1.CoordinatorLayout是什么？
CoordinatorLayout is a super-powered FrameLayout.
在谷歌官方文档中解释CoordinatorLayout是一个超级帧布局，可在两种情况下使用：
1.作为窗口布局的顶层父布局
2.作为与一个或者多个子视图进行交互的容器
通过给CoordinatorLayout的直接子控件指定一个Behavior可以在不同的兄弟控件之间得到许多不同的交互，Behaviors可以被用来实现各种各样的交互动效，
例如：滑动列表时的悬浮按钮自动显示与隐藏，滑动时头部控件的缩放、位移等。
2.CoordinatorLayout是如何协调子控件进行交互的？
CoordinatorLayout本身不具备实际交互能力，所有的交互行为都会被分发给子view的Behavior去实现，如果子view都没有指定Behavior，只能作为一个FrameLayout存在。
CoordinatorLayout实现了自己的LayoutParams，而Behavior就存储在LayoutParams中，由于子控件的LayoutParams类型是由父控件决定的，所以能拥有CoordinatorLayout.LayoutParams的控件只能是CoordinatorLayout的直接子view。
这在一定程度上也使得交互产生了一定的局限性。可以弥补这个局限性的功能是CoordinatorLayout实现了NestedScrollingParent接口，而嵌套滚动的实现是可以多层级传递事件的。和普通实现NestedScrollingParent的父控件向上传递事件的行为不同的是
CoordinatorLayout会向下传递滚动事件，依然是传递到Behavior中。
所以一个实现了NestedScrollingChild接口的view可以直接包含在CoordinatorLayout的直接子view中，且可以多层级包含，而CoordinatorLayout的直接子view不需要再次实现NestedScrollingParent接口。
总结一句话就是CoordinatorLayout会拦截作用于它的一切行为并分发给它的所有直接子view。这使得不同子view之间的行为相互影响成为可能。
3.CoordinatorLayout是如何拦截行为并分发的？
这里我们可以进入源码先观察一下CoordinatorLayout是如何测量并布局子view的，如下：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
prepareChildren(); //准备子view
ensurePreDrawListener();
//省略若干代码...
final int childCount = mDependencySortedChildren.size();
for (int i = 0; i < childCount; i++) {
final View child = mDependencySortedChildren.get(i);
if (child.getVisibility() == GONE) {
// If the child is GONE, skip...
continue;
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//省略若干代码...
final Behavior b = lp.getBehavior();
//如果子view的Behavior为空，或者子view的测量方法返回false就使用CoordinatorLayout默认的
//onMeasureChild方式进行测量。如果子view的onMeasureChild返回true表示子view自己已经进行了测量。
if (b == null || !b.onMeasureChild(this, child, childWidthMeasureSpec, keylineWidthUsed,
childHeightMeasureSpec, 0)) {
onMeasureChild(child, childWidthMeasureSpec, keylineWidthUsed,
childHeightMeasureSpec, 0);
}
}
//省略若干代码...
setMeasuredDimension(width, height);
}
以上可看见CoordinatorLayout实际上在自己的各个行为处理方法中优先通知询问子view是否要先处理，如果子view处理了，则不会再处理。
但是这只是把处理权限赋予了子view，在每个子view之间并没有建立相互的的行为关联。但在以上方法中有两处关键信息不能忽视，就是prepareChildren()和ensurePreDrawListener()两个方法。
下面继续追踪源码：
private void prepareChildren() {
//mDependencySortedChildren用于存储有依赖于其他子view行为的子view
mDependencySortedChildren.clear();
mChildDag.clear();
for (int i = 0, count = getChildCount(); i < count; i++) {
final View view = getChildAt(i);
final LayoutParams lp = getResolvedLayoutParams(view);
lp.findAnchorView(this, view);
mChildDag.addNode(view);
// Now iterate again over the other children, adding any dependencies to the graph
for (int j = 0; j < count; j++) {
if (j == i) {
continue;
}
final View other = getChildAt(j);
if (lp.dependsOn(this, view, other)) {
if (!mChildDag.contains(other)) {
// Make sure that the other node is added
mChildDag.addNode(other);
}
// Now add the dependency to the graph
mChildDag.addEdge(other, view);
}
}
}
// Finally add the sorted graph list to our list
mDependencySortedChildren.addAll(mChildDag.getSortedList());
// We also need to reverse the result since we want the start of the list to contain
// Views which have no dependencies, then dependent views after that
Collections.reverse(mDependencySortedChildren);
}
/**
* Add or remove the pre-draw listener as necessary.
