【移動應用開發(fā)技術(shù)】android中事件分發(fā)機制的實現(xiàn)原理是什么

android中事件分發(fā)機制的實現(xiàn)原理是什么，針對這個問題，這篇文章詳細介紹了相對應的分析和解答，希望可以幫助更多想解決這個問題的小伙伴找到更簡單易行的方法。android中的事件處理，以及解決滑動沖突問題都離不開事件分發(fā)機制，android中的事件流，即MotionEvent都會經(jīng)歷一個從分發(fā)，攔截到處理的一個過程。即dispatchTouchEvent(),onInterceptEvent()到onTouchEvent()的一個過程，在dispatchTouchEvent()負責了事件的分發(fā)過程，在dispatchTouchEvent()中會調(diào)用onInterceptEvent()與onTouchEvent()，如果onInterceptEvent()返回true，那么會調(diào)用到當前view的onTouchEvent()方法，如果不攔截，事件就會下發(fā)到子view的dispatchTouchEvent()中進行同樣的操作。本文將帶領大家從源碼角度來分析android是如何進行事件分發(fā)的。android中的事件分發(fā)流程最先從activity的dispatchTouchEvent()開始:public

boolean

dispatchTouchEvent(MotionEvent

ev)

{

if

(ev.getAction()

==

MotionEvent.ACTION_DOWN)

{

onUserInteraction();

}

if

(getWidow().superDispatchTouchEvent(ev))

{

return

true;

}

return

onTouchEvent(ev);

}這里調(diào)用了getWindow().superDispatchTouchEvent(ev)，這里可以看出activity將MotionEvent傳寄給了Window。而Window是一個抽象類，superDispatchTouchEvent()也是一個抽象方法，這里用到的是window的子類phoneWindow。@Override

public

boolean

superDispatchTouchEvent(MotionEvent

event)

{

return

mDecor.superDispatchTouchEvent(event);

}從這里可以看出，event事件被傳到了DecorView,也就是我們的頂層view.我們繼續(xù)跟蹤：public

boolean

superDispatchTouchEvent(MotionEvent

event)

{

return

super.dispatchTouchEvent(event);

}這里調(diào)用到了父類的dispatchTouchEvent()方法，而DecorView是繼承自FrameLayout，F(xiàn)rameLayout繼承了ViewGroup,所以這里會調(diào)用到ViewGroup的dispatchTouchEvent()方法。所以整個事件流從activity開始，傳遞到window，最后再到我們的view(viewGroup也是繼承自view)中，而view才是我們整個事件處理的核心階段。我們來看一下viewGroup的dispatchTouchEvent()中的實現(xiàn)：if

(actionMasked

==

MotionEvent.ACTION_DOWN)

{

//

Throw

away

all

previous

state

when

starting

a

new

touch

gesture.

//

The

framework

may

have

dropped

the

up

or

cancel

event

for

the

previous

gesture

//

due

to

an

app

switch,

ANR,

or

some

other

state

change.

cancelAndClearTouchTargets(ev);

resetTouchState();

}這是dispatchTouchEvent()開始時截取的一段代碼，我們來看一下，首先，當我們手指按下view時，會調(diào)用到resetTouchState()方法，在resetTouchState()中：private

void

resetTouchState()

{

clearTouchTargets();

resetCancelNextUpFlag(this);

mGroupFlags

&=

~FLAG_DISALLOW_INTERCEPT;

mNestedScrollAxes

=

SCROLL_AXIS_NONE;

}我們繼續(xù)跟蹤clearTouchTargets()方法：private

void

clearTouchTargets()

{

TouchTarget

target

=

mFirstTouchTarget;

if

(target

!=

null)

{

do

{

TouchTarget

next

=

target.next;

target.recycle();

target

=

next;

}

while

(target

!=

null);

mFirstTouchTarget

=

null;

}

}在clearTouchTargets()方法中，我們最終將mFirstTouchTarget賦值為null，我們繼續(xù)回到dispatchTouchEvent()中，接著執(zhí)行了下段代碼：//

