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SwipeRefreshLayout，用最少的代码定制最美的上下拉刷新样式

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Android 前端工具

下拉刷新框架其实有很多，而且质量都比较高。但是在日常开发中，每一款产品都会有一套自己独特的一套刷新样式。相信有很多小伙伴在个性化定制中都或多或少的遇到过麻烦。今天我就给大家推荐一个在定制方面很出彩的一个刷新框架SwipeToLoadLayout，该框架自身完成了下拉刷新与上拉加载功能，同时将顶部视图与底部视图的UI定制功能通过接口很方便的提供给使用者自行定义。
相关代码已经上传到github上，欢迎star、fork

基本流程

先简单了解一下SwipeToLoadLayout的使用流程，以下拉刷新为例：

完成Header部分，实现SwipeRefreshTrigger与SwipeRefreshTrigger接口
完成activity或fragment的布局，在SwipeToLoadLayout节点下配置好Header与下拉目标组件（如RecyclerView等）

这里还是要稍微说一下，因为这个布局过程还是有一定的规则的
首先布局的id是固定的，这个我们在ids.xml中就能看出。框架提供三个View：Header、Target、Footer，分别对应三个位置的View

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item name="swipe_target" type="id" />
    <item name="swipe_refresh_header" type="id" />
    <item name="swipe_load_more_footer" type="id" />
</resources>

其次onFinishInflate()方法告诉我们，最多只能同时存在这三个View，不能有更多的子View了

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        final int childNum = getChildCount();
        if (childNum == 0) {
            // no child return
            return;
        } else if (0 < childNum && childNum < 4) {
            mHeaderView = findViewById(R.id.swipe_refresh_header);
            mTargetView = findViewById(R.id.swipe_target);
            mFooterView = findViewById(R.id.swipe_load_more_footer);
        } else {
            // more than three children: unsupported!
            throw new IllegalStateException("Children num must equal or less than 3");
        }
        if (mTargetView == null) {
            return;
        }
        if (mHeaderView != null && mHeaderView instanceof SwipeTrigger) {
            mHeaderView.setVisibility(GONE);
        }
        if (mFooterView != null && mFooterView instanceof SwipeTrigger) {
            mFooterView.setVisibility(GONE);
        }
    }

这样你就能得出下一步该怎么来实现了吧？没错肯定是这样的

<?xml version="1.0" encoding="utf-8"?>
<com.aspsine.swipetoloadlayout.SwipeToLoadLayout >
    <View
        android:id="@id/swipe_refresh_header" />
    <android.support.v7.widget.RecyclerView
        android:id="@id/swipe_target" />
    <View
        android:id="@id/swipe_load_more_footer" />
</com.aspsine.swipetoloadlayout.SwipeToLoadLayout>

Header的部分尤为重要。我们需在Header上实现SwipeTrigger与SwipeRefreshTrigger接口，接口中的方法分别对应滑动刷新在各个状态下的回调。它们分别为
onPrepare：代表下拉刷新开始的状态
onMove：代表正在滑动过程中的状态
onRelease：代表手指松开后，下拉刷新进入松开刷新的状态
onComplete：代表下拉刷新完成的状态
onReset：代表下拉刷新重置恢复的状态
onRefresh：代表正在刷新中的状态
有了这几个接口，我们就可以完成Header部分的任何动画效果了。当然上拉加载更多的场景，只是把SwipeRefreshTrigger接口换成SwipeLoadMoreTrigger接口而已，其他跟下拉刷新情况完全相同

在activity或fragment中配置下拉监听事件，并在数据获取完成后主动触发刷新swipeToLoadLayout.setRefreshing(false);完成功能

更深入的部分我们放到源码分析里面再说

看起来好像很简单，那么我们就通过几个小Demo了解一下如何使用吧

仿新浪微博

之所以第一个范例选择新浪微博，是因为它是最传统刷新风格：根据箭头和文字的不同来表明当前不同的状态

如果你在早期研究过PullToRefresh，那么很容易在这个框架基础上实现相应的视图更新功能

先完成头部的定义。WeiboRefreshHeaderView作为头，其实际为一个LinearLayout

class WeiboRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger

头部布局很简单

<?xml version="1.0" encoding="utf-8"?>
<com.renyu.swipetoloadlayoutdemo.view.WeiboRefreshHeaderView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="60dip"
    android:gravity="center"
    android:orientation="horizontal">
    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">
        <ProgressBar
            android:id="@+id/pb_weibo"
            style="?android:attr/progressBarStyleSmallInverse"
            android:layout_centerInParent="true"
            android:visibility="gone"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
        <ImageView
            android:id="@+id/iv_weibo"
            android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@mipmap/tableview_pull_refresh_arrow_down"
            android:layout_centerInParent="true"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
    </RelativeLayout>
    <TextView
        android:id="@+id/tv_weibo"
        android:layout_marginStart="10dip"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="下拉刷新"/>
</com.renyu.swipetoloadlayoutdemo.view.WeiboRefreshHeaderView>

activity的布局也很简单，把头跟身子一起加在SwipeToLoadLayout里

<?xml version="1.0" encoding="utf-8"?>
<com.aspsine.swipetoloadlayout.SwipeToLoadLayout
    xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/swipe_weibo">

