Android NestedScrolling機制

安卓開發 · 發表 2018-09-26 10:07:35

摘要： NestedScrolling機制現在在App的作用越來越重要，許多很漂亮的互動都是基於NestedScrolling機制進行完成的。 NestedScrolling機制主要是能夠讓父View和子View在滾動時互相協調配合。其中有兩個重要的類，分別是：介面類 Neste...

NestedScrolling機制現在在App的作用越來越重要，許多很漂亮的互動都是基於NestedScrolling機制進行完成的。

NestedScrolling機制主要是能夠讓父View和子View在滾動時互相協調配合。其中有兩個重要的類，分別是：

介面類
NestedScrollingParent(最新:NestedScrollingParent2)
NestedScrollingChild(最新:NestedScrollingChild2)

幫助類
NestedScrollingChildHelper
NestedScrollingParentHelper

父類繼承NestedScrollingParent介面，而子類繼承NestedScrollingChild介面，同時讓父類包含子類，而不是自接父子關係，就搭起了NestedScrollingParent機制的基本骨架。

其主要流程是：

子類滑動，把滑動產生的事件和引數傳給父類
父類根據子類傳過來的引數進行各種互動操作，如變大縮小之類的

而NestedScrollingChildHelper和NestedScrollingParentHelper是兩個幫助類，在實現NestedScrollingChild和NestedScrollingParent介面時，使用這兩個幫助類可以簡化我們的工作。

NestedScrollingChild 介面類

public interface NestedScrollingChild {

/**
* 設定巢狀滑動是否能用
*/
@Override
public void setNestedScrollingEnabled(boolean enabled);

/**
* 判斷巢狀滑動是否可用
*/
@Override
public boolean isNestedScrollingEnabled();

/**
* 開始巢狀滑動
*
* @param axes 表示方向軸，有橫向和豎向
*/
@Override
public boolean startNestedScroll(int axes);

/**
* 停止巢狀滑動
*/
@Override
public void stopNestedScroll();

/**
* 判斷是否有父View 支援巢狀滑動
*/
@Override
public boolean hasNestedScrollingParent() ;

/**
* 滑行時呼叫
* @param velocityX x 軸上的滑動速率
* @param velocityY y 軸上的滑動速率
* @param consumed 是否被消費
* @returntrue if the nested scrolling parent consumed or otherwise reacted to the fling
*/
@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) ;

/**
* 進行滑行前呼叫
* @param velocityX x 軸上的滑動速率
* @param velocityY y 軸上的滑動速率
* @return true if a nested scrolling parent consumed the fling
*/
@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) ;

/**
* 子view處理scroll後呼叫
* @param dxConsumed x軸上被消費的距離（橫向）
* @param dyConsumed y軸上被消費的距離（豎向）
* @param dxUnconsumed x軸上未被消費的距離
* @param dyUnconsumed y軸上未被消費的距離
* @param offsetInWindow 子View的窗體偏移量
* @returntrue if the event was dispatched, false if it could not be dispatched.
*/
@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow) ;

/**
* 在子View的onInterceptTouchEvent或者onTouch中，呼叫該方法通知父View滑動的距離
* @param dxx軸上滑動的距離
* @param dyy軸上滑動的距離
* @param consumed 父view消費掉的scroll長度
* @param offsetInWindow子View的窗體偏移量
* @return 支援的巢狀的父View 是否處理了 滑動事件
*/
@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow);

}

上面這個方法方法和代表的意思我都已經貼出來，然後是隻是一個介面類上面的方法要怎麼實現呢，這時候就要用到上面的幫助類NestedScrollingChildHelper，一個完整的實現模板如下：

public class MyNestedScrollingChild extends LinearLayout implements NestedScrollingChild {
private NestedScrollingChildHelper mNestedScrollingChildHelper;

public MyNestedScrollingChild(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
mNestedScrollingChildHelper = new NestedScrollingChildHelper(this);
mNestedScrollingChildHelper.setNestedScrollingEnabled(true);
}

@Override
public void setNestedScrollingEnabled(boolean enabled) {
mNestedScrollingChildHelper.setNestedScrollingEnabled(enabled);
}

@Override
public boolean isNestedScrollingEnabled() {
return mNestedScrollingChildHelper.isNestedScrollingEnabled();
}