Check

for

interception.

final

boolean

intercepted;

if

(actionMasked

==

MotionEvent.ACTION_DOWN

||

mFirstTouchTarget

!=

null)

{

final

boolean

disallowIntercept

=

(mGroupFlags

&

FLAG_DISALLOW_INTERCEPT)

!=

0;

if

(!disallowIntercept)

{

intercepted

=

onInterceptTouchEvent(ev);

ev.setAction(action);

//

restore

action

in

case

it

was

changed

}

else

{

intercepted

=

false;

}

}

else

{

//

There

are

no

touch

targets

and

this

action

is

not

an

initial

down

//

so

this

view

group

continues

to

intercept

touches.

intercepted

=

true;

}當view被按下或mFirstTouchTarget!=null的時候，從前面可以知道，當每次view被按下時，也就是重新開始一次事件流的處理時，mFirstTouchTarget都會被設置成null，一會我們看mFirstTouchTarget是什么時候被賦值的。從disallowIntercept屬性我們大概能猜到是用來判斷是否需要坐攔截處理，而我們知道可以通過調(diào)用父view的requestDisallowInterceptTouchEvent(true)可以讓我們的父view不能對事件進行攔截，我們先來看看requestDisallowInterceptTouchEvent()方法中的實現(xiàn)：@Override

public

void

requestDisallowInterceptTouchEvent(boolean

disallowIntercept)

{

if

(disallowIntercept

==

((mGroupFlags

&

FLAG_DISALLOW_INTERCEPT)

!=

0))

{

//

We're

already

in

this

state,

assume

our

ancestors

are

too

return;

}

if

(disallowIntercept)

{

mGroupFlags

|=

FLAG_DISALLOW_INTERCEPT;

}

else

{

mGroupFlags

&=

~FLAG_DISALLOW_INTERCEPT;

}

//

Pass

it

up

to

our

parent

if

(mParent

!=

null)

{

mParent.requestDisallowInterceptTouchEvent(disallowIntercept);

}

}這里也是通過設置標志位做判斷處理，所以這里是通過改變mGroupFlags標志，然后在dispatchTouchEvent()剛發(fā)中變更disallowIntercept的值判斷是否攔截，當為true時，即需要攔截，這個時候便會跳過onInterceptTouchEvent()攔截判斷，并標記為不攔截，即intercepted=false，我們繼續(xù)看viewGroup的onInterceptTouchEvent()處理：public

boolean

onInterceptTouchEvent(MotionEvent

ev)

{

if

(ev.isFromSource(InputDevice.SOURCE_MOUSE)

&&

ev.getAction()

==

MotionEvent.ACTION_DOWN

&&

ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)

&&

isOnScrollbarThumb(ev.getX(),

ev.getY()))

{

return

true;

}

return

false;

}即默認情況下，只有在ACTION_DOWN時，viewGroup才會表現(xiàn)為攔截。我們繼續(xù)往下看：final

int

childrenCount

=

mChildrenCount;

if

(newTouchTarget

==

null

&&

childrenCount

!=

0)

{

final

float

x

=

ev.getX(actionIndex);

final

float

y

=

ev.getY(actionIndex);

//

Find

a

child

that

can

receive

the

event.

//

Scan

children

from

front

to

back.

final

ArrayList<View>

preorderedList

=

buildTouchDispatchChildList();

final

boolean

customOrder

=

preorderedList

==

null

&&

isChildrenDrawingOrderEnabled();

final

View[]

children

=

mChildren;

for

(int

i

=

childrenCount

-

1;

i

>=

0;

i--)

{

final

int

childIndex

=

getAndVerifyPreorderedIndex(

childrenCount,

i,

customOrder);

final

View

child

=

getAndVerifyPreorderedView(

preorderedList,

children,

childIndex);

//

If

there

is

a

view

that

has

accessibility

focus

we

want

it

//

to

get

the

event

first

and

if

not

handled

we

will

perform

a

//

normal

dispatch.