    <include
        layout="@layout/header_weibo"
        android:id="@id/swipe_refresh_header" />
    <TextView
        android:id="@id/swipe_target"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="下拉刷新"/>
</com.aspsine.swipetoloadlayout.SwipeToLoadLayout>

下面就是完成头部动画效果了。新浪微博的这个效果就是视图被下拉到头部高度之后，将箭头位置旋转一下同时更换文字，刷新时展现progressbar即可

class WeiboRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger {

    var pb_weibo: ProgressBar? = null
    var iv_weibo: ImageView? = null
    var tv_weibo: TextView? = null

    // 是否发生旋转
    var rotated = false

    private val rotate_up: Animation by lazy {
        AnimationUtils.loadAnimation(context, R.anim.rotate_up)
    }

    private val rotate_down: Animation by lazy {
        AnimationUtils.loadAnimation(context, R.anim.rotate_down)
    }

    constructor(context: Context) : super(context)
    constructor(context: Context, attributeSet: AttributeSet) : super(context, attributeSet)
    constructor(context: Context, attributeSet: AttributeSet, defStyleAttr: Int) : super(context, attributeSet, defStyleAttr)

    override fun onFinishInflate() {
        super.onFinishInflate()

        pb_weibo = findViewById(R.id.pb_weibo)
        iv_weibo = findViewById(R.id.iv_weibo)
        tv_weibo = findViewById(R.id.tv_weibo)
    }

    override fun onReset() {
        pb_weibo?.visibility = View.GONE
        iv_weibo?.visibility = View.VISIBLE
        tv_weibo?.text = "下拉刷新"
    }

    override fun onComplete() {
        tv_weibo?.text = "刷新完成"
        pb_weibo?.visibility = View.GONE
    }

    override fun onRelease() {

    }

    override fun onMove(p0: Int, p1: Boolean, p2: Boolean) {
        if (p0 > SizeUtils.dp2px(60f)) {
            if (!rotated) {
                rotated = true
                tv_weibo?.text = "释放更新"
                iv_weibo?.clearAnimation()
                iv_weibo?.startAnimation(rotate_up)
            }
        }
        else {
            if (rotated) {
                rotated = false
                tv_weibo?.text = "下拉刷新"
                iv_weibo?.clearAnimation()
                iv_weibo?.startAnimation(rotate_down)
            }
        }
    }

    override fun onPrepare() {

    }

    override fun onRefresh() {
        tv_weibo?.text = "加载中"
        iv_weibo?.clearAnimation()
        iv_weibo?.visibility = View.GONE
        pb_weibo?.visibility = View.VISIBLE
    }
}

对照一下上文的刷新周期，应该很好理解

美团外卖

美团外卖是利用ImageView直接播放一段animation直到刷新完成停止。在下拉过程中，该ImageView随着位移的距离变化而发生相应的大小变化

美团外卖动画效果是由一系列的图片组成的，所以与新浪微博效果相比更为简单一些

一样要完成头部视图的定义

class MTRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger

<?xml version="1.0" encoding="utf-8"?>
<com.renyu.swipetoloadlayoutdemo.view.MTRefreshHeaderView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="wrap_content"
    android:gravity="center"
    android:padding="10dip">
    <ImageView
        android:id="@+id/iv_mt"
        android:layout_width="112dp"
        android:layout_height="44dp"
        android:background="@drawable/animation_list_refresh_mt"
        android:transformPivotX="56dp"
        android:transformPivotY="22dp"
        android:scaleY="0.3"
        android:scaleX="0.3"/>
</com.renyu.swipetoloadlayoutdemo.view.MTRefreshHeaderView>

剩下就是完成动画的播放与缩放的处理了

class MTRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger {

    var iv_mt: ImageView? = null

    val animationDrawable: AnimationDrawable by lazy {
        iv_mt?.background as AnimationDrawable
    }

    constructor(context: Context) : super(context)
    constructor(context: Context, attributeSet: AttributeSet) : super(context, attributeSet)
    constructor(context: Context, attributeSet: AttributeSet, defStyleAttr: Int) : super(context, attributeSet, defStyleAttr)

    override fun onFinishInflate() {
        super.onFinishInflate()

        iv_mt = findViewById(R.id.iv_mt)
    }

    override fun onReset() {

    }

    override fun onComplete() {
        animationDrawable.stop()
    }

    override fun onRelease() {

    }

    override fun onMove(p0: Int, p1: Boolean, p2: Boolean) {
        val percent = if (p0 * 1.0f / SizeUtils.dp2px(44f) > 1) 1f else p0 * 1.0f / SizeUtils.dp2px(44f)