@Override
public boolean startNestedScroll(int axes) {
return mNestedScrollingChildHelper.startNestedScroll(axes);
}

@Override
public void stopNestedScroll() {
mNestedScrollingChildHelper.stopNestedScroll();
}

@Override
public boolean hasNestedScrollingParent() {
return mNestedScrollingChildHelper.hasNestedScrollingParent();
}

@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return mNestedScrollingChildHelper.dispatchNestedFling(velocityX,velocityY,consumed);
}

@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return mNestedScrollingChildHelper.dispatchNestedPreFling(velocityX,velocityY);
}

@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, @Nullable int[] offsetInWindow) {
return mNestedScrollingChildHelper.dispatchNestedScroll(dxConsumed,dyConsumed,dxUnconsumed,dyUnconsumed,offsetInWindow);
}

@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow) {
return dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow);
}

}

NestedScrollingParent 介面

public interface NestedScrollingParent {

@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes);

@Override
public void onStopNestedScroll(View child);

@Override
public void onNestedScrollAccepted(View child, View target, int axes);

@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed);

@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed);

@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed);

@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY);

@Override
public int getNestedScrollAxes();
}

從上面的程式碼可以看出NestedScrollingChild的方法執行之後就會回撥父View的各個方法，從方法名也知道作用和NestedScrollingChild的用作大同小異。當子View執行startNestedScroll時，就會回撥父View的onStartNestedScroll、onNestedScrollAccepted方法，當子View執行dispatchNestedPreScroll方法時，就會回撥父View的onNestedPreScroll，當子View執行dispatchNestedScroll方法時，就會回撥父View的onNestedScroll方法，由此類推，dispatchNestedPreFling回撥父View的onNestedPreFling方法，dispatchNestedFling回撥父View的onNestedFling方法，等。

同時也有幾個介面是需要幫助類進行實現的，模板程式碼如下：

public class MyNestedScrollingParent extends LinearLayout implements NestedScrollingParent {
private NestedScrollingParentHelper mNestedScrollingParentHelper;

public MyNestedScrollingParent(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
mNestedScrollingParentHelper = new NestedScrollingParentHelper(this);
}

@Override
public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes) {
return super.onStartNestedScroll(child, target, nestedScrollAxes);
}

@Override
public void onNestedScroll(View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
super.onNestedScroll(target, dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed);
}

@Override
public void onNestedPreScroll(View target, int dx, int dy, int[] consumed) {
super.onNestedPreScroll(target, dx, dy, consumed);
}

@Override
public boolean onNestedFling(View target, float velocityX, float velocityY, boolean consumed) {
return super.onNestedFling(target, velocityX, velocityY, consumed);
}

@Override
public boolean onNestedPreFling(View target, float velocityX, float velocityY) {
return super.onNestedPreFling(target, velocityX, velocityY);
}

@Override
public void onStopNestedScroll(View child) {
mNestedScrollingParentHelper.onStopNestedScroll(child);
}

@Override
public void onNestedScrollAccepted(View child, View target, int axes) {
mNestedScrollingParentHelper.onNestedScrollAccepted(child,target,axes);
}

@Override
public int getNestedScrollAxes() {
return mNestedScrollingParentHelper.getNestedScrollAxes();
}

}

最後總結，子View通過startNestedScroll()發起巢狀滑動，同時父View也會回撥自己的onStartNestedScroll()方法，接著子View每次在滾動前都會呼叫dispatchNestedPreScroll()方法,父View的onNestedPreScroll()也會操作，父View決定是否熬滑動，然後才是子View自己滑動，之後子View也可以呼叫上面的其它方法做相應的處理，最後呼叫stopNestedScroll()結束。

最後舉一個例項吧

public class MyNestedScrollChild extends LinearLayout implements NestedScrollingChild2 {
private NestedScrollingChildHelper mNestedScrollingChildHelper;
private int[] offset=new int[2];
private int[] consumed=new int[2];
private TextView scrollText;
private int showHeight;
private int lastY;
privateboolean srcollTop=false;

public MyNestedScrollChild(Context context) {
super(context);
}

public MyNestedScrollChild(Context context, AttributeSet attrs) {
super(context, attrs);
setBackgroundColor(context.getResources().getColor(R.color.colorffffff));
}

