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。

WindowInsets 在View下的的分发（二）

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：

@Override
public 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WindowInsets
protected 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。