android 自定义View(一) View的事件分发与绘制

为什么需要View事件分发与绘制

  在做android开发的过程中，Android提供的控件不一定全能满足我们的需求，因此我们需要去自定义属于我们自己的控件。如为定义一些控件的属性，样式，功能等。为了实现这些效果。我们有必要先了解一下自定义控件时候经常需要重写的几个函数，这几个函数涉及到了View事件的分发和绘制。

View的事件分发

  View的事件分发是指当我们在屏幕上产生点击后，产生了一个触摸事件，这个事件通过层层分发最后到相应的view去执行。要知道事件是怎么分发的，首先我们需要知道Android中显示出来的View是怎么组成的。

（1）View的组成

  在Android中，所有的控件都是View,而在一个页面上这些View都是以树形结构进行组成，以一个View为根节点，根节点下依次是儿子节点，孙子节点等。这些节点可以是View，也可以上ViewGroup。当我们点击屏幕后，这些系统通过TW到驱动到Framwork处理，将事件封装成一个MontionEvent事件。如在一个Activity中，这个Activity接受到一个触摸事件则会有一个MontionEvent从页面的根View开始，进行层层分发，开始寻找能够处理这个事件的View。

（2）事件分发需要重写的几个方法

在自定义View中，通常有几种情况：

1、继承一个已有的View。如继承一个TextView之类的。

2、直接继承一个View。

3、继承一个ViewGroup。

其中，在1和2 提供了两个方法

dispatchTouchEvent(MotionEvent ev)：用来进行事件的分发

onTouchEvent(MotionEvent ev)：用来处理点击事件。

而在3中多出了一个

onInterceptTouchEvent(MotionEvent ev)：用来进行事件的拦截。

先来看看在View的dispatchTouchEvent(MotionEvent ev)方法进行了什么。

/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}

boolean result = false;

if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}

final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
android.util.SeempLog.record(3);
// Defensive cleanup for new gesture
stopNestedScroll();
}

if (onFilterTouchEventForSecurity(event)) {
//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;
}
}

if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}

// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}

return result;
}

这个方法的返回值代表着View是否需要处理这个事件。true则代表消费掉这个事件。同时注意

if (onFilterTouchEventForSecurity(event)) {
//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;
}
}

可以看出 OnTouchListener 是优先于onTouchEvent处理的。接下来看看View中的onTouchEvent方法。

public boolean onTouchEvent(MotionEvent event) {
android.util.SeempLog.record(3);
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
//viewFlags  的值为CLICKABLE 或者LONG_CLICKABLE或者CONTEXT_CLICKABLE 这个事件就讲被消费，消费的方式根据对应的值处理
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_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
......
}

return true;
}

return false;
}

接着在看看在ViewGroup中的dispatchTouchEvent 有何不同。

// 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. //下面没有处理该事件的View 则有该控件拦截
intercepted = true;
}

首先对是否拦截该事件进行了处理。在onInterceptTouchEvent中，默认了不进行拦截。

public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}

接下来看看dispatchTouchEvent 对其中的子View处理。

final View[] children = mChildren;

for (int i = childrenCount - 1; i >= 0; i–) {

final int childIndex = customOrder

? getChildDrawingOrder(childrenCount, i) : i;

final View child = (preorderedList == null)

? children[childIndex] : preorderedList.get(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);
}

在for循环中，遍历ViewGroup的子View,一旦遍历到合适的View 则将事件传递下去。事件通过dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign) 进行传递。

final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event); //子View不存在，调用父类的dispatch,ViewGroup继承于View 即调用View的dispatch
} else {
handled = child.dispatchTouchEvent(event);//子View存在，调用子View的dispatchTouchEvent，如果子Vie可ViewGropu可View
}
event.setAction(oldAction);
return handled;
}