        iv_mt?.scaleY = (0.3f + 0.7 * percent).toFloat()
        iv_mt?.scaleX = (0.3f + 0.7 * percent).toFloat()
    }

    override fun onPrepare() {
        if (!animationDrawable.isRunning) {
            animationDrawable.start()
        }

        iv_mt?.scaleY = 0.3f
        iv_mt?.scaleX = 0.3f
    }

    override fun onRefresh() {
        if (!animationDrawable.isRunning) {
            animationDrawable.start()
        }

        iv_mt?.scaleY = 1f
        iv_mt?.scaleX = 1f
    }
}

代码都很简单，很容易理解

饿了么

饿了么的效果是通过SVG来实现的

饿了么app对资源进行了混淆，所以我拿不到图片，只能随便从其他地方找一个了

一样是Header的编写，这里面有一点不同，我用android-pathview这个开源框架实现SVG播放进度控制功能

我需要将这个动画效果在下拉刷新的过程中实现

class ElemeRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger

<?xml version="1.0" encoding="utf-8"?>
<com.renyu.swipetoloadlayoutdemo.view.ElemeRefreshHeaderView xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center">
    <com.eftimoff.androipathview.PathView
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/pathView_ele"
        android:layout_width="58dp"
        android:layout_height="58dp"
        app:pathColor="@android:color/black"
        app:svg="@raw/issues"
        app:pathWidth="2dp"/>
</com.renyu.swipetoloadlayoutdemo.view.ElemeRefreshHeaderView>

下面就是根据滑动偏移量来处理SVG播放的进度

class ElemeRefreshHeaderView : LinearLayout, SwipeTrigger, SwipeRefreshTrigger {

    var pathView_ele: PathView? = null

    constructor(context: Context) : super(context)
    constructor(context: Context, attributeSet: AttributeSet) : super(context, attributeSet)
    constructor(context: Context, attributeSet: AttributeSet, defStyleAttr: Int) : super(context, attributeSet, defStyleAttr)

    override fun onFinishInflate() {
        super.onFinishInflate()

        pathView_ele = findViewById(R.id.pathView_ele)
    }

    override fun onReset() {

    }

    override fun onComplete() {
        pathView_ele?.setPercentage(1f)
    }

    override fun onRelease() {
    }

    override fun onMove(p0: Int, p1: Boolean, p2: Boolean) {
        val percent = 1 - (SizeUtils.dp2px(58f) - p0) * 1.0f / SizeUtils.dp2px(58f)
        val value = if (percent >= 1) 1f else percent
        pathView_ele?.setPercentage(value)
    }

    override fun onPrepare() {
        pathView_ele?.setPercentage(0f)
    }

    override fun onRefresh() {
        pathView_ele?.setPercentage(1f)
    }
}

这里你会发出一个疑问，怎么效果与饿了么有的差距？饿了么是滑动到Header完成展开之后就不再继续下滑了，那咱们这个怎么实现呢？那我只能说不好意思，在现有条件下咱们实现不了，只能通过改源码完成

那我们就顺带来阅读源码，看看这个地方怎么改进吧？

源码分析

之前的onFinishInflate咱们就不说了，那个就是告诉我们只能有三个View，分别是Header、Target、Footer

然后是测量阶段，在测量阶段可以得到两个重要的变量mHeaderHeight与mFooterHeight，他们分别代表Header与Footer的高度。同时如果定义的mRefreshTriggerOffset（松开刷新的高度）比Header或Footer的高度小，则修正这个刷新位置

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        // header
        if (mHeaderView != null) {
            final View headerView = mHeaderView;
            measureChildWithMargins(headerView, widthMeasureSpec, 0, heightMeasureSpec, 0);
            MarginLayoutParams lp = ((MarginLayoutParams) headerView.getLayoutParams());
            mHeaderHeight = headerView.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;
            if (mRefreshTriggerOffset < mHeaderHeight) {
                mRefreshTriggerOffset = mHeaderHeight;
            }
        }
        // target
        if (mTargetView != null) {
            final View targetView = mTargetView;
            measureChildWithMargins(targetView, widthMeasureSpec, 0, heightMeasureSpec, 0);
        }
        // footer
        if (mFooterView != null) {
            final View footerView = mFooterView;
            measureChildWithMargins(footerView, widthMeasureSpec, 0, heightMeasureSpec, 0);
            MarginLayoutParams lp = ((MarginLayoutParams) footerView.getLayoutParams());
            mFooterHeight = footerView.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;
            if (mLoadMoreTriggerOffset < mFooterHeight) {
                mLoadMoreTriggerOffset = mFooterHeight;
            }
        }
    }

在onLayout中对三个视图进行布局

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        layoutChildren();

        mHasHeaderView = (mHeaderView != null);
        mHasFooterView = (mFooterView != null);
    }