We

may

do

a

double

iteration

but

this

is

//

safer

given

the

timeframe.

if

(childWithAccessibilityFocus

!=

null)

{

if

(childWithAccessibilityFocus

!=

child)

{

continue;

}

childWithAccessibilityFocus

=

null;

i

=

childrenCount

-

1;

}

if

(!canViewReceivePointerEvents(child)

||

!isTransformedTouchPointInView(x,

y,

child,

null))

{

ev.setTargetAccessibilityFocus(false);

continue;

}

newTouchTarget

=

getTouchTarget(child);

if

(newTouchTarget

!=

null)

{

//

Child

is

already

receiving

touch

within

its

bounds.

//

Give

it

the

new

pointer

in

addition

to

the

ones

it

is

handling.

newTouchTarget.pointerIdBits

|=

idBitsToAssign;

break;

}

resetCancelNextUpFlag(child);

if

(dispatchTransformedTouchEvent(ev,

false,

child,

idBitsToAssign))

{

//

Child

wants

to

receive

touch

within

its

bounds.

mLastTouchDownTime

=

ev.getDownTime();

if

(preorderedList

!=

null)

{

//

childIndex

points

into

presorted

list,

find

original

index

for

(int

j

=

0;

j

<

childrenCount;

j++)

{

if

(children[childIndex]

==

mChildren[j])

{

mLastTouchDownIndex

=

j;

break;

}

}

}

else

{

mLastTouchDownIndex

=

childIndex;

}

mLastTouchDownX

=

ev.getX();

mLastTouchDownY

=

ev.getY();

newTouchTarget

=

addTouchTarget(child,

idBitsToAssign);

alreadyDispatchedToNewTouchTarget

=

true;

break;

}

//

The

accessibility

focus

didn't

handle

the

event,

so

clear

//

the

flag

and

do

a

normal

dispatch

to

all

children.

ev.setTargetAccessibilityFocus(false);

}

if

(preorderedList

!=

null)

preorderedList.clear();

}這段代碼首先會通過一個循環(huán)去遍歷所有的子view，最終會調(diào)用到dispatchTransformedTouchEvent()方法，我們繼續(xù)看dispatchTransformedTouchEvent()的實現(xiàn)：private

boolean

dispatchTransformedTouchEvent(MotionEvent

event,

boolean

cancel,

View

child,

int

desiredPointerIdBits)

{

final

boolean

handled;

//

Canceling

motions

is

a

special

case.

We

don't

need

to

perform

any

transformations

//

or

filtering.

The

important

part

is

the

action,

not

the

contents.

final

int

oldAction

=

event.getAction();

if

(cancel

||

oldAction

==

MotionEvent.ACTION_CANCEL)

{

event.setAction(MotionEvent.ACTION_CANCEL);

if

(child

==

null)

{

handled

=

super.dispatchTouchEvent(event);

}

else

{

handled

=

child.dispatchTouchEvent(event);

}

event.setAction(oldAction);

return

handled;

}

//

Calculate

the

number

of

pointers

to

deliver.

final

int

oldPointerIdBits

=

event.getPointerIdBits();

final

int

newPointerIdBits

=

oldPointerIdBits

&

desiredPointerIdBits;

//

If

for

some

reason

we

ended

up

in

an

inconsistent

state

where

it

looks

like

we

//

might

produce

a

motion

event

with

no

pointers

in

it,

then

drop

the

event.

if

(newPointerIdBits

==

0)

{

return

false;

}

//

If

the

number

of

pointers

is

the

same

and

we

don't

need

to

perform

any

fancy

//

irreversible

transformations,

then

we

can

reuse

the

motion

event

for

this

//

dispatch

as

long

as

we

are

careful

to

revert

any

changes

we

make.

