android:fitSystemWindows详解

从Android 4.4开始，Android系统加入了一个比较酷的功能，就是我们可以设置状态栏的的颜色了，有个这个功能，状态栏就不再是黑乎乎的了，我们就可以根据我们应用的主色去设置状态栏的颜色，使得应用体验变得好一些，所以我们通过如下方式设置状态栏透明。

window.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS, WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);
window.setFlags(WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION, WindowManager.LayoutParams.FLAG_TRANSLUCENT_NAVIGATION);

但是设置了状态栏和导航栏透明之后，发现Activity的contentView超出了ActionBar，那么我们就要使用fitSystemWindws来解决这个问题，关于具体如何解决这个问题，在Android4.4新的特性，在应用内开启透明状态栏和透明虚拟按钮这篇博客中有详细介绍。



那么android:fitSystemWindows到底是什么东西啊，它是怎样计算的？

在Android Framework的源代码中查看View.java，有这几个重要方法，如下：

dispatchApplyWindowInsets

onApplyWindowInsets

fitSystemWindows

是一个自定义的View，并且覆盖View的这三个方法，打印出Log，可以发现这三个方法的调用顺序是

dispatchApplyWindowInsets

onApplyWindowInsets

fitSystemWindows

这里我们通过代码调试的方法来查看Android Framework的方法调用堆栈，如图：



根据堆栈可以发现，dispatchApplyWindowInsets方法是在ViewRootImpl.performMeasure(int,int)方法中调用的。也就是说，dispatchApplyWindowInsets是在整个View Hierarchy的measure过程中调用的。

在从layout文件中解释到fitSystemWindows为true设置标志位。那这个标志为在什么时候起的作用。

case com.android.internal.R.styleable.View_fitsSystemWindows:
if (a.getBoolean(attr, false)) {
viewFlagValues |= FITS_SYSTEM_WINDOWS;
viewFlagMasks |= FITS_SYSTEM_WINDOWS;
}
break;

再看上面那幅堆栈图，在ActionBarOverlayLayout.measure()中开始调用fitSystemWindows的相关方法，进入ActionBarOverlayLayout的源码。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//通过findViewById方法，得到布局文件中的view
pullChildren();

int maxHeight = 0;
int maxWidth = 0;
int childState = 0;

int topInset = 0;
int bottomInset = 0;
//测量ActionBar的高度和宽度
measureChildWithMargins(mActionBarTop, widthMeasureSpec, 0, heightMeasureSpec, 0);
LayoutParams lp = (LayoutParams) mActionBarTop.getLayoutParams();
maxWidth = Math.max(maxWidth,
mActionBarTop.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
mActionBarTop.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = ViewUtils.combineMeasuredStates(childState,
ViewCompat.getMeasuredState(mActionBarTop));

// xlarge screen layout doesn't have bottom action bar.
//测量ActionBar底部区域的高度和宽度
if (mActionBarBottom != null) {
measureChildWithMargins(mActionBarBottom, widthMeasureSpec, 0, heightMeasureSpec, 0);
lp = (LayoutParams) mActionBarBottom.getLayoutParams();
maxWidth = Math.max(maxWidth,
mActionBarBottom.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
mActionBarBottom.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = ViewUtils.combineMeasuredStates(childState,
ViewCompat.getMeasuredState(mActionBarBottom));
}

final int vis = ViewCompat.getWindowSystemUiVisibility(this);
final boolean stable = (vis & SYSTEM_UI_FLAG_LAYOUT_STABLE) != 0;

if (stable) {
// This is the standard space needed for the action bar.  For stable measurement,
// we can't depend on the size currently reported by it -- this must remain constant.
topInset = mActionBarHeight;
//考虑到ActionbarTab的高度，计算topInset
if (mHasNonEmbeddedTabs) {
final View tabs = mActionBarTop.getTabContainer();
if (tabs != null) {
// If tabs are not embedded, increase space on top to account for them.
topInset += mActionBarHeight;
}
}
} else if (mActionBarTop.getVisibility() != GONE) {
// This is the space needed on top of the window for all of the action bar
// and tabs.
topInset = mActionBarTop.getMeasuredHeight();
}
//如果ActionBar是split模式，考虑底部的高度，计算insetBottom
if (mDecorToolbar.isSplit()) {
// If action bar is split, adjust bottom insets for it.
if (mActionBarBottom != null) {
if (stable) {
bottomInset = mActionBarHeight;
} else {
bottomInset = mActionBarBottom.getMeasuredHeight();
}
}
}

// If the window has not requested system UI layout flags, we need to
// make sure its content is not being covered by system UI...  though it
// will still be covered by the action bar if they have requested it to
// overlay.
mContentInsets.set(mBaseContentInsets);
mInnerInsets.set(mBaseInnerInsets);
if (!mOverlayMode && !stable) {
mContentInsets.top += topInset;
mContentInsets.bottom += bottomInset;
} else {
mInnerInsets.top += topInset;
mInnerInsets.bottom += bottomInset;

dc56
}
//在ActionBar为非Overlay模式下，应用计算好的ContentInsets
applyInsets(mContent, mContentInsets, true, true, true, true);
//
if (!mLastInnerInsets.equals(mInnerInsets)) {
// If the inner insets have changed, we need to dispatch this down to
// the app's fitSystemWindows().  We do this before measuring the content
// view to keep the same semantics as the normal fitSystemWindows() call.
mLastInnerInsets.set(mInnerInsets);