这里有一个重要的方法layoutChildren，这个方法就是改变三个视图的位置的。当然这个位置要根据不同的类型来处理，默认情况下我们都是STYLE.CLASSIC类型。

private void layoutChildren() {
        final int width = getMeasuredWidth();
        final int height = getMeasuredHeight();

        final int paddingLeft = getPaddingLeft();
        final int paddingTop = getPaddingTop();
        final int paddingRight = getPaddingRight();
        final int paddingBottom = getPaddingBottom();

        if (mTargetView == null) {
            return;
        }

        // layout header
        if (mHeaderView != null) {
            final View headerView = mHeaderView;
            MarginLayoutParams lp = (MarginLayoutParams) headerView.getLayoutParams();
            final int headerLeft = paddingLeft + lp.leftMargin;
            final int headerTop;
            switch (mStyle) {
                case STYLE.CLASSIC:
                    // classic
                    headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset;
                    break;
                case STYLE.ABOVE:
                    // classic
                    headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset;
                    break;
                case STYLE.BLEW:
                    // blew
                    headerTop = paddingTop + lp.topMargin;
                    break;
                case STYLE.SCALE:
                    // scale
                    headerTop = paddingTop + lp.topMargin - mHeaderHeight / 2 + mHeaderOffset / 2;
                    break;
                case STYLE.BLEW2CLASSIC:
                    // blew2classic
                    if (mHeaderOffset > mHeaderHeight) {
                        headerTop = paddingTop + lp.topMargin;
                    }
                    else {
                        headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset;
                    }
                    break;
                default:
                    // classic
                    headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset;
                    break;
            }
            final int headerRight = headerLeft + headerView.getMeasuredWidth();
            final int headerBottom = headerTop + headerView.getMeasuredHeight();
            headerView.layout(headerLeft, headerTop, headerRight, headerBottom);
        }

        // layout target
        if (mTargetView != null) {
            final View targetView = mTargetView;
            MarginLayoutParams lp = (MarginLayoutParams) targetView.getLayoutParams();
            final int targetLeft = paddingLeft + lp.leftMargin;
            final int targetTop;

            switch (mStyle) {
                case STYLE.CLASSIC:
                    // classic
                    targetTop = paddingTop + lp.topMargin + mTargetOffset;
                    break;
                case STYLE.ABOVE:
                    // above
                    targetTop = paddingTop + lp.topMargin;
                    break;
                case STYLE.BLEW:
                    // classic
                    targetTop = paddingTop + lp.topMargin + mTargetOffset;
                    break;
                case STYLE.SCALE:
                    // classic
                    targetTop = paddingTop + lp.topMargin + mTargetOffset;
                    break;
                case STYLE.BLEW2CLASSIC:
                    // classic
                    targetTop = paddingTop + lp.topMargin + mTargetOffset;
                    break;
                default:
                    // classic
                    targetTop = paddingTop + lp.topMargin + mTargetOffset;
                    break;
            }
            final int targetRight = targetLeft + targetView.getMeasuredWidth();
            final int targetBottom = targetTop + targetView.getMeasuredHeight();
            targetView.layout(targetLeft, targetTop, targetRight, targetBottom);
        }

        // layout footer
        if (mFooterView != null) {
            final View footerView = mFooterView;
            MarginLayoutParams lp = (MarginLayoutParams) footerView.getLayoutParams();
            final int footerLeft = paddingLeft + lp.leftMargin;
            final int footerBottom;
            switch (mStyle) {
                case STYLE.CLASSIC:
                    // classic
                    footerBottom = height - paddingBottom - lp.bottomMargin + mFooterHeight + mFooterOffset;
                    break;
                case STYLE.ABOVE:
                    // classic
                    footerBottom = height - paddingBottom - lp.bottomMargin + mFooterHeight + mFooterOffset;
                    break;
                case STYLE.BLEW:
                    // blew
                    footerBottom = height - paddingBottom - lp.bottomMargin;
                    break;
                case STYLE.SCALE:
                    // scale
                    footerBottom = height - paddingBottom - lp.bottomMargin + mFooterHeight / 2 + mFooterOffset / 2;
                    break;
                case STYLE.BLEW2CLASSIC:
                    // blew2classic
                    if (mFooterOffset > mFooterHeight) {
                        footerBottom = height - paddingBottom - lp.bottomMargin;
                    }
                    else {
                        footerBottom = height - paddingBottom - lp.bottomMargin + mFooterHeight + mFooterOffset;
                    }
                    break;
                default:
                    // classic
                    footerBottom = height - paddingBottom - lp.bottomMargin + mFooterHeight + mFooterOffset;
                    break;
            }
            final int footerTop = footerBottom - footerView.getMeasuredHeight();
            final int footerRight = footerLeft + footerView.getMeasuredWidth();

            footerView.layout(footerLeft, footerTop, footerRight, footerBottom);
        }

        if (mStyle == STYLE.CLASSIC
                || mStyle == STYLE.ABOVE) {
            if (mHeaderView != null) {
                mHeaderView.bringToFront();
            }
            if (mFooterView != null) {
                mFooterView.bringToFront();
            }
        } else if (mStyle == STYLE.BLEW || mStyle == STYLE.SCALE || mStyle == STYLE.BLEW2CLASSIC) {
            if (mTargetView != null) {
                mTargetView.bringToFront();
            }
        }
    }