//

Otherwise

we

need

to

make

a

copy.

final

MotionEvent

transformedEvent;

if

(newPointerIdBits

==

oldPointerIdBits)

{

if

(child

==

null

||

child.hasIdentityMatrix())

{

if

(child

==

null)

{

handled

=

super.dispatchTouchEvent(event);

}

else

{

final

float

offsetX

=

mScrollX

-

child.mLeft;

final

float

offsetY

=

mScrollY

-

child.mTop;

event.offsetLocation(offsetX,

offsetY);

handled

=

child.dispatchTouchEvent(event);

event.offsetLocation(-offsetX,

-offsetY);

}

return

handled;

}

transformedEvent

=

MotionEvent.obtain(event);

}

else

{

transformedEvent

=

event.split(newPointerIdBits);

}

//

Perform

any

necessary

transformations

and

dispatch.

if

(child

==

null)

{

handled

=

super.dispatchTouchEvent(transformedEvent);

}

else

{

final

float

offsetX

=

mScrollX

-

child.mLeft;

final

float

offsetY

=

mScrollY

-

child.mTop;

transformedEvent.offsetLocation(offsetX,

offsetY);

if

(!

child.hasIdentityMatrix())

{

transformedEvent.transform(child.getInverseMatrix());

}

handled

=

child.dispatchTouchEvent(transformedEvent);

}

//

Done.

transformedEvent.recycle();

return

handled;

}這段代碼就比較明顯了，如果child不為null，始終會調(diào)用到child.dispatchTouchEvent();否則調(diào)用super.dispatchTouchEvent();如果child不為null時，事件就會向下傳遞，如果子view處理了事件，即dispatchTransformedTouchEvent()即返回true。繼續(xù)向下執(zhí)行到addTouchTarget()方法，我們繼續(xù)看addTouchTarget()方法的執(zhí)行結(jié)果：private

TouchTarget

addTouchTarget(@NonNull

View

child,

int

pointerIdBits)

{

final

TouchTarget

target

=

TouchTarget.obtain(child,

pointerIdBits);

target.next

=

mFirstTouchTarget;

mFirstTouchTarget

=

target;

return

target;

}這個時候我們發(fā)現(xiàn)mFirstTouchTarget又出現(xiàn)了，這時候會給mFirstTouchTarget重新賦值，即mFirstTouchTarget不為null。也就是說，如果事件被當前view或子view消費了，那么在接下來的ACTION_MOVE或ACTION_UP事件中，mFirstTouchTarget就不為null。但如果我們繼承了該viewGroup，并在onInterceptTouchEvent()的ACTION_MOVE中攔截了事件，那么后續(xù)事件將不會下發(fā)，將由該viewGroup直接處理，從下面代碼我們可以得到：//

Dispatch

to

touch

targets,

excluding

the

new

touch

target

if

we

already

//

dispatched

to

it.

Cancel

touch

targets

if

necessary.

TouchTarget

predecessor

=

null;

TouchTarget

target

=

mFirstTouchTarget;

while

(target

!=

null)

{

final

TouchTarget

next

=

target.next;

if

(alreadyDispatchedToNewTouchTarget

&&

target

==

newTouchTarget)

{

handled

=

true;

}

else

{

final

boolean

cancelChild

=

resetCancelNextUpFlag(target.child)

||

intercepted;

if

(dispatchTransformedTouchEvent(ev,

cancelChild,

target.child,

target.pointerIdBits))

{

handled

=

true;

}

if

(cancelChild)

{

if

(predecessor

==

null)

{

mFirstTouchTarget

=

next;

}

else

{

predecessor.next

=

next;

}

target.recycle();

target

=

next;

continue;

}

}

predecessor

=

target;

target

=

next;

}當存在子view并且事件被子view消費時，即在ACTION_DOWN階段mFirstTouchTarget會被賦值，即在接下來的ACTION_MOVE事件中，由于intercepted為true，所以將ACTION_CANCEL事件傳遞過去，從dispatchTransformedTouchEvent()中可以看到：if

(cancel

||

oldAction

==

MotionEvent.ACTION_CANCEL)

{

event.setAction(MotionEvent.ACTION_CANCEL);

if

(child

==

null)

{

handled

=

super.dispatchTouchEvent(event);

}

else

{

handled

=

child.dispatchTouchEvent(event);

}

event.setAction(oldAction);

return

handled;