*/
void ensurePreDrawListener() {
boolean hasDependencies = false;
final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (hasDependencies(child)) {
hasDependencies = true;
break;
}
}
if (hasDependencies != mNeedsPreDrawListener) {
if (hasDependencies) {
//如果有依赖则添加相应的视图绘制监听回调
addPreDrawListener();
} else {
removePreDrawListener();
}
}
}
/**
* Add the pre-draw listener if we're attached to a window and mark that we currently
* need it when attached.
*/
void addPreDrawListener() {
if (mIsAttachedToWindow) {
// Add the listener
if (mOnPreDrawListener == null) {
mOnPreDrawListener = new OnPreDrawListener();
}
//获取视图树观察者，添加CoordinatorLayout自身实现的mOnPreDrawListener监听
final ViewTreeObserver vto = getViewTreeObserver();
vto.addOnPreDrawListener(mOnPreDrawListener);
}
// Record that we need the listener regardless of whether or not we're attached.
// We'll add the real listener when we become attached.
mNeedsPreDrawListener = true;
}
//CoordinatorLayout内部类OnPreDrawListener，相当简洁，只是调用了onChildViewsChanged方法
class OnPreDrawListener implements ViewTreeObserver.OnPreDrawListener {
@Override
public boolean onPreDraw() {
onChildViewsChanged(EVENT_PRE_DRAW);
return true;
}
}
从上面可以追踪到监听被依赖子view的绘制之后会调用onChildViewsChanged方法，下面来看一下onChildViewsChanged中做了什么
final void onChildViewsChanged(@DispatchChangeEvent final int type) {
//省略代码若干...
for (int i = 0; i < childCount; i++) {
final View child = mDependencySortedChildren.get(i);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (type == EVENT_PRE_DRAW && child.getVisibility() == View.GONE) {
// Do not try to update GONE child views in pre draw updates.
continue;
}
//省略代码若干...
// Update any behavior-dependent views for the change
for (int j = i + 1; j < childCount; j++) {
final View checkChild = mDependencySortedChildren.get(j);
final LayoutParams checkLp = (LayoutParams) checkChild.getLayoutParams();
final Behavior b = checkLp.getBehavior();
//获取Behavior是否是依赖于当前遍历的子view，如果是则调用Behavior相关方法进行回调通知依赖view的改变
if (b != null && b.layoutDependsOn(this, checkChild, child)) {
//在通知依赖view改变之前先检查是否存在嵌套滚动，如果存在则此次不通知，因为此次事件
//并没有作用于当前依赖view而是通过嵌套滚动机制传递给嵌套view。
if (type == EVENT_PRE_DRAW && checkLp.getChangedAfterNestedScroll()) {
// If this is from a pre-draw and we have already been changed
// from a nested scroll, skip the dispatch and reset the flag
checkLp.resetChangedAfterNestedScroll();
continue;
}
final boolean handled;
switch (type) {
case EVENT_VIEW_REMOVED:
// EVENT_VIEW_REMOVED means that we need to dispatch
// onDependentViewRemoved() instead
b.onDependentViewRemoved(this, checkChild, child);
handled = true;
break;
default:
// Otherwise we dispatch onDependentViewChanged()
handled = b.onDependentViewChanged(this, checkChild, child);
break;
}
if (type == EVENT_NESTED_SCROLL) {
// If this is from a nested scroll, set the flag so that we may skip
// any resulting onPreDraw dispatch (if needed)
checkLp.setChangedAfterNestedScroll(handled);
}
}
}
}
releaseTempRect(inset);
releaseTempRect(drawRect);
releaseTempRect(lastDrawRect);
}
从以上代码分析我们知道在测量子view时的大致流程如下：
微信截图_20180104150129.png
4.一个实现自定义Behavior的小例子
ezgif-4-5e4a8abb69.gif
以上的例子实现了红色控件依赖于按钮的缩放属性，当按钮缩放时跟着缩放，同时获取依赖控件的文本值计算结果。主要展示了依赖控件可以监听到被依赖控件的重新绘制变化，以及可以获取被依赖控件的相关属性值等。
以下是例子的具体实现：
package com.example.myapplication;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import java.util.Random;
public class TestBehaviorActivity extends AppCompatActivity {
Button button;
Random random = new Random();
boolean flag;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test_behavior);
button = findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
button.setText(random.nextInt(10) + " + " + random.nextInt(10));
//改变控件属性使控件重新绘制，以便CoordinatorLayout通知Behavior的回调函数
if (flag) {
button.setScaleX(1);
} else {
button.setScaleX(2);
}
flag = !flag;
}
});
}
}
package com.example.myapplication;