以下拉刷新为例，看这行代码。
paddingTop与lp.topMargin都是0，mHeaderHeight是Header的高度，mHeaderOffset就是手指滑动的距离（这个稍后会有说明）。在下拉过程中，mHeaderOffset的值会越来越大，所以headerTop的值是从-mHeaderHeight开始逐渐增大的，所以headerView会向下逐步移动

headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset

而Target更为简单，你手指滑动多少它就跟着滑动多少

targetTop = paddingTop + lp.topMargin + mTargetOffset;

这样能够想象出饿了么滑动到mHeaderHeight高度之后如何处理的吧，请参考我自己定义的style--BLEW2CLASSIC

if (mHeaderOffset > mHeaderHeight) {
    headerTop = paddingTop + lp.topMargin;
}
else {
    headerTop = paddingTop + lp.topMargin - mHeaderHeight + mHeaderOffset;
}

继续往下来到事件分发部分了

@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        final int action = MotionEventCompat.getActionMasked(ev);
        switch (action) {
            case MotionEvent.ACTION_CANCEL:
            case MotionEvent.ACTION_UP:
                // swipeToRefresh -> finger up -> finger down if the status is still swipeToRefresh
                // in onInterceptTouchEvent ACTION_DOWN event will stop the scroller
                // if the event pass to the child view while ACTION_MOVE(condition is false)
                // in onInterceptTouchEvent ACTION_MOVE the ACTION_UP or ACTION_CANCEL will not be
                // passed to onInterceptTouchEvent and onTouchEvent. Instead It will be passed to
                // child view's onTouchEvent. So we must deal this situation in dispatchTouchEvent
                onActivePointerUp();
                break;
        }
        return super.dispatchTouchEvent(ev);
    }

获取事件之后，在手指释放的时候执行onActivePointerUp()，咱们来看看。分别判断了当前是处在下拉以刷新、上拉以加载更多、松开以刷新、松开以加载更多，然后滚动到响应的位置上去。注意在松开状态时，执行了onRelease()回调

    private void onActivePointerUp() {
        if (STATUS.isSwipingToRefresh(mStatus)) {
            // simply return
            scrollSwipingToRefreshToDefault();

        } else if (STATUS.isSwipingToLoadMore(mStatus)) {
            // simply return
            scrollSwipingToLoadMoreToDefault();

        } else if (STATUS.isReleaseToRefresh(mStatus)) {
            // return to header height and perform refresh
            mRefreshCallback.onRelease();
            scrollReleaseToRefreshToRefreshing();

        } else if (STATUS.isReleaseToLoadMore(mStatus)) {
            // return to footer height and perform loadMore
            mLoadMoreCallback.onRelease();
            scrollReleaseToLoadMoreToLoadingMore();

        }
    }

随后就是事件拦截的判断。只要你向下滑动时Target确实不能再向下移动了或者向上滑动时Target确实不能再向上移动了，那么SwipeRefreshLayout就把事件拦截，执行onTouchEvent里面的位移操作了

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        final int action = MotionEventCompat.getActionMasked(event);
        switch (action) {
            case MotionEvent.ACTION_DOWN:

                mActivePointerId = MotionEventCompat.getPointerId(event, 0);
                mInitDownY = mLastY = getMotionEventY(event, mActivePointerId);
                mInitDownX = mLastX = getMotionEventX(event, mActivePointerId);

                // if it isn't an ing status or default status
                if (STATUS.isSwipingToRefresh(mStatus) || STATUS.isSwipingToLoadMore(mStatus) ||
                        STATUS.isReleaseToRefresh(mStatus) || STATUS.isReleaseToLoadMore(mStatus)) {
                    // abort autoScrolling, not trigger the method #autoScrollFinished()
                    mAutoScroller.abortIfRunning();
                    if (mDebug) {
                        Log.i(TAG, "Another finger down, abort auto scrolling, let the new finger handle");
                    }
                }

                if (STATUS.isSwipingToRefresh(mStatus) || STATUS.isReleaseToRefresh(mStatus)
                        || STATUS.isSwipingToLoadMore(mStatus) || STATUS.isReleaseToLoadMore(mStatus)) {
                    return true;
                }

                // let children view handle the ACTION_DOWN;

                // 1\. children consumed:
                // if at least one of children onTouchEvent() ACTION_DOWN return true.
                // ACTION_DOWN event will not return to SwipeToLoadLayout#onTouchEvent().
                // but the others action can be handled by SwipeToLoadLayout#onInterceptTouchEvent()

                // 2\. children not consumed:
                // if children onTouchEvent() ACTION_DOWN return false.
                // ACTION_DOWN event will return to SwipeToLoadLayout's onTouchEvent().
                // SwipeToLoadLayout#onTouchEvent() ACTION_DOWN return true to consume the ACTION_DOWN event.

