Android源碼進(jìn)階之ViewDragHelper原理機(jī)制解析
本文轉(zhuǎn)載自微信公眾號(hào)「Android開發(fā)編程」，作者Android開發(fā)編程 。轉(zhuǎn)載本文請(qǐng)聯(lián)系A(chǔ)ndroid開發(fā)編程公眾號(hào)。
前言
ViewDragHelper類，是用來處理View邊界拖動(dòng)相關(guān)的類;
主要功能處理在View上的觸摸事件，記錄觸摸點(diǎn)、計(jì)算距離、滾動(dòng)動(dòng)畫、狀態(tài)回調(diào)等，如果我們自己手動(dòng)實(shí)現(xiàn)自然會(huì)很麻煩還可能出錯(cuò)，而這個(gè)類會(huì)幫助我們大大簡化工作量;
今天我們就來分析一波;
一、ViewDragHelper的中主要API介紹
1、ViewDragHelper create(ViewGroup forParent, Callback cb)
一個(gè)靜態(tài)的創(chuàng)建方法;
- 參數(shù)1：出入的是相應(yīng)的ViewGroup;
- 參數(shù)2：是一個(gè)回掉，需要自己實(shí)現(xiàn);
2、shouldInterceptTouchEvent(MotionEvent ev)
處理事件分發(fā)的(怎么說這個(gè)方法呢?主要是將ViewGroup的事件分發(fā)，委托給ViewDragHelper進(jìn)行處理);
- 參數(shù)1：MotionEvent ev 主要是ViewGroup的事件;
3、processTouchEvent(MotionEvent event)
處理相應(yīng)TouchEvent的方法，這里要注意一個(gè)問題，處理相應(yīng)的TouchEvent的時(shí)候要將結(jié)果返回為true，消費(fèi)本次事件,否則將無法使用ViewDragHelper處理相應(yīng)的拖拽事件;
4、ViewDragHelper.Callback的API
tryCaptureView(View child, int pointerId) 這是一個(gè)抽象類，必須去實(shí)現(xiàn)，也只有在這個(gè)方法返回true的時(shí)候下面的方法才會(huì)生效;
onViewDragStateChanged(int state) 當(dāng)狀態(tài)改變的時(shí)候回調(diào)，返回相應(yīng)的狀態(tài)(這里有三種狀態(tài));
- STATE_IDLE 閑置狀態(tài);
- STATE_DRAGGING 正在拖動(dòng);
- STATE_SETTLING 放置到某個(gè)位置;
onViewPositionChanged(View changedView, int left, int top, int dx, int dy) 當(dāng)你拖動(dòng)的View位置發(fā)生改變的時(shí)候回調(diào);
- 參數(shù)1：你當(dāng)前拖動(dòng)的這個(gè)View
- 參數(shù)2：距離左邊的距離
- 參數(shù)3：距離右邊的距離
- 參數(shù)4：x軸的變化量
- 參數(shù)5：y軸的變化量
onViewCaptured(View capturedChild, int activePointerId)捕獲View的時(shí)候調(diào)用的方法
- 參數(shù)1：捕獲的View(也就是你拖動(dòng)的這個(gè)View);
- 參數(shù)2：這個(gè)參數(shù)我也不知道什么意思API中寫的一個(gè)什么指針，這里沒有到也沒有注意;
onViewReleased(View releasedChild, float xvel, float yvel) 當(dāng)View停止拖拽的時(shí)候調(diào)用的方法
- 參數(shù)1：你拖拽的這個(gè)View
- 參數(shù)2：x軸的速率
- 參數(shù)3：y軸的速率
clampViewPositionVertical(View child, int top, int dy) 豎直拖拽的時(shí)候回調(diào)的方法
- 參數(shù)1：拖拽的View
- 參數(shù)2：距離頂部的距離
- 參數(shù)3：變化量
clampViewPositionHorizontal(View child, int left, int dx) 水平拖拽的時(shí)候回調(diào)的方法
- 參數(shù)1：拖拽的View
- 參數(shù)2：距離左邊的距離
- 參數(shù)3：變化量
二、實(shí)現(xiàn)原理介紹
1、初始化
- private ViewDragHelper(Context context, ViewGroup forParent, Callback cb) {
- ...