@Override
protected void onFinishInflate() {
super.onFinishInflate();
scrollText=(TextView)getChildAt(0);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
showHeight = getMeasuredHeight();
heightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

public boolean canChildScrollUp() {
return srcollTop;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
switch(event.getAction()){
case MotionEvent.ACTION_DOWN:
lastY=(int)event.getRawY();
break;
case MotionEvent.ACTION_MOVE:
int y=(int)(event.getRawY());
int dy=y-lastY;
lastY=y;
if(startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL)
&&dispatchNestedPreScroll(0,dy,consumed,offset)){
int remain = dy - consumed[1];
if (remain != 0) {
scrollBy(0, -remain);
}
}else{
scrollBy(0, -dy);
}
break;
}
return true;
}

//限制滾動範圍
@Override
public void scrollTo(int x, int y) {
int maxY = getMeasuredHeight()- showHeight;
if (y > maxY) {
y = maxY;
srcollTop=false;
}else if (y < 0) {
y = 0;
srcollTop=true;
}else{
srcollTop=false;
}
super.scrollTo(x, y);
}

public NestedScrollingChildHelper getNestedScrollingChildHelper(){
if(mNestedScrollingChildHelper==null){
mNestedScrollingChildHelper=new NestedScrollingChildHelper(this);
mNestedScrollingChildHelper.setNestedScrollingEnabled(true);
}
return mNestedScrollingChildHelper;
}

@Override
public boolean startNestedScroll(int axes, int type) {
return getNestedScrollingChildHelper().startNestedScroll(axes,type);
}

@Override
public void stopNestedScroll(int type) {
getNestedScrollingChildHelper().stopNestedScroll(type);
}

@Override
public boolean hasNestedScrollingParent(int type) {
return getNestedScrollingChildHelper().hasNestedScrollingParent(type);
}

@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed,
int dyUnconsumed, @Nullable int[] offsetInWindow, int type) {
return getNestedScrollingChildHelper().dispatchNestedScroll(dxConsumed,dyConsumed,
dxUnconsumed,dyUnconsumed,offsetInWindow,type);
}

@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow, int type) {
return getNestedScrollingChildHelper().dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow,type);
}

@Override
public void setNestedScrollingEnabled(boolean enabled) {
getNestedScrollingChildHelper().setNestedScrollingEnabled(enabled);
}

@Override
public boolean isNestedScrollingEnabled() {
return getNestedScrollingChildHelper().isNestedScrollingEnabled();
}

@Override
public boolean startNestedScroll(int axes) {
return getNestedScrollingChildHelper().startNestedScroll(axes);
}

@Override
public void stopNestedScroll() {
getNestedScrollingChildHelper().stopNestedScroll();
}

@Override
public boolean hasNestedScrollingParent() {
return getNestedScrollingChildHelper().hasNestedScrollingParent();
}

@Override
public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed,
@Nullable int[] offsetInWindow) {
return getNestedScrollingChildHelper().dispatchNestedScroll(dxConsumed,dyConsumed,dxUnconsumed,dyUnconsumed,offsetInWindow);
}

@Override
public boolean dispatchNestedPreScroll(int dx, int dy, @Nullable int[] consumed, @Nullable int[] offsetInWindow) {
return getNestedScrollingChildHelper().dispatchNestedPreScroll(dx,dy,consumed,offsetInWindow);
}

@Override
public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) {
return getNestedScrollingChildHelper().dispatchNestedFling(velocityX,velocityY,consumed);
}

@Override
public boolean dispatchNestedPreFling(float velocityX, float velocityY) {
return getNestedScrollingChildHelper().dispatchNestedPreFling(velocityX,velocityY);
}

}

首先給出NestedScrollingChild子View,重點看一下onTouchEvent()方法,當MotionEvent.ACTION_MOVE時,不斷的呼叫startNestedScroll()和dispatchNestedPreScroll()向父View傳送直接，然後滾動通過scrollBy()滾動觸發事件的View,這就是最核心的程式碼了，接著看父View程式碼如下：

public class MyNestedScrollParent extends LinearLayout implements NestedScrollingParent2 {
private NestedScrollingParentHelper mNestedScrollingParentHelper;
private MyNestedScrollChild scrollChildView;
private ImageView foodIV;
private TextView titleTV;
private int imageHeight;
private int titleHeight;
private int imageMargin;
private int scrollY;

public MyNestedScrollParent(Context context) {
this(context,null);
}

public MyNestedScrollParent(Context context, AttributeSet attrs) {
super(context, attrs);
mNestedScrollingParentHelper=new NestedScrollingParentHelper(this);
}