import android.content.Context;
import android.support.design.widget.CoordinatorLayout;
import android.util.AttributeSet;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class TestBehavior extends CoordinatorLayout.Behavior {
//在布局中使用 app:layout_behavior=".TestBehavior"方式，必须实现的一个方法
public TestBehavior(Context context, AttributeSet attrs) {
super(context, attrs);
}
// 设置依赖于哪个控件，这里设置依赖于类型为Button的控件，返回true表示依赖
@Override
public boolean layoutDependsOn(CoordinatorLayout parent, TextView child, View dependency) {
return dependency instanceof Button;
}
//依赖的view发生改变，这里的dependency和layoutDependsOn设定的依赖控件一致
@Override
public boolean onDependentViewChanged(CoordinatorLayout parent, TextView child, View dependency) {
Button button = (Button) dependency;
String text = button.getText().toString();
child.setScaleX(button.getScaleX());
try {
int[] num = getNum(text);
child.setText("两数之和 ：" + (num[0] + num[1]));
} catch (Exception e) {
e.printStackTrace();
}
return true;
}
//依赖的view在窗体中被移除消失
@Override
public void onDependentViewRemoved(CoordinatorLayout parent, TextView child, View dependency) {
super.onDependentViewRemoved(parent, child, dependency);
}
public int[] getNum(String text) throws Exception {
int[] numArray = new int[2];
Pattern pattern = Pattern.compile("\\d+");
Matcher matcher = pattern.matcher(text);
matcher.find();
String num = matcher.group();
numArray[0] = Integer.valueOf(num);
matcher.find();
num = matcher.group();
numArray[1] = Integer.valueOf(num);
return numArray;
}
}
5.CoordinatorLayout.Behavior相关信息列举
微信图片_20180104153129.png
6.嵌套滚动简析
1、什么是嵌套滚动？ 嵌套滚动就是存在两个滚动行为的相互嵌套，当一个开始滚动另一个可以随着滚动的行为。
在一般的交互过程中 ，嵌套的滚动是单一的行为，子view的滚动影响这父view，父view并不影响子view。
所以一般的嵌套滚动交互流程图部分如下：
微信截图_20180104163733.png
2、关于嵌套滚动相关核心类有以下两组：
被滚动类需实现：NestedScrollingParent接口并实例化NestedScrollingParentHelper帮助类
滚动类需实现：NestedScrollingChild接口并实例化NestedScrollingChildHelper帮助类
NestedScrollingParentHelper、NestedScrollingChildHelper承担了实现相应接口的具体嵌套操作。在实现接口的相关类中调用帮助类的对应方法即可完成嵌套流程。
3、NestedScrollingParent和NestedScrollingChild接口简析图
7.一个实现嵌套滚动的小例子
ezgif-4-c5520d9b19.gif
上面的例子滚动流程分为两块，向上滑动和向下滑动处理逻辑的先后顺序正好完全相反。
向上滑动：先滑动父view，再滑动子view，再滑动子view中的内容。
向下滑动，先滑动子view中的内容，再滑动子view，再滑动父view。
下面来看下具体实现：
package com.nestedscroll.nesteddemo;
import android.content.Context;
import android.support.annotation.Nullable;
import android.support.v4.view.NestedScrollingChild;
import android.support.v4.view.NestedScrollingChildHelper;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.widget.LinearLayout;
public class NestedScrollChildView extends LinearLayout implements NestedScrollingChild {
private NestedScrollingChildHelper helper;
private int[] consumed = new int[2];
private int[] offsetInWindow = new int[2];
private float lastY;
private float initY;
public NestedScrollChildView(Context context) {
super(context);
init();
}
public NestedScrollChildView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
init();
}
public NestedScrollChildView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
helper = new NestedScrollingChildHelper(this);
setNestedScrollingEnabled(true);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
initY = getY();
}
@Override
public void setNestedScrollingEnabled(boolean enabled) {
helper.setNestedScrollingEnabled(enabled);
}
@Override
public boolean isNestedScrollingEnabled() {
return helper.isNestedScrollingEnabled();
}
@Override
public boolean startNestedScroll(int axes) {
return helper.startNestedScroll(axes);
}
@Override
public void stopNestedScroll() {
helper.stopNestedScroll();
}
@Override
public boolean hasNestedScrollingParent() {
return helper.hasNestedScrollingParent();
}
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int
dyUnconsumed, int[] offsetInWindow) {
return helper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed, offsetInWindow);
}
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