                // anyway: handle action down in onInterceptTouchEvent() to init is an good option
                break;
            case MotionEvent.ACTION_MOVE:
                if (mActivePointerId == INVALID_POINTER) {
                    return false;
                }
                float y = getMotionEventY(event, mActivePointerId);
                float x = getMotionEventX(event, mActivePointerId);
                final float yInitDiff = y - mInitDownY;
                final float xInitDiff = x - mInitDownX;
                mLastY = y;
                mLastX = x;
                boolean moved = Math.abs(yInitDiff) > Math.abs(xInitDiff)
                        && Math.abs(yInitDiff) > mTouchSlop;
                boolean triggerCondition =
                        // refresh trigger condition
                        (yInitDiff > 0 && moved && onCheckCanRefresh()) ||
                                //load more trigger condition
                                (yInitDiff < 0 && moved && onCheckCanLoadMore());
                if (triggerCondition) {
                    // if the refresh's or load more's trigger condition  is true,
                    // intercept the move action event and pass it to SwipeToLoadLayout#onTouchEvent()
                    return true;
                }
                break;
            case MotionEvent.ACTION_POINTER_UP: {
                onSecondaryPointerUp(event);
                mInitDownY = mLastY = getMotionEventY(event, mActivePointerId);
                mInitDownX = mLastX = getMotionEventX(event, mActivePointerId);
                break;
            }
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                mActivePointerId = INVALID_POINTER;
                break;
        }
        return super.onInterceptTouchEvent(event);
    }

下面就是位移过程。
如果当期处于初始STATUS_DEFAULT状态，则进入STATUS_SWIPING_TO_REFRESH，同时回调onPrepare()方法
如果在下拉刷新流程中向上滑动并且滑动偏移量小于0，为了不让Target部分移动到屏幕之外，则将体系流程恢复到初始STATUS_DEFAULT状态，同时使用fixCurrentStatusLayout()方法调整三个View的位置。上拉加载更多流程同理
在正常下拉刷新流程中，如果当期状态是STATUS_SWIPING_TO_REFRESH或者是STATUS_RELEASE_TO_REFRESH，即处于下拉以刷新、松开以刷新状态，如果下拉的距离超过mRefreshTriggerOffset，则进入松开以刷新状态，反之则进入下拉以刷新状态。上拉加载更多流程同理
这时候会触发位移发生fingerScroll()

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        final int action = MotionEventCompat.getActionMasked(event);

        switch (action) {
            case MotionEvent.ACTION_DOWN:
                mActivePointerId = MotionEventCompat.getPointerId(event, 0);
                return true;

            case MotionEvent.ACTION_MOVE:
                // take over the ACTION_MOVE event from SwipeToLoadLayout#onInterceptTouchEvent()
                // if condition is true
                final float y = getMotionEventY(event, mActivePointerId);
                final float x = getMotionEventX(event, mActivePointerId);

                final float yDiff = y - mLastY;
                final float xDiff = x - mLastX;
                mLastY = y;
                mLastX = x;

                if (Math.abs(xDiff) > Math.abs(yDiff) && Math.abs(xDiff) > mTouchSlop) {
                    return true;
                }

                if (STATUS.isStatusDefault(mStatus)) {
                    if (yDiff > 0 && onCheckCanRefresh()) {
                        mRefreshCallback.onPrepare();
                        setStatus(STATUS.STATUS_SWIPING_TO_REFRESH);
                    } else if (yDiff < 0 && onCheckCanLoadMore()) {
                        mLoadMoreCallback.onPrepare();
                        setStatus(STATUS.STATUS_SWIPING_TO_LOAD_MORE);
                    }
                } else if (STATUS.isRefreshStatus(mStatus)) {
                    if (mTargetOffset <= 0) {
                        setStatus(STATUS.STATUS_DEFAULT);
                        fixCurrentStatusLayout();
                        return true;
                    }
                } else if (STATUS.isLoadMoreStatus(mStatus)) {
                    if (mTargetOffset >= 0) {
                        setStatus(STATUS.STATUS_DEFAULT);
                        fixCurrentStatusLayout();
                        return true;
                    }
                }

