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NESTED SCROLLING LIVING LIFE ON THE EDGE +ChrisBanes @chrisbanes

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User swipes up, collapsing the AppBarLayout and scrolling the list

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User swipes down, scrolling the list

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User needs to swipe down again, to expand the AppBarLayout

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The problem: Flinging does not transfer left over velocity

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The goal: Flinging transfers velocity when it reaches the end

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> NESTED SCROLLING: LIVING LIFE ON THE EDGE Nested Scrolling NESTED SCROLLING

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Nested scrolling views was difficult

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Nested scrolling views was difficult

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No content

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Nested scrolling was created to solve this

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An event system where scroll events are dispatched up the view hierarchy

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ScrollView LinearLayout ImageView ListView

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void setNestedScrollingEnabled(boolean enabled); boolean isNestedScrollingEnabled(); boolean startNestedScroll(int axes); boolean hasNestedScrollingParent(); boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed,
 int[] offsetInWindow); boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); boolean dispatchNestedPreFling(float velocityX, float velocityY); boolean dispatchNestedFling(float velocityX, float velocityY,
 boolean consumed); void stopNestedScroll(); Sending

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void setNestedScrollingEnabled(boolean enabled); boolean isNestedScrollingEnabled(); boolean startNestedScroll(int axes); boolean hasNestedScrollingParent(); boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed,
 int[] offsetInWindow); boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); boolean dispatchNestedPreFling(float velocityX, float velocityY); boolean dispatchNestedFling(float velocityX, float velocityY,
 boolean consumed); void stopNestedScroll(); Sending

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boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed,
 int[] offsetInWindow); boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); boolean dispatchNestedPreFling(float velocityX, float velocityY); boolean dispatchNestedFling(float velocityX, float velocityY,
 boolean consumed); Sending

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boolean onStartNestedScroll(View child, View target,
 int nestedScrollAxes); void onNestedScrollAccepted(View child, View target,
 int nestedScrollAxes); void onStopNestedScroll(View target); int getNestedScrollAxes(); Receiving void onNestedPreScroll(View target, int dx, int dy, int[] consumed); void onNestedScroll(View target, int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed); boolean onNestedPreFling(View target, float velocityX,
 float velocityY); boolean onNestedFling(View target, float velocityX,
 float velocityY, boolean consumed);

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boolean onStartNestedScroll(View child, View target,
 int nestedScrollAxes); void onNestedScrollAccepted(View child, View target,
 int nestedScrollAxes); void onStopNestedScroll(View target); int getNestedScrollAxes(); Receiving void onNestedPreScroll(View target, int dx, int dy, int[] consumed); void onNestedScroll(View target, int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed); boolean onNestedPreFling(View target, float velocityX,
 float velocityY); boolean onNestedFling(View target, float velocityX,
 float velocityY, boolean consumed);

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Receiving void onNestedPreScroll(View target, int dx, int dy, int[] consumed); void onNestedScroll(View target, int dxConsumed, int dyConsumed,
 int dxUnconsumed, int dyUnconsumed); boolean onNestedPreFling(View target, float velocityX,
 float velocityY); boolean onNestedFling(View target, float velocityX,
 float velocityY, boolean consumed);

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Pre vs Non-Pre

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Pre vs Non-Pre Pre-scroll events allow a parent to intercept and consume the event

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Pre vs Non-Pre The view then scrolls the remainder Pre-scroll events allow a parent to intercept and consume the event

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Pre vs Non-Pre Scroll events allow the parent to observe what the view scrolled The view then scrolls the remainder Pre-scroll events allow a parent to intercept and consume the event

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Let’s see how it happens…

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ACTION_DOWN

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ACTION_DOWN // Initialise VelocityTracker and add event mVelocityTracker.addMovement(ev); int y = ev.getY(); // 1020 mInitialMotionY = y; mLastMotionY = y; startNestedScroll(SCROLL_AXIS_VERTICAL);

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ACTION_DOWN startNestedScroll(SCROLL_AXIS_VERTICAL); // Initialise VelocityTracker and add event mVelocityTracker.addMovement(ev); int y = ev.getY(); // 1020 mInitialMotionY = y; mLastMotionY = y;

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ACTION_DOWN startNestedScroll(SCROLL_AXIS_VERTICAL); @Override boolean onStartNestedScroll(View child, View target, int axes) { // child == LinearLayout // target == ListView // axes == SCROLL_AXIS_VERTICAL return true; }

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ACTION_DOWN startNestedScroll(SCROLL_AXIS_VERTICAL); @Override void onNestedScrollAccepted(View child, View target, int axes) { // child == LinearLayout // target == ListView // axes == SCROLL_AXIS_VERTICAL }

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ACTION_MOVE

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ACTION_MOVE int[] mScrollConsumed = new int[2]; int y = ev.getY(); // 1010 int dy = mLastMotionY - y; // -10 if (dispatchNestedPreScroll(0, dy, mScrollConsumed, ...)) { dy -= mScrollConsumed[1]; } // INSERT Move children by remaining dy // TODO call dispatchNestedScroll()

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ACTION_MOVE int[] mScrollConsumed = new int[2]; int y = ev.getY(); // 1010 int dy = mLastMotionY - y; // -10 if (dispatchNestedPreScroll(0, dy, mScrollConsumed, ...)) { dy -= mScrollConsumed[1]; } // INSERT Move children by remaining dy // TODO call dispatchNestedScroll()

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ACTION_MOVE @Override void onNestedPreScroll(View target, int dx, int dy, int[] consumed) { // dx = 0, dy = -10, consumed = int[2] int amountWeConsume = doSomethingWithPreScroll(dy); // consumed[0] = y, consumed[1] = x consumed[1] = amountWeConsume; } dispatchNestedPreScroll(0, dy, mScrollConsumed, ...)

