/** * @see View.OnApplyWindowInsetsListener * @see View#onApplyWindowInsets(WindowInsets) */

深入View的源码可以发现，如果View设定了OnApplyWindowInsetsListener后，会采用OnApplyWindowInsetsListener的实现来处理WindowInsets，否则才会使用onApplyWindowInsets(WindowInsets)方法来处理WindowInsets，在dispatchApplyWindowInsets(WindowInsets)中进行分发处理，代码如下：

public WindowInsets dispatchApplyWindowInsets(WindowInsets insets) {    try {        mPrivateFlags3 |= PFLAG3_APPLYING_INSETS;        if (mListenerInfo != null && mListenerInfo.mOnApplyWindowInsetsListener != null) {            return mListenerInfo.mOnApplyWindowInsetsListener.onApplyWindowInsets(this, insets);        } else {            return onApplyWindowInsets(insets);        }    } finally {        mPrivateFlags3 &= ~PFLAG3_APPLYING_INSETS;    }}

可以发现，对于View而言，会在dispatchApplyWindowInset的过程中Apply WindowInsets。对应的ViewGroup代码如下：

@Overridepublic WindowInsets dispatchApplyWindowInsets(WindowInsets insets) {    insets = super.dispatchApplyWindowInsets(insets);    if (!insets.isConsumed()) {        final int count = getChildCount();        for (int i = 0; i < count; i++) {            insets = getChildAt(i).dispatchApplyWindowInsets(insets);            if (insets.isConsumed()) {                break;            }        }    }    return insets;}

ViewGroup自身也会Apply WindowInsets，如果该过程中没有消耗掉WindowInsets，则会继续传递给 child 处理WindwInsets，如果child中消耗了WindowInsets, 则会退出分发循环。

再看一下，View自身是如何处理WindowInsets的

public WindowInsets onApplyWindowInsets(WindowInsets insets) {    if ((mPrivateFlags3 & PFLAG3_FITTING_SYSTEM_WINDOWS) == 0) {        // We weren't called from within a direct call to fitSystemWindows,        // call into it as a fallback in case we're in a class that overrides it        // and has logic to perform.        if (fitSystemWindows(insets.getSystemWindowInsets())) {            return insets.consumeSystemWindowInsets();        }    } else {        // We were called from within a direct call to fitSystemWindows.        if (fitSystemWindowsInt(insets.getSystemWindowInsets())) {            return insets.consumeSystemWindowInsets();        }    }    return insets;}

查看fitSystemWindows(Rect insets)方法

//当if条件成立时，会进入 dispatchApply逻辑，不成立则进入实际的处理逻辑fitSystemWindowsInt(insets)//如此设置的原因在于，不直接调用fitSystemWindowsInt(insets)方法，而是要经过dispatchApply后再调用protected boolean fitSystemWindows(Rect insets) {    if ((mPrivateFlags3 & PFLAG3_APPLYING_INSETS) == 0) {        if (insets == null) {            // Null insets by definition have already been consumed.            // This call cannot apply insets since there are none to apply,            // so return false.            return false;        }        // If we're not in the process of dispatching the newer apply insets call,        // that means we're not in the compatibility path. Dispatch into the newer        // apply insets path and take things from there.        try {            mPrivateFlags3 |= PFLAG3_FITTING_SYSTEM_WINDOWS;            return dispatchApplyWindowInsets(new WindowInsets(insets)).isConsumed();        } finally {            mPrivateFlags3 &= ~PFLAG3_FITTING_SYSTEM_WINDOWS;        }    } else {        // We're being called from the newer apply insets path.        // Perform the standard fallback behavior.        return fitSystemWindowsInt(insets);    }}

再看fitSystemWindowsInt(Rect insets)方法

//当View设置了fitSystemWindow = true 后， 才会处理 WindowInsets，否则，直接返回false。//private boolean fitSystemWindowsInt(Rect insets) {    if ((mViewFlags & FITS_SYSTEM_WINDOWS) == FITS_SYSTEM_WINDOWS) {        mUserPaddingStart = UNDEFINED_PADDING;        mUserPaddingEnd = UNDEFINED_PADDING;        Rect localInsets = sThreadLocal.get();        if (localInsets == null) {            localInsets = new Rect();            sThreadLocal.set(localInsets);        }        boolean res = computeFitSystemWindows(insets, localInsets);        mUserPaddingLeftInitial = localInsets.left;        mUserPaddingRightInitial = localInsets.right;        internalSetPadding(localInsets.left, localInsets.top,                    localInsets.right, localInsets.bottom);        return res;    }    return false;}//计算是否消费WindowInsetsprotected boolean computeFitSystemWindows(Rect inoutInsets, Rect outLocalInsets) {    if ((mViewFlags & OPTIONAL_FITS_SYSTEM_WINDOWS) == 0            || mAttachInfo == null            || ((mAttachInfo.mSystemUiVisibility & SYSTEM_UI_LAYOUT_FLAGS) == 0            && !mAttachInfo.mOverscanRequested)) {        outLocalInsets.set(inoutInsets);        inoutInsets.set(0, 0, 0, 0);        return true;    } else {        // The application wants to take care of fitting system window for        // the content...  however we still need to take care of any overscan here.        final Rect overscan = mAttachInfo.mOverscanInsets;        outLocalInsets.set(overscan);        inoutInsets.left -= overscan.left;        inoutInsets.top -= overscan.top;        inoutInsets.right -= overscan.right;        inoutInsets.bottom -= overscan.bottom;        return false;    }}//重新调整View的padding值protected void internalSetPadding(int left, int top, int right, int bottom) {    //省略部分非关键代码    ...    if (mPaddingLeft != left) {        changed = true;        mPaddingLeft = left;    }    if (mPaddingTop != top) {        changed = true;        mPaddingTop = top;    }    if (mPaddingRight != right) {        changed = true;        mPaddingRight = right;    }    if (mPaddingBottom != bottom) {        changed = true;        mPaddingBottom = bottom;    }    if (changed) {        requestLayout();        invalidateOutline();    }}

以上便是WindowInsets在View和ViewGroup中处理WindowInsets的过程，ViewGroup与View之间处理WindowInsets的区别在于dispatchApplyWindowInsets(…)函数，网上大数文章将其当成一个分发逻辑看待，其实更准确的说法应该是一个消费或者分发逻辑。

总结：

WindowInsets是一个描述了屏幕上的各个插入空间的一个类，其在后期中可以扩展，WindowInsets在消耗后将不再继续传递。对于普通的View而言，要消耗WindowInsets必须先设置View的 fitsSystemWindows 的属性为true。这也是为什么对普通View层级设置fitsSystemWindows属性为true却只有一个顶层的生效而已。单对于一些特殊的View而言，则是另外一番情况了，具体将在下篇中说明。