                if (STATUS.isRefreshStatus(mStatus)) {
                    if (STATUS.isSwipingToRefresh(mStatus) || STATUS.isReleaseToRefresh(mStatus)) {
                        if (mTargetOffset >= mRefreshTriggerOffset) {
                            setStatus(STATUS.STATUS_RELEASE_TO_REFRESH);
                        } else {
                            setStatus(STATUS.STATUS_SWIPING_TO_REFRESH);
                        }
                        fingerScroll(yDiff);
                    }
                } else if (STATUS.isLoadMoreStatus(mStatus)) {
                    if (STATUS.isSwipingToLoadMore(mStatus) || STATUS.isReleaseToLoadMore(mStatus)) {
                        if (-mTargetOffset >= mLoadMoreTriggerOffset) {
                            setStatus(STATUS.STATUS_RELEASE_TO_LOAD_MORE);
                        } else {
                            setStatus(STATUS.STATUS_SWIPING_TO_LOAD_MORE);
                        }
                        fingerScroll(yDiff);
                    }
                }
                return true;

            case MotionEvent.ACTION_POINTER_DOWN: {
                final int pointerIndex = MotionEventCompat.getActionIndex(event);
                final int pointerId = MotionEventCompat.getPointerId(event, pointerIndex);
                if (pointerId != INVALID_POINTER) {
                    mActivePointerId = pointerId;
                }
                mInitDownY = mLastY = getMotionEventY(event, mActivePointerId);
                mInitDownX = mLastX = getMotionEventX(event, mActivePointerId);
                break;
            }
            case MotionEvent.ACTION_POINTER_UP: {
                onSecondaryPointerUp(event);
                mInitDownY = mLastY = getMotionEventY(event, mActivePointerId);
                mInitDownX = mLastX = getMotionEventX(event, mActivePointerId);
                break;
            }
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                if (mActivePointerId == INVALID_POINTER) {
                    return false;
                }
                mActivePointerId = INVALID_POINTER;
                break;
            default:
                break;
        }
        return super.onTouchEvent(event);
    }

位移无非就是对mTargetOffset进行赋值，同时调整三个View的位置。注意这里调用了onMove()回调

    private void fingerScroll(final float yDiff) {
        float ratio = mDragRatio;
        float yScrolled = yDiff * ratio;

        // make sure (targetOffset>0 -> targetOffset=0 -> default status)
        // or (targetOffset<0 -> targetOffset=0 -> default status)
        // forbidden fling (targetOffset>0 -> targetOffset=0 ->targetOffset<0 -> default status)
        // or (targetOffset<0 -> targetOffset=0 ->targetOffset>0 -> default status)
        // I am so smart :)

        float tmpTargetOffset = yScrolled + mTargetOffset;
        if ((tmpTargetOffset > 0 && mTargetOffset < 0)
                || (tmpTargetOffset < 0 && mTargetOffset > 0)) {
            yScrolled = -mTargetOffset;
        }

        if (mRefreshFinalDragOffset >= mRefreshTriggerOffset && tmpTargetOffset > mRefreshFinalDragOffset) {
            yScrolled = mRefreshFinalDragOffset - mTargetOffset;
        } else if (mLoadMoreFinalDragOffset >= mLoadMoreTriggerOffset && -tmpTargetOffset > mLoadMoreFinalDragOffset) {
            yScrolled = -mLoadMoreFinalDragOffset - mTargetOffset;
        }

        if (STATUS.isRefreshStatus(mStatus)) {
            mRefreshCallback.onMove(mTargetOffset, false, false);
        } else if (STATUS.isLoadMoreStatus(mStatus)) {
            mLoadMoreCallback.onMove(mTargetOffset, false, false);
        }
        updateScroll(yScrolled);
    }

    private void updateScroll(final float yScrolled) {
        if (yScrolled == 0) {
            return;
        }
        mTargetOffset += yScrolled;

        if (STATUS.isRefreshStatus(mStatus)) {
            mHeaderOffset = mTargetOffset;
            mFooterOffset = 0;
        } else if (STATUS.isLoadMoreStatus(mStatus)) {
            mFooterOffset = mTargetOffset;
            mHeaderOffset = 0;
        }

        if (mDebug) {
            Log.i(TAG, "mTargetOffset = " + mTargetOffset);
        }
        layoutChildren();
        invalidate();
    }

最后就是执行结束刷新操作，完成闭环。结束的时候，refreshing值为false，执行onComplete()回调，同时回滚到初始位置

    public void setRefreshing(boolean refreshing) {
        if (!isRefreshEnabled() || mHeaderView == null) {
            return;
        }
        this.mAutoLoading = refreshing;
        if (refreshing) {
            if (STATUS.isStatusDefault(mStatus)) {
                setStatus(STATUS.STATUS_SWIPING_TO_REFRESH);
                scrollDefaultToRefreshing();
            }
        } else {
            if (STATUS.isRefreshing(mStatus)) {
                mRefreshCallback.onComplete();
                postDelayed(new Runnable() {
                    @Override
                    public void run() {
                        scrollRefreshingToDefault();
                    }
                }, mRefreshCompleteDelayDuration);
            }
        }
    }