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ACTION_MOVE @Override void onNestedPreScroll(View target, int dx, int dy, int[] consumed) { // dx = 0, dy = -10, consumed = int[2] int amountWeConsume = doSomethingWithPreScroll(dy); // consumed[0] = y, consumed[1] = x consumed[1] = amountWeConsume; } dispatchNestedPreScroll(0, dy, mScrollConsumed, ...) // -3

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ACTION_MOVE int[] mScrollConsumed = new int[2]; int y = ev.getY(); // 1010 int dy = mLastMotionY - y; // -10 if (dispatchNestedPreScroll(0, dy, mScrollConsumed, ...)) { dy -= mScrollConsumed[1]; } // INSERT Move children by remaining dy // TODO call dispatchNestedScroll() // -3 remember // dy = -7

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ACTION_MOVE int[] mScrollConsumed = new int[2]; int y = ev.getY(); // 1010 int dy = mLastMotionY - y; // -10 if (dispatchNestedPreScroll(0, dy, mScrollConsumed, ...)) { dy -= mScrollConsumed[1]; } // INSERT Move children by remaining dy // TODO call dispatchNestedScroll()

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ACTION_MOVE // INSERT Move children by remaining dy dispatchNestedScroll(0, dy, 0, unconsumedY, ...) @Override void onNestedScroll(View target,
 int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) { // Do something if you wish }

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ACTION_UP

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ACTION_UP float velY = ...; if (!dispatchNestedPreFling(0, velY)) { // Parent hasn't consumed fling, lets fling // and let the parent observe dispatchNestedFling(0, velY, ...); fling(velY); } stopNestedScroll();

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ACTION_UP float velY = ...; if (!dispatchNestedPreFling(0, velY)) { // Parent hasn't consumed fling, lets fling // and let the parent observe dispatchNestedFling(0, velY, ...); fling(velY); } stopNestedScroll();

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ACTION_UP dispatchNestedPreFling(0, velY); @Override boolean onNestedPreFling(View target, float velX, float velY) { // return true to consume the whole fling, // false to let the view handle it }

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ACTION_UP float velY = ...; if (!dispatchNestedPreFling(0, velY)) { // Parent hasn't consumed fling, lets fling // and let the parent observe dispatchNestedFling(0, velY, ...); fling(velY); } stopNestedScroll();

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ACTION_UP float velY = ...; if (!dispatchNestedPreFling(0, velY)) { // Parent hasn't consumed fling, lets fling // and let the parent observe dispatchNestedFling(0, velY, ...); fling(velY); } stopNestedScroll();

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The problem: Flinging does not transfer left over velocity

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The problem: Nested flinging does not allow interception like scrolling

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> NESTED SCROLLING: LIVING LIFE ON THE EDGE Indirect Nested Scrolling INDIRECT NESTED SCROLLING Work in progress

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Nested scrolling

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Direct nested scrolling

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Direct nested scrolling User directly touching screen

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User not directly touching screen Indirect nested scrolling

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Settling from fling Setting scroll position programmatically Scrolling from KeyEvents ?

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API is mostly the same as nested scrolling

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void setNestedIndirectScrollingEnabled(boolean enabled); boolean isNestedIndirectScrollingEnabled(); boolean startNestedIndirectScroll(int axes); boolean dispatchNestedIndirectPreScroll(int dx, int dy,
 int[] consumed); boolean dispatchNestedIndirectScroll(int dxConsumed,
 int dyConsumed, int dxUnconsumed, int dyUnconsumed); boolean hasNestedIndirectScrollingParent(); void stopNestedIndirectScroll(); Sending

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boolean onStartNestedIndirectScroll(View child, View target,
 int axes); void onNestedIndirectScrollAccepted(View child, View target,
 int axes); void onNestedIndirectPreScroll(View target, int dx, int dy,
 int[] consumed); void onNestedIndirectScroll(View target, int dxConsumed,
 int dyConsumed, int dxUnconsumed, int dyUnconsumed); void onStopNestedIndirectScroll(View child); Receiving

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Fling API is mostly the same as nested scrolling

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Fling API is mostly the same as nested scrolling Works with nested scrolling

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No content

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startNestedScroll(...) ACTION_DOWN

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dispatchNestedPreScroll(...) dispatchNestedScroll(...) ACTION_MOVE

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ACTION_MOVE

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stopNestedScroll() ACTION_UP fling() startNestedIndirectScroll(...) ↳

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dispatchNestedIndirectPreScroll(...) dispatchNestedIndirectScroll(...)

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stopNestedIndirectScroll()

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Full demo

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Over and out… +ChrisBanes @chrisbanes