}并將mFirstTouchTarget最終賦值為next，而此時mFirstTouchTarget位于TouchTarget鏈表尾部，所以mFirstTouchTarget會賦值為null，那么接下來的事件將不會進入到onInterceptTouchEvent()中。也就會直接交由該view處理。如果我們沒有進行事件的攔截，而是交由子view去處理，由于ViewGroup的onInterceptTouchEvent()默認并不會攔截除了ACTION_DOWN以外的事件，所以后續(xù)事件將繼續(xù)交由子view去處理，如果存在子view且事件位于子view內(nèi)部區(qū)域的話。所以無論是否進行攔截，事件流都會交由view的dispatchTouchEvent()中進行處理，我們接下來跟蹤一下view中的dispatchTouchEvent()處理過程：if

(actionMasked

==

MotionEvent.ACTION_DOWN)

{

//

Defensive

cleanup

for

new

gesture

stopNestedScroll();

}

if

(onFilterTouchEventForSecurity(event))

{

if

((mViewFlags

&

ENABLED_MASK)

==

ENABLED

&&

handleScrollBarDragging(event))

{

result

=

true;

}

//noinspection

SimplifiableIfStatement

ListenerInfo

li

=

mListenerInfo;

if

(li

!=

null

&&

li.mOnTouchListener

!=

null

&&

(mViewFlags

&

ENABLED_MASK)

==

ENABLED

&&

li.mOnTouchListener.onTouch(this,

event))

{

result

=

true;

}

if

(!result

&&

onTouchEvent(event))

{

result

=

true;

}

}當被按下時，即ACTION_DOWN時，view會停止內(nèi)部的滾動，如果view沒有被覆蓋或遮擋時，首先會進行mListenerInfo是否為空的判斷，我們看下mListenerInfo是在哪里初始化的：ListenerInfo

getListenerInfo()

{

if

(mListenerInfo

!=

null)

{

return

mListenerInfo;

}

mListenerInfo

=

new

ListenerInfo();

return

mListenerInfo;

}這里可以看出，mListenerInfo一般不會是null，知道在我們使用它時調(diào)用過這段代碼，而當view被加入window中的時候，會調(diào)用下面這段代碼，從注釋中也可以看出來：/**

*

Add

a

listener

for

attach

state

changes.

*

*

This

listener

will

be

called

whenever

this

view

is

attached

or

detached

*

from

a

window.

Remove

the

listener

using

*

{@link

#removeOnAttachStateChangeListener(OnAttachStateChangeListener)}.

*

*

@param

listener

Listener

to

attach

*

@see

#removeOnAttachStateChangeListener(OnAttachStateChangeListener)

*/

public

void

addOnAttachStateChangeListener(OnAttachStateChangeListener

listener)

{

ListenerInfo

li

=

getListenerInfo();

if

(li.mOnAttachStateChangeListeners

==

null)

{

li.mOnAttachStateChangeListeners

=

new

CopyOnWriteArrayList<OnAttachStateChangeListener>();

}

li.mOnAttachStateChangeListeners.add(listener);

}到這里我們就知道，mListenerInfo一開始就是被初始化好了的，所以li不可能為null，li.mOnTouchListener!=null即當設置了TouchListener時不為null，并且view是enabled狀態(tài)，一般情況view都是enable的。這個時候會調(diào)用到onTouch()事件，當onTouch()返回true時，這個時候result會賦值true。而當result為true時，onTouchEvent()將不會被調(diào)用。從這里可以看出，onTouch()會優(yōu)先onTouchEvent()調(diào)用；當view設置touch監(jiān)聽并返回true時，那么它的onTouchEvent()將被屏蔽。否則會調(diào)用onTouchEvent()處理。那么讓我們繼續(xù)來看看onTouchEvent()中的事件處理：if

((viewFlags

&

ENABLED_MASK)

==

DISABLED)

{

if

(action

==

MotionEvent.ACTION_UP

&&

(mPrivateFlags

&

PFLAG_PRESSED)

!=

0)

{

setPressed(false);