- mParentView = forParent;//BaseView
- mCallback = cb;//callback
- final ViewConfiguration vc = ViewConfiguration.get(context);
- final float density = context.getResources().getDisplayMetrics().density;
- mEdgeSize = (int) (EDGE_SIZE * density + 0.5f);//邊界拖動(dòng)距離范圍
- mTouchSlop = vc.getScaledTouchSlop();//拖動(dòng)距離閾值
- mScroller = new OverScroller(context, sInterpolator);//滾動(dòng)器
- }
- mParentView是指基于哪個(gè)View進(jìn)行觸摸處理;
- mCallback是觸摸處理的各個(gè)階段的回調(diào);
- mEdgeSize是指在邊界多少距離內(nèi)算作拖動(dòng)，默認(rèn)為20dp;
- mTouchSlop指滑動(dòng)多少距離算作拖動(dòng)，用的系統(tǒng)默認(rèn)值;
- mScroller是View滾動(dòng)的Scroller對(duì)象，用于處理釋觸摸放后，View的滾動(dòng)行為，比如滾動(dòng)回原始位置或者滾動(dòng)出屏幕;
2.攔截事件處理
該類提供了boolean shouldInterceptTouchEvent(MotionEvent)方法：
- override fun onInterceptTouchEvent(ev: MotionEvent?) =
- dragHelper?.shouldInterceptTouchEvent(ev) ?: super.onInterceptTouchEvent(ev)
該方法用于處理mParentView是否攔截此次事件
- public boolean shouldInterceptTouchEvent(MotionEvent ev) {
- ...
- switch (action) {
- ...
- case MotionEvent.ACTION_MOVE: {
- if (mInitialMotionX == null || mInitialMotionY == null) break;
- // First to cross a touch slop over a draggable view wins. Also report edge drags.
- final int pointerCount = ev.getPointerCount();
- for (int i = 0; i < pointerCount; i++) {
- final int pointerId = ev.getPointerId(i);
- // If pointer is invalid then skip the ACTION_MOVE.
- if (!isValidPointerForActionMove(pointerId)) continue;
- final float x = ev.getX(i);
- final float y = ev.getY(i);
- final float dx = x - mInitialMotionX[pointerId];
- final float dy = y - mInitialMotionY[pointerId];
- final View toCapture = findTopChildUnder((int) x, (int) y);
- final boolean pastSlop = toCapture != null && checkTouchSlop(toCapture, dx, dy);
- ...
- //判斷pointer的拖動(dòng)邊界
- reportNewEdgeDrags(dx, dy, pointerId);
- ...
- }
- saveLastMotion(ev);
- break;
- }
- ...
- }
- return mDragState == STATE_DRAGGING;
- }
攔截事件的前提是mDragState為STATE_DRAGGING，也就是正在拖動(dòng)狀態(tài)下才會(huì)攔截，那么什么時(shí)候會(huì)變?yōu)橥蟿?dòng)狀態(tài)呢?當(dāng)ACTION_MOVE時(shí)，調(diào)用reportNewEdgeDrags方法:
- private void reportNewEdgeDrags(float dx, float dy, int pointerId) {
- int dragsStarted = 0;
- //判斷是否在Left邊緣進(jìn)行滑動(dòng)
- if (checkNewEdgeDrag(dx, dy, pointerId, EDGE_LEFT)) {
- dragsStarted |= EDGE_LEFT;
- }
- if (checkNewEdgeDrag(dy, dx, pointerId, EDGE_TOP)) {
- dragsStarted |= EDGE_TOP;
- }
- ...
- if (dragsStarted != 0) {
- mEdgeDragsInProgress[pointerId] |= dragsStarted;
- //回調(diào)拖動(dòng)的邊
- mCallback.onEdgeDragStarted(dragsStarted, pointerId);
- }
- }
- private boolean checkNewEdgeDrag(float delta, float odelta, int pointerId, int edge) {
- final float absDelta = Math.abs(delta);
- final float absODelta = Math.abs(odelta);
- //是否支持edge的拖動(dòng)以及是否滿足拖動(dòng)距離的閾值
- if ((mInitialEdgesTouched[pointerId] & edge) != edge || (mTrackingEdges & edge) == 0
- || (mEdgeDragsLocked[pointerId] & edge) == edge
- || (mEdgeDragsInProgress[pointerId] & edge) == edge
- || (absDelta <= mTouchSlop && absODelta <= mTouchSlop)) {
- return false;
- }
- if (absDelta < absODelta * 0.5f && mCallback.onEdgeLock(edge)) {
- mEdgeDragsLocked[pointerId] |= edge;
- return false;
- }
- return (mEdgeDragsInProgress[pointerId] & edge) == 0 && absDelta > mTouchSlop;
- }
可以看到，當(dāng)ACTION_MOVE時(shí)，會(huì)嘗試找到pointer對(duì)應(yīng)的拖動(dòng)邊界，這個(gè)邊界可以由我們來制定，比如側(cè)滑關(guān)閉頁面是從左側(cè)開始的，所以我們可以調(diào)用setEdgeTrackingEnabled(ViewDragHelper.EDGE_LEFT)來設(shè)置只支持左側(cè)滑動(dòng)。而一旦有滾動(dòng)發(fā)生，就會(huì)回調(diào)callback的onEdgeDragStarted方法，交由我們做如下操作：
- override fun onEdgeDragStarted(edgeFlags: Int, pointerId: Int) {
- super.onEdgeDragStarted(edgeFlags, pointerId)
- dragHelper?.captureChildView(getChildAt(0), pointerId)
- }
- 我們調(diào)用了ViewDragHelper的captureChildView方法：
- public void captureChildView(View childView, int activePointerId) {
- mCapturedView = childView;//記錄拖動(dòng)view
- mActivePointerId = activePointerId;
- mCallback.onViewCaptured(childView, activePointerId);
- setDragState(STATE_DRAGGING);//設(shè)置狀態(tài)為開始拖動(dòng)
- }
此時(shí)，就記錄了拖動(dòng)的View，并將狀態(tài)置為拖動(dòng)，那么在下次ACTION_MOVE的時(shí)候，該mParentView就會(huì)攔截事件，交由自己的onTouchEvent方法處理拖動(dòng)了;
3.拖動(dòng)事件處理
該類提供了void processTouchEvent(MotionEvent)方法，通常我們需要這么寫：
- override fun onTouchEvent(event: MotionEvent?): Boolean {
- dragHelper?.processTouchEvent(event)//交由ViewDragHelper處理
- return true
- }
該方法用于處理mParentView攔截事件后的拖動(dòng)處理：
- public void processTouchEvent(MotionEvent ev) {
- ...