return helper.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
}
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return helper.dispatchNestedFling(velocityX, velocityY, consumed);
}
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return helper.dispatchNestedPreFling(velocityX, velocityY);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
switch (event.getActionMasked()) {
case MotionEvent.ACTION_DOWN:
lastY = event.getRawY();
//通知父view要开始滑动了
startNestedScroll(SCROLL_AXIS_VERTICAL);
break;
case MotionEvent.ACTION_MOVE:
float deltaY = lastY - event.getRawY();
lastY = event.getRawY();
if (deltaY > 0) { //嵌套子view向上滑动时
//通知父view滑动
if (dispatchNestedPreScroll(0, (int) deltaY, consumed, offsetInWindow)) {
// 如果父view消耗了滑动，需减去消耗距离
deltaY -= consumed[1];
}
//父view消耗的距离为int类型，损耗了精度，这里向下取整去掉剩余值
if (Math.floor(deltaY) == 0) {
break;
}
//滑动子view
if (getY() - deltaY >= 0) {
setY(getY() - deltaY);
deltaY = 0;
} else {
deltaY -= getY();
setY(0);
}
//滑动子view内容
if (getScrollY() + deltaY <= (getMeasuredHeight() - 40) / 2) {
scrollBy(0, (int) deltaY);
} else {
scrollTo(0, (getMeasuredHeight() - 40) / 2);
}
} else { // 嵌套子view向下滑动时
float dyConsumed; //子view滑动消耗距离
//滑动子view内容
if (getScrollY() + deltaY >= -(getMeasuredHeight() - 40) / 2) {
scrollBy(0, (int) deltaY);
dyConsumed = deltaY;
} else {
scrollTo(0, -(getMeasuredHeight() - 40) / 2);
dyConsumed = getScrollY() + (getMeasuredHeight() - 40) / 2;
}
float dyUnconsumed = deltaY - dyConsumed;
//滑动子view
if (getY() - dyUnconsumed <= initY) {
setY(getY() - dyUnconsumed);
dyConsumed += dyUnconsumed;
} else {
dyConsumed = (int) (dyConsumed + (initY - getY()));
setY(initY);
}
//通知父view滑动
dispatchNestedScroll(0, (int) dyConsumed, 0, (int) (deltaY - dyConsumed), offsetInWindow);
}
break;
case MotionEvent.ACTION_CANCEL:
case MotionEvent.ACTION_UP:
//通知父view停止本次嵌套滑动
stopNestedScroll();
break;
}
return true;
}
}
package com.nestedscroll.nesteddemo;
import android.content.Context;
import android.support.annotation.Nullable;
import android.support.v4.view.NestedScrollingParent;
import android.support.v4.view.NestedScrollingParentHelper;
import android.util.AttributeSet;
import android.view.View;
import android.widget.LinearLayout;
public class NestedScrollParentView extends LinearLayout implements NestedScrollingParent {
NestedScrollingParentHelper helper;
private float initY;
public NestedScrollParentView(Context context) {
super(context);
init();
}
public NestedScrollParentView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
init();
}
public NestedScrollParentView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
private void init() {
helper = new NestedScrollingParentHelper(this);
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
initY = getY();
}
@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
return (nestedScrollAxes & SCROLL_AXIS_VERTICAL) != 0;
}
@Override
public void onNestedScrollAccepted(View child, View target, int nestedScrollAxes) {
helper.onNestedScrollAccepted(child, target, nestedScrollAxes);
}
@Override
public void onStopNestedScroll(View target) {
helper.onStopNestedScroll(target);
}
@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
if (getY() - dyUnconsumed <= initY) {
setY(getY() - dyUnconsumed);
} else {
setY(initY);
}
}
@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
if (getY() - dy >= 0) {
setY(getY() - dy);
consumed[1] = dy;
} else {
consumed[1] = (int) getY();
setY(0);
}
}
@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
return false;
}
@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
return false;
}
@Override
public int getNestedScrollAxes() {
return helper.getNestedScrollAxes();
}
}
8.在CoordinatorLayout中的嵌套滚动
前面分析CoordinatorLayout实现了NestedScrollingParent接口，而RecyclerView实现了NestedScrollingChild接口，
利用CoordinatorLayout和RecyclerView可以快速实现上面例子的嵌套滚动，以及需要的视图依赖变换。