10fitsSystemWindows对CoordinatorLayout的影响

10fitsSystemWindows对CoordinatorLayout的影响

在提到过，为了让CollapsingToolbarLayout内部的伪状态栏和真正的statusbar重合， CoordinatorLayout和AppBarLayout的fitsSystemWindows应该一样，同时为true或者false。

现在来具体分析一下，各种case。对应Activity： CollapsFitSystemActivity

CoordinatorLayout和AppBarLayout的fitsSystemWindows都为true

此时CoordinatorLayout包含状态栏，AppBarLayout也包含状态，CollapsingToolbarLayout内的伪状态栏和实际状态栏在同样位置。

CoordinatorLayout的fitsSystemWindows为true，AppBarLayout的fitsSystemWindows为false

此时CoordinatorLayout包含状态栏，AppBarLayout不包含状态栏，会出现伪状态栏和实际状态栏不在一起，就显示了2条状态栏，

CoordinatorLayout的fitsSystemWindows为false，AppBarLayout的fitsSystemWindows为false

此时CoordinatorLayout不包括状态栏，AppBarLayout不包含状态栏,伪状态栏不会绘制。

为何CoordinatorLayout的fitsSystemWindows为false，CoordinatorLayout就不包括状态栏

CoordinatorLayout的fitsSystemWindows

这个值设置为true或者false，会有什么意义呢？为true，CoordinatorLayout就包括状态栏，否则就不包括状态栏。简单一句话，背后有一大堆代码来实现这个。

影响attachInfo.mSystemUiVisibility

首先看CoordinatorLayout构造函数里面有下边这段代码，setupForWindowInsets这明显就是处理系统边界的。

//CoordinatorLayout构造函数
if (INSETS_HELPER != null) {
INSETS_HELPER.setupForWindowInsets(this, new ApplyInsetsListener());
}

看INSETS_HELPER.setupForWindowInsets的具体实现，在CoordinatorLayoutInsetsHelperLollipop内，这里可以看出如果设置为true，那么就会加上View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN这个flag顾名思义代表全屏的意思。所以CoordinatorLayout 的mSystemUiVisibility内有SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN标志。

//CoordinatorLayoutInsetsHelperLollipop
public void setupForWindowInsets(View view, OnApplyWindowInsetsListener insetsListener) {
if (ViewCompat.getFitsSystemWindows(view)) {
// First apply the insets listener
ViewCompat.setOnApplyWindowInsetsListener(view, insetsListener);
// Now set the sys ui flags to enable us to lay out in the window insets
view.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_STABLE
| View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN);
}
}

而在view的performCollectViewAttributes代码里，把 attachInfo.mSystemUiVisibility |= mSystemUiVisibility;，所以attachInfo.mSystemUiVisibility里面也设置了SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN标志。attachInfo是所有view共享的，从ViewRootImpl dispatch下来。

void performCollectViewAttributes(AttachInfo attachInfo, int visibility) {
if ((visibility & VISIBILITY_MASK) == VISIBLE) {
if ((mViewFlags & KEEP_SCREEN_ON) == KEEP_SCREEN_ON) {
attachInfo.mKeepScreenOn = true;
}
attachInfo.mSystemUiVisibility |= mSystemUiVisibility;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnSystemUiVisibilityChangeListener != null) {
attachInfo.mHasSystemUiListeners = true;
}
}
}

dispatchApplyInsets

DecorView的直接子view LinearLayout

ViewRootImpl在performTraversals时会调dispatchApplyInsets，内调DecorView的dispatchApplyWindowInsets,然后调DecorView的child的dispatchApplyWindowInsets，DecorView主要child是LinearLayout。所以会调用LinearLayout的dispatchApplyWindowInsets。LinearLayout的dispatchApplyWindowInsets会调用super.dispatchApplyWindowInsets。

//ViewGroup
@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;
}

所以调了view的dispatchApplyWindowInsets，会走L7 onApplyWindowInsets

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;
}
}

这里走L6 fitSystemWindows，然后内部调用fitSystemWindowsInt，

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;
}

fitSystemWindowsInt里主要看 computeFitSystemWindows和internalSetPadding。

//View
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;
//根据localInsets设置padding
internalSetPadding(localInsets.left, localInsets.top,
localInsets.right, localInsets.bottom);
return res;
}
return false;
}