//Overlay模式下，将InnerInsets分发到子View中。            mContent.dispatchFitSystemWindows(mInnerInsets);
}

measureChildWithMargins(mContent, widthMeasureSpec, 0, heightMeasureSpec, 0);
lp = (LayoutParams) mContent.getLayoutParams();
maxWidth = Math.max(maxWidth,
mContent.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
mContent.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = ViewUtils.combineMeasuredStates(childState,
ViewCompat.getMeasuredState(mContent));

// Account for padding too
maxWidth += getPaddingLeft() + getPaddingRight();
maxHeight += getPaddingTop() + getPaddingBottom();

// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

setMeasuredDimension(
ViewCompat.resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
ViewCompat.resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
}

根据上面的堆栈图知道mContent是NativeActionModeAwareLayout类型，而NativeActionModeAwareLayout，没有dispatchFitSystemWindows方法，那么查看其父类的dispatchFitSystemWindows方法。NativeActionModeAwareLayout的父类是ContentFrameLayout类型，看它的dispatchFitSystemWindows方法。

public void dispatchFitSystemWindows(Rect insets) {
fitSystemWindows(insets);
}

它直接调用View的fitSystemWindows方法。看View的fitSystemWindows方法。

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;
}

try {
mPrivateFlags3 |= PFLAG3_FITTING_SYSTEM_WINDOWS;
return dispatchApplyWindowInsets(new WindowInsets(insets)).isConsumed();
} finally {
mPrivateFlags3 &= ~PFLAG3_FITTING_SYSTEM_WINDOWS;
}
} else {
return fitSystemWindowsInt(insets);
}
}

由于第一次调用，这里的mPrivateFlags3 的PFLAG3_APPLYING_INSETS标志为不为1，所以进入if条件。进入dispatchApplyWindowInsets方法。并将mPrivateFlags3 的PFLAG3_APPLYING_INSETS标志为置为1。由于ContentFrameLayout继承了Framelayout ,所以进入了ViewGroup的dispatchApplyWindowInsets方法。

@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;
}

首先调用了 super.dispatchApplyWindowInsets方法，也就是View的 dispatchApplyWindowInsets，

然后如果insets没有被消费掉的话，分别调用每个view child的dispatchApplyWindowInsets方法，让子view去消费它，如果子view消费了，那么到此结束。先执行 super.dispatchApplyWindowInsets方法。

下面是View.dispatchApplyWindowInsets方法。

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;
}
}

在此方法中首先将mPrivateFlags3 的PFLAG3_APPLYING_INSETS标志位置为1，然后如果开发者设置了listener的话就调用listener，否则调用onApplyWindowInsets方法。

public WindowInsets onApplyWindowInsets(WindowInsets insets) {
if ((mPrivateFlags3 & PFLAG3_FITTING_SYSTEM_WINDOWS) == 0) {
if (fitSystemWindows(insets.getSystemWindowInsets())) {
return insets.consumeSystemWindowInsets();
}
} else {
if (fitSystemWindowsInt(insets.getSystemWindowInsets())) {
return insets.consumeSystemWindowInsets();
}
}
return insets;
}

在第一次调用fitSystemWindows方法后，mPrivateFlags3 得 PFLAG3_FITTING_SYSTEM_WINDOWS标志为被置位1了，所以进入fitSystemWindowsInt方法。

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;
}

在这个方法中，两个关键函数

computeFitSystemWindows

internalSetPadding

先看computeFitSystemWindows。官方解释是，计算insets应该被此view消费掉还是继续传递。

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;
}
}

再看internalSetPadding

此方法是设置view的padding。

protected void internalSetPadding(int left, int top, int right, int bottom) {
mUserPaddingLeft = left;
mUserPaddingRight = right;
mUserPaddingBottom = bottom;

final int viewFlags = mViewFlags;
boolean changed = false;

// Common case is there are no scroll bars.
if ((viewFlags & (SCROLLBARS_VERTICAL|SCROLLBARS_HORIZONTAL)) != 0) {
if ((viewFlags & SCROLLBARS_VERTICAL) != 0) {
final int offset = (viewFlags & SCROLLBARS_INSET_MASK) == 0
? 0 : getVerticalScrollbarWidth();
switch (mVerticalScrollbarPosition) {
case SCROLLBAR_POSITION_DEFAULT:
if (isLayoutRtl()) {
left += offset;
} else {
right += offset;
}
break;
case SCROLLBAR_POSITION_RIGHT:
right += offset;
break;
case SCROLLBAR_POSITION_LEFT:
left += offset;
break;
}
}
if ((viewFlags & SCROLLBARS_HORIZONTAL) != 0) {
bottom += (viewFlags & SCROLLBARS_INSET_MASK) == 0
? 0 : getHorizontalScrollbarHeight();
}
}

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();
}
}