综上可知，ViewGroup与View的事件传递规律如下：

1、ViewGroup 通过dispatchTouchEvent 来分发事件，如果该事件被拦截，则调用父类的onTouchEvent进行处理，不拦截则将事件传递给子View处理，子View则调用自己onTouchEvent进行处理。如果子View不进行处理，则会返回给ViewGroup 进行处理。

View的绘制

  View的绘制是通过三个方法来进行。分别是测量、布局以及绘制。在view中分别对应了Measure(int widthMeasureSpec, int heightMeasureSpec)， layout(int l, int t, int r, int b)以及draw(Canvas canvas)方法。

   如上文所说，整个视图是一个树形结构，View的绘制也是重根节点开始进行递归。绘制的发起者为ViewRootImpl.Java类的performTraversals()函数展开。通过判断是否需要measure、layout、draw来决定View的绘制。通常会调用到View的onMeasure,onLayout(ViewGroup中进行重写)，以及onDrwa方法。

(1)onMeasure方法

  在View的绘制中onMeadure方法主要用于对自身宽高的测量。在View.java中，可以看到

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

这里的意图是在onMeasure中调用了setMeasuredDimension函数去set值。

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth  = insets.left + insets.right;
int opticalHeight = insets.top  + insets.bottom;

measuredWidth  += optical ? opticalWidth  : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;

mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

很明显，这里去设置了View的宽和高。再来看看作为参数的getDefaultSize具体做了什么。这个函数中分别以getSuggestedMinimumWidth()和widthMeasureSpec作为参数。先来分析getDefaultSize

public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);//测量模式
int specSize = MeasureSpec.getSize(measureSpec);//测量大小

switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}

在getDefaultSize出现了specMode，在MeasureSpec类中，一共有三种模式

public static final int UNSPECIFIED = 0 << MODE_SHIFT; //父容器不做约束，View可任意大小

public static final int EXACTLY = 1 << MODE_SHIFT;//父容器确定view的宽高。

public static final int AT_MOST = 2 << MODE_SHIFT;//对应于wrap_content属性,不能超过父容器

再来看看getSuggestedMinimumWidth()。

return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());

如果有背景则返回mMinWidth 否则返回mMinWidth 与Drawable宽度的最大值。当然在getMinimumWidth中，Drawable在没设定固有宽度则返回0；

(2)layout方法

  在layout(int l, int t, int r, int b)方法中一共有4个参数。分别表示了该view相对于父容器的左边距，上边距，右边距，下边距。

public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}

int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;

boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}

mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}

这里通过setFrame(l, t, r, b)，View将自己在父容器的位置进行了设置。

(3)onDraw方法

  党在重写View的时候，我们可以重写onDraw方法，来实现我们自己绘制。

/**
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
protected void onDraw(Canvas canvas) {
}

可以看出，我们可以有自己的draw只要去重写该方法。其实这个方法在View的绘制里也被回调。在View的Draw方法里

public void draw(Canvas canvas)

int saveCount;
// Step 1, draw the background, if needed
if (!dirtyOpaque) {
drawBackground(canvas);
}

// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);

// Step 4, draw the children
dispatchDraw(canvas);

// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}

// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);

// we're done...
return;
}

1、Draw一个View的时候先要需要则先绘制一个背景。

2、保存canvas层

3、绘制自身内容

4、如果有子元素则绘制子元素

5、绘制效果

6、绘制装饰品

但是第二步和第五步在一般情况下都会被省略掉。

可以看见onDraw方法在第三步被回调。

(4)View绘制总结

  View的绘制包括了measure ,layout,draw几个步骤，同时在这几个步骤中都会onMeasure ,onLayout,onDraw进行了回调。

ViewGroup的绘制

  ViewGroup中，没有measure方法但是提供了measureChildren()和measureChild()方法，这位ViewGroup提供了测量子View的方式。

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}

measureChildren调用measureChild方法让子View进行自我测量。

protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();

final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

在measureChild中由调用了View的measure方法进行了测量。

以上便是View的事件分发与绘制的原理分析，下一章将给出具体的demo来展示怎么通过这几个方法来自定义View。