}

//

A

disabled

view

that

is

clickable

still

consumes

the

touch

//

events,

it

just

doesn't

respond

to

them.

return

(((viewFlags

&

CLICKABLE)

==

CLICKABLE

||

(viewFlags

&

LONG_CLICKABLE)

==

LONG_CLICKABLE)

||

(viewFlags

&

CONTEXT_CLICKABLE)

==

CONTEXT_CLICKABLE);

}首先，當view狀態(tài)是DISABLED時，只要view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE，都會返回true,而button默認是CLICKABLE的，textview默認不是CLICKABLE的，而view一般默認都不是LONG_CLICKABLE的。我們繼續(xù)向下看：if

(mTouchDelegate

!=

null)

{

if

(mTouchDelegate.onTouchEvent(event))

{

return

true;

}

}如果有代理事件，仍然會返回true.if

(((viewFlags

&

CLICKABLE)

==

CLICKABLE

||

(viewFlags

&

LONG_CLICKABLE)

==

LONG_CLICKABLE)

||

(viewFlags

&

CONTEXT_CLICKABLE)

==

CONTEXT_CLICKABLE)

{

switch

(action)

{

case

MotionEvent.ACTION_UP:

boolean

prepressed

=

(mPrivateFlags

&

PFLAG_PREPRESSED)

!=

0;

if

((mPrivateFlags

&

PFLAG_PRESSED)

!=

0

||

prepressed)

{

//

take

focus

if

we

don't

have

it

already

and

we

should

in

//

touch

mode.

boolean

focusTaken

=

false;

if

(isFocusable()

&&

isFocusableInTouchMode()

&&

!isFocused())

{

focusTaken

=

requestFocus();

}

if

(prepressed)

{

//

The

button

is

being

released

before

we

actually

//

showed

it

as

pressed.

Make

it

show

the

pressed

//

state

now

(before

scheduling

the

click)

to

ensure

//

the

user

sees

it.

setPressed(true,

x,

y);

}

if

(!mHasPerformedLongPress

&&

!mIgnoreNextUpEvent)

{

//

This

is

a

tap,

so

remove

the

longpress

check

removeLongPressCallback();

//

Only

perform

take

click

actions

if

we

were

in

the

pressed

state

if

(!focusTaken)

{

//

Use

a

Runnable

and

post

this

rather

than

calling

//

performClick

directly.

This

lets

other

visual

state

//

of

the

view

update

before

click

actions

start.

if

(mPerformClick

==

null)

{

mPerformClick

=

new

PerformClick();

}

if

(!post(mPerformClick))

{

performClick();

}

}

}

if

(mUnsetPressedState

==

null)

{

mUnsetPressedState

=

new

UnsetPressedState();

}

if

(prepressed)

{

postDelayed(mUnsetPressedState,

ViewConfiguration.getPressedStateDuration());

}

else

if

(!post(mUnsetPressedState))

{

//

If

the

post

failed,

unpress

right

now

mUnsetPressedState.run();

}

removeTapCallback();

}

mIgnoreNextUpEvent

=

false;

break;

case

MotionEvent.ACTION_DOWN:

mHasPerformedLongPress

=

false;

if

(performButtonActionOnTouchDown(event))

{

break;

}

//

Walk

up

the

hierarchy

to

determine

if

we're

inside

a

scrolling

container.

boolean

isInScrollingContainer

=

isInScrollingContainer();

//

For

views

inside

a

scrolling

container,

delay

the

pressed

feedback

for

//

a

short

period

in

case

this

is

a

scroll.

if

(isInScrollingContainer)

{

mPrivateFlags

|=

PFLAG_PREPRESSED;

if

(mPendingCheckForTap

==

null)

{

mPendingCheckForTap

=

new

CheckForTap();

}

mPendingCheckForTap.x

=

event.getX();

mPendingCheckForTap.y

=

event.getY();

postDelayed(mPendingCheckForTap,

ViewConfiguration.getTapTimeout());

}

else

{

//

Not

inside

a

scrolling

container,

so

show

the

feedback

right

away

setPressed(true,

x,