- switch (action) {
- ...
- case MotionEvent.ACTION_MOVE: {
- if (mDragState == STATE_DRAGGING) {
- // If pointer is invalid then skip the ACTION_MOVE.
- if (!isValidPointerForActionMove(mActivePointerId)) break;
- final int index = ev.findPointerIndex(mActivePointerId);
- final float x = ev.getX(index);
- final float y = ev.getY(index);
- //計(jì)算距離上次的拖動(dòng)距離
- final int idx = (int) (x - mLastMotionX[mActivePointerId]);
- final int idy = (int) (y - mLastMotionY[mActivePointerId]);
- dragTo(mCapturedView.getLeft() + idx, mCapturedView.getTop() + idy, idx, idy);//處理拖動(dòng)
- saveLastMotion(ev);//記錄當(dāng)前觸摸點(diǎn)
- }...
- break;
- }
- ...
- case MotionEvent.ACTION_UP: {
- if (mDragState == STATE_DRAGGING) {
- releaseViewForPointerUp();//釋放拖動(dòng)view
- }
- cancel();
- break;
- }...
- }
- }
(1)拖動(dòng)
ACTION_MOVE時(shí)，會(huì)計(jì)算出pointer距離上次的位移，然后計(jì)算出capturedView的目標(biāo)位置，進(jìn)行拖動(dòng)處理;
- private void dragTo(int left, int top, int dx, int dy) {
- int clampedX = left;
- int clampedY = top;
- final int oldLeft = mCapturedView.getLeft();
- final int oldTop = mCapturedView.getTop();
- if (dx != 0) {
- clampedX = mCallback.clampViewPositionHorizontal(mCapturedView, left, dx);//通過callback獲取真正的移動(dòng)值
- ViewCompat.offsetLeftAndRight(mCapturedView, clampedX - oldLeft);//進(jìn)行位移
- }
- if (dy != 0) {
- clampedY = mCallback.clampViewPositionVertical(mCapturedView, top, dy);
- ViewCompat.offsetTopAndBottom(mCapturedView, clampedY - oldTop);
- }
- if (dx != 0 || dy != 0) {
- final int clampedDx = clampedX - oldLeft;
- final int clampedDy = clampedY - oldTop;
- mCallback.onViewPositionChanged(mCapturedView, clampedX, clampedY,
- clampedDx, clampedDy);//callback回調(diào)移動(dòng)后的位置
- }
- }
通過callback的clampViewPositionHorizontal方法決定實(shí)際移動(dòng)的水平距離，通常都是返回left值，即拖動(dòng)了多少就移動(dòng)多少;
通過callback的onViewPositionChanged方法，可以對(duì)View拖動(dòng)后的新位置做一些處理，如;
- override fun onViewPositionChanged(changedView: View?, left: Int, top: Int, dx: Int, dy: Int) {
- super.onViewPositionChanged(changedView, left, top, dx, dy)
- //當(dāng)新的left位置到達(dá)width時(shí)，即滑動(dòng)除了界面，關(guān)閉頁面
- if (left >= width && context is Activity && !context.isFinishing) {
- context.finish()
- }
- }
(2)釋放
而ACTION_UP動(dòng)作時(shí)，要釋放拖動(dòng)View
- private void releaseViewForPointerUp() {
- ...
- dispatchViewReleased(xvel, yvel);
- }
- private void dispatchViewReleased(float xvel, float yvel) {
- mReleaseInProgress = true;
- mCallback.onViewReleased(mCapturedView, xvel, yvel);//callback回調(diào)釋放
- mReleaseInProgress = false;
- if (mDragState == STATE_DRAGGING) {
- // onViewReleased didn't call a method that would have changed this. Go idle.