@Override
protected void onFinishInflate() {
super.onFinishInflate();
FrameLayout frameLayout=(FrameLayout) getChildAt(0);
scrollChildView=(MyNestedScrollChild) getChildAt(1);
foodIV=frameLayout.findViewById(R.id.foodIV);
titleTV=frameLayout.findViewById(R.id.titleTV);
foodIV.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
imageHeight=foodIV.getHeight();
}
});
titleTV.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
titleHeight=titleTV.getHeight();
}
});
}

@Override
public boolean onStartNestedScroll(@NonNull View child, @NonNull View target, int axes, int type) {
if(target instanceof MyNestedScrollChild) {
return true;
}
return false;
}

@Override
public void onNestedScrollAccepted(@NonNull View child, @NonNull View target, int axes, int type) {
mNestedScrollingParentHelper.onNestedScrollAccepted(child,target,axes,type);
}

@Override
public void onStopNestedScroll(@NonNull View target, int type) {
mNestedScrollingParentHelper.onStopNestedScroll(target,type);
}

@Override
public int getNestedScrollAxes() {
return mNestedScrollingParentHelper.getNestedScrollAxes();
}

@Override
public void onStopNestedScroll(@NonNull View target) {
mNestedScrollingParentHelper.onStopNestedScroll(target);
}

@Override
public void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
int dxUnconsumed, int dyUnconsumed, int type) {

}

@Override
public void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed, int type) {
imageMargin=titleHeight-imageHeight;
scrollY+=dy;
if(scrollY<=imageMargin){
scrollY=imageMargin;
scrollChildView.setTranslationY(scrollY);
}else{
if(dy<0){
//上滑
consumed[1]=dy;
scrollChildView.setTranslationY(scrollY);
}else{
//下滑
if(!scrollChildView.canChildScrollUp()){
scrollY-=dy;
}
if(scrollY>=0){
scrollY=0;
}
scrollChildView.setTranslationY(scrollY);
}
}
}

@Override
public boolean onStartNestedScroll(@NonNull View child, @NonNull View target, int axes) {
return false;
}

}

前面的子View的dispatchNestedPreScroll()對應這個父View的onNestedScroll()如上面的程式碼，通過View.setTranslationY()來滑動整個子View，consumed[1]=dy;表示子View和滑動的View一起滑。最後看一下佈局檔案：

<?xml version="1.0" encoding="utf-8"?>
<com.jack.meituangoodsdetails.view.MyNestedScrollParent
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<FrameLayout
android:layout_width="match_parent"
android:layout_height="170dp">

<ImageView
android:id="@+id/foodIV"
android:layout_width="match_parent"
android:layout_height="170dp"
android:src="@mipmap/food_bg"
android:scaleType="fitXY"/>

<TextView
android:id="@+id/titleTV"
android:layout_width="match_parent"
android:layout_height="50dp"
android:gravity="center"
android:textColor="@android:color/white"
android:background="@color/color2e8b57"
android:text="MyTitle"/>

</FrameLayout>

<com.jack.meituangoodsdetails.view.MyNestedScrollChild
android:id="@+id/scroll_view"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:id="@+id/scrollText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="123\n456\n789\n111\n222\n333\n444\n555\n666\n777\n888\n999\n14\n12\n13\n44\n55\n66\n77\n88\n99\n11\n22\n33\n44\n55\n66\n77\n88\n99\n77\n88\n88\n8\n88\n88\n" />
</com.jack.meituangoodsdetails.view.MyNestedScrollChild>

</com.jack.meituangoodsdetails.view.MyNestedScrollParent>

父View包裹子View，以達成依賴關係。

講了NestedScrolling,就有必要講解CoordinatorLayout.Behavior，下回講吧，最後奉上原始碼吧ofollow,noindex">https://github.com/jack921/MeiTuanGoodsDetails

Android NestedScrolling機制

NestedScrollingChild 介面類

NestedScrollingParent 介面

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