内调computeFitSystemWindows(此时this为DecorView的子view LinearLayout)，看这里，这里的if有3个条件，我们可以简单点无视前面2个条件，这样就变为

((mAttachInfo.mSystemUiVisibility & SYSTEM_UI_LAYOUT_FLAGS) == 0

&& !mAttachInfo.mOverscanRequested)

其实!mAttachInfo.mOverscanRequested一般为true，所以进一步简化条件变为(mAttachInfo.mSystemUiVisibility & SYSTEM_UI_LAYOUT_FLAGS) == 0

再看

public static final int SYSTEM_UI_LAYOUT_FLAGS =
SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
| SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN;

SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION肯定是false，所以我们只要看SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN就好了，条件进一步简化

mAttachInfo.mSystemUiVisibility & SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN==0

而由上文知，如果fitsSystemWindows 设置为true的话，mAttachInfo.mSystemUiVisibility 内会设置SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN，所以此时if不满足走下边else。

如果fitsSystemWindows设置为false，那就走if，把 outLocalInsets.set(inoutInsets);，这样上边的localInsets.top就是63（状态栏高度），在internalSetPadding内会设置LinearLayout的paddingTop为63，这样CoordinatorLayout就不可能包括状态栏了。

而如果CoordinatorLayout的fitsSystemWindows为true，那么下边会走else， LinearLayout的paddingTop也为0，所以CoordinatorLayout就包括状态栏了。

/**
* @hide Compute the insets that should be consumed by this view and the ones
* that should propagate to those under it.
*/
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)) {
//CoordinatorLayout的fitsSystemWindows为false
outLocalInsets.set(inoutInsets);
inoutInsets.set(0, 0, 0, 0);
return true;
} else {
//CoordinatorLayout的fitsSystemWindows为true
// 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;
}
}

fitsSystemWindows设为true和false的区别主要在于DecorView的子view LinearLayout的padding设置不同。所以第三层的FrameLayout的高度就不一样了。这就是CoordinatorLayout是否包含状态栏的原因。

为何CoordinatorLayout的fitsSystemWindows为false，AppBarLayout的fitsSystemWindows为false，不会出现双状态栏呢？

首先我们要看看CollapsingToolbarLayout的伪状态栏绘制有什么前提，必须topInset>0, mLastInsets不为null。那mLastInsets是什么时候赋值的呢？

//android.support.design.widget.CollapsingToolbarLayout#draw
// Now draw the status bar scrim
if (mStatusBarScrim != null && mScrimAlpha > 0) {
final int topInset = mLastInsets != null ? mLastInsets.getSystemWindowInsetTop() : 0;
if (topInset > 0) {
mStatusBarScrim.setBounds(0, -mCurrentOffset, getWidth(),
topInset - mCurrentOffset);
mStatusBarScrim.mutate().setAlpha(mScrimAlpha);
mStatusBarScrim.draw(canvas);
}
}

构造函数里注册了个回调，这是在dispatchApplyInsets调用的，必须调到CollapsingToolbarLayout的dispatchApplyInsets才可能有mLastInsets。

//CollapsingToolbarLayout构造函数
ViewCompat.setOnApplyWindowInsetsListener(this,
new android.support.v4.view.OnApplyWindowInsetsListener() {
@Override
public WindowInsetsCompat onApplyWindowInsets(View v,
WindowInsetsCompat insets) {
mLastInsets = insets;
requestLayout();
return insets.consumeSystemWindowInsets();
}
});

在ViewRootImpl的dispatchApplyInsets过程中，会发给DecorView的直接子view，LinearLayout，如果CoordinatorLayout的fitsSystemWindows为false，那么 LinearLayout在L3会消耗掉这个inset，直接返回true，而不会继续往

LinearLayout的子view分发，所以CollapsingToolbarLayout内的mLastInsets必定为空，那伪状态栏根本不会绘制。

@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;
}

总结

1、CoordinatorLayout的fitsSystemWindows为false，那CollapsingToolbarLayout就必定不会绘制伪状态栏，因为mLastInsets为null。

参考资料

https://my.oschina.net/KobeGong/blog/469059