- setDragState(STATE_IDLE);//重置狀態(tài)
- }
- }
通常在callback的onViewReleased方法中，我們可以判斷當(dāng)前釋放點(diǎn)的位置，從而決定是要回彈頁面還是滑出屏幕
- override fun onViewReleased(releasedChild: View?, xvel: Float, yvel: Float) {
- super.onViewReleased(releasedChild, xvel, yvel)
- //滑動(dòng)速度到達(dá)一定值時(shí)直接關(guān)閉
- if (xvel >= 300) {//滑動(dòng)頁面到屏幕外，關(guān)閉頁面
- dragHelper?.settleCapturedViewAt(width, 0)
- } else {//回彈頁面
- dragHelper?.settleCapturedViewAt(0, 0)
- }
- //刷新，開始關(guān)閉或重置動(dòng)畫
- invalidate()
- }
如滑動(dòng)速度大于300時(shí)，我們調(diào)用settleCapturedViewAt方法將頁面滾動(dòng)出屏幕，否則調(diào)用該方法進(jìn)行回彈
(3)滾動(dòng)
- public boolean settleCapturedViewAt(int finalLeft, int finalTop) {
- return forceSettleCapturedViewAt(finalLeft, finalTop,
- (int) mVelocityTracker.getXVelocity(mActivePointerId),
- (int) mVelocityTracker.getYVelocity(mActivePointerId));
- }
- private boolean forceSettleCapturedViewAt(int finalLeft, int finalTop, int xvel, int yvel) {
- //當(dāng)前位置
- final int startLeft = mCapturedView.getLeft();
- final int startTop = mCapturedView.getTop();
- //偏移量
- final int dx = finalLeft - startLeft;
- final int dy = finalTop - startTop;
- ...
- final int duration = computeSettleDuration(mCapturedView, dx, dy, xvel, yvel);
- //使用Scroller對(duì)象開始滾動(dòng)
- mScroller.startScroll(startLeft, startTop, dx, dy, duration);
- //重置狀態(tài)為滾動(dòng)
- setDragState(STATE_SETTLING);
- return true;
- }
- 其內(nèi)部使用的是Scroller對(duì)象：是View的滾動(dòng)機(jī)制，其回調(diào)是View的computeScroll()方法，在其內(nèi)部通過Scroller對(duì)象的computeScrollOffset方法判斷是否滾動(dòng)完畢，如仍需滾動(dòng)，需要調(diào)用invalidate方法進(jìn)行刷新;
- ViewDragHelper據(jù)此提供了一個(gè)類似的方法continueSettling，需要在computeScroll中調(diào)用，判斷是否需要invalidate;
- public boolean continueSettling(boolean deferCallbacks) {
- if (mDragState == STATE_SETTLING) {
- //是否滾動(dòng)結(jié)束
- boolean keepGoing = mScroller.computeScrollOffset();
- //當(dāng)前滾動(dòng)值
- final int x = mScroller.getCurrX();
- final int y = mScroller.getCurrY();
- //偏移量
- final int dx = x - mCapturedView.getLeft();
- final int dy = y - mCapturedView.getTop();
- //便宜操作
- if (dx != 0) {
- ViewCompat.offsetLeftAndRight(mCapturedView, dx);
- }
- if (dy != 0) {
- ViewCompat.offsetTopAndBottom(mCapturedView, dy);
- }
- //回調(diào)
- if (dx != 0 || dy != 0) {
- mCallback.onViewPositionChanged(mCapturedView, x, y, dx, dy);
- }
- //滾動(dòng)結(jié)束狀態(tài)
- if (!keepGoing) {
- if (deferCallbacks) {
- mParentView.post(mSetIdleRunnable);
- } else {
- setDragState(STATE_IDLE);
- }
- }
- }
- return mDragState == STATE_SETTLING;
- }
在我們的View中
- override fun computeScroll() {
- super.computeScroll()
- if (dragHelper?.continueSettling(true) == true) {
- invalidate()
- }
- }
- 以上，就是ViewDragHelper的實(shí)現(xiàn)原理和使用方式
- override fun computeScroll() {
- super.computeScroll()
- if (dragHelper?.continueSettling(true) == true) {
- invalidate()
- }
- }
以上，就是ViewDragHelper的實(shí)現(xiàn)原理和使用方式
總結(jié)
ViewDragHelper本質(zhì)上是對(duì)MotionEvent的分析及處理，并提供了一系列的監(jiān)聽回調(diào)方法，來幫助我們減輕開發(fā)負(fù)擔(dān)，更為方便地處理控件的滑動(dòng)拖拽邏輯;
是不是覺得很簡單，一起加油，各位老鐵們;