这里还有一个补充，关于自动滑动方面。自动滚动一般都是通过AutoScroller类，调用其autoScroll()方法来完成，而实际上也是调用Scroller.startScroll()。但是不知道你有没有注意到post(this)，它在反复调用这个Runnable的run()来判断滑动是否已经结束。如果没有结束，则通过autoScroll()方法来调用move()回调；如果已经结束，则通过autoScrollFinished()方法来判断下一步应该到达何种状态

private class AutoScroller implements Runnable {

        private Scroller mScroller;

        private int mmLastY;

        private boolean mRunning = false;

        private boolean mAbort = false;

        public AutoScroller() {
            mScroller = new Scroller(getContext());
        }

        @Override
        public void run() {
            boolean finish = !mScroller.computeScrollOffset() || mScroller.isFinished();
            int currY = mScroller.getCurrY();
            int yDiff = currY - mmLastY;
            if (finish) {
                finish();
            } else {
                mmLastY = currY;
                SwipeToLoadLayout.this.autoScroll(yDiff);
                post(this);
            }
        }

        /**
         * remove the post callbacks and reset default values
         */
        private void finish() {
            mmLastY = 0;
            mRunning = false;
            removeCallbacks(this);
            // if abort by user, don't call
            if (!mAbort) {
                autoScrollFinished();
            }
        }

        /**
         * abort scroll if it is scrolling
         */
        public void abortIfRunning() {
            if (mRunning) {
                if (!mScroller.isFinished()) {
                    mAbort = true;
                    mScroller.forceFinished(true);
                }
                finish();
                mAbort = false;
            }
        }

        /**
         * The param yScrolled here isn't final pos of y.
         * It's just like the yScrolled param in the
         * {@link #updateScroll(float yScrolled)}
         *
         * @param yScrolled
         * @param duration
         */
        private void autoScroll(int yScrolled, int duration) {
            removeCallbacks(this);
            mmLastY = 0;
            if (!mScroller.isFinished()) {
                mScroller.forceFinished(true);
            }
            mScroller.startScroll(0, 0, 0, yScrolled, duration);
            post(this);
            mRunning = true;
        }
    }

如果是松开以刷新，则进入刷新状态，同时回调onRefresh()方法
如果是正在刷新状态，则复原，执行onReset()方法
如果是松开以刷新并且通过setRefresh(true)方法进来的，则进入正在刷新状态，执行onRefresh()方法；反之则执行复原操作，执行onReset()方法。
上拉加载更多流程同理

private void autoScrollFinished() {
        int mLastStatus = mStatus;

        if (STATUS.isReleaseToRefresh(mStatus)) {
            setStatus(STATUS.STATUS_REFRESHING);
            fixCurrentStatusLayout();
            mRefreshCallback.onRefresh();

        } else if (STATUS.isRefreshing(mStatus)) {
            setStatus(STATUS.STATUS_DEFAULT);
            fixCurrentStatusLayout();
            mRefreshCallback.onReset();

        } else if (STATUS.isSwipingToRefresh(mStatus)) {
            if (mAutoLoading) {
                mAutoLoading = false;
                setStatus(STATUS.STATUS_REFRESHING);
                fixCurrentStatusLayout();
                mRefreshCallback.onRefresh();
            } else {
                setStatus(STATUS.STATUS_DEFAULT);
                fixCurrentStatusLayout();
                mRefreshCallback.onReset();
            }
        } else if (STATUS.isStatusDefault(mStatus)) {

        } else if (STATUS.isSwipingToLoadMore(mStatus)) {
            if (mAutoLoading) {
                mAutoLoading = false;
                setStatus(STATUS.STATUS_LOADING_MORE);
                fixCurrentStatusLayout();
                mLoadMoreCallback.onLoadMore();
            } else {
                setStatus(STATUS.STATUS_DEFAULT);
                fixCurrentStatusLayout();
                mLoadMoreCallback.onReset();
            }
        } else if (STATUS.isLoadingMore(mStatus)) {
            setStatus(STATUS.STATUS_DEFAULT);
            fixCurrentStatusLayout();
            mLoadMoreCallback.onReset();
        } else if (STATUS.isReleaseToLoadMore(mStatus)) {
            setStatus(STATUS.STATUS_LOADING_MORE);
            fixCurrentStatusLayout();
            mLoadMoreCallback.onLoadMore();
        } else {
            throw new IllegalStateException("illegal state: " + STATUS.getStatus(mStatus));
        }

        if (mDebug) {
            Log.i(TAG, STATUS.getStatus(mLastStatus) + " -> " + STATUS.getStatus(mStatus));
        }
    }

源码分析到此结束。怎么样，是不是很简单

参考文章
MNSwipeToLoadDemo

链接：https://www.jianshu.com/p/fc8c73db72b3