自定义view中onMeasure、onLayout、onDraw、onFinishInflate、onSizeChanged方法调用时机

一般自定义view或viewGroup基本上都会去实现onMeasure、onLayout、onDraw方法，还有另外两个方法是onFinishInflate和onSizeChanged。

onFinishInflate方法只有在布局文件中加载view实例会回调，如果直接new一个view的话是不会回调的。

比如一个ViewGroup，只有它和它的子view完全被加载实例化了之后才回去回调该viewGroup的这个方法。因为在LayoutInflater的inflate执行过程中最终的调用路径是：inflate --》 rInflateChildren --》 rInflate ，

其中inflate的调用是这样的：

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {synchronized (mConstructorArgs) {Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");final Context inflaterContext = mContext;final AttributeSet attrs = Xml.asAttributeSet(parser);Context lastContext = (Context) mConstructorArgs[0];mConstructorArgs[0] = inflaterContext;View result = root;try {// Look for the root node.int type;while ((type = parser.next()) != XmlPullParser.START_TAG &&type != XmlPullParser.END_DOCUMENT) {// Empty}if (type != XmlPullParser.START_TAG) {throw new InflateException(parser.getPositionDescription()+ ": No start tag found!");}final String name = parser.getName();if (DEBUG) {System.out.println("**************************");System.out.println("Creating root view: "+ name);System.out.println("**************************");}if (TAG_MERGE.equals(name)) {if (root == null || !attachToRoot) {throw new InflateException("<merge /> can be used only with a valid "+ "ViewGroup root and attachToRoot=true");}rInflate(parser, root, inflaterContext, attrs, false);} else {// Temp is the root view that was found in the xmlfinal View temp = createViewFromTag(root, name, inflaterContext, attrs);ViewGroup.LayoutParams params = null;if (root != null) {if (DEBUG) {System.out.println("Creating params from root: " +root);}// Create layout params that match root, if suppliedparams = root.generateLayoutParams(attrs);if (!attachToRoot) {// Set the layout params for temp if we are not// attaching. (If we are, we use addView, below)temp.setLayoutParams(params);}}if (DEBUG) {System.out.println("-----> start inflating children");}// Inflate all children under temp against its context.rInflateChildren(parser, temp, attrs, true);if (DEBUG) {System.out.println("-----> done inflating children");}// We are supposed to attach all the views we found (int temp)// to root. Do that now.if (root != null && attachToRoot) {root.addView(temp, params);}// Decide whether to return the root that was passed in or the// top view found in xml.if (root == null || !attachToRoot) {result = temp;}}} catch (XmlPullParserException e) {final InflateException ie = new InflateException(e.getMessage(), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} catch (Exception e) {final InflateException ie = new InflateException(parser.getPositionDescription()+ ": " + e.getMessage(), e);ie.setStackTrace(EMPTY_STACK_TRACE);throw ie;} finally {// Don't retain static reference on context.mConstructorArgs[0] = lastContext;mConstructorArgs[1] = null;Trace.traceEnd(Trace.TRACE_TAG_VIEW);}return result;}
}

然后，通过递归调用rInflate解析所有的xml节点，如下：

void rInflate(XmlPullParser parser, View parent, Context context,AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {final int depth = parser.getDepth();int type;while (((type = parser.next()) != XmlPullParser.END_TAG ||parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {if (type != XmlPullParser.START_TAG) {continue;}final String name = parser.getName();if (TAG_REQUEST_FOCUS.equals(name)) {parseRequestFocus(parser, parent);} else if (TAG_TAG.equals(name)) {parseViewTag(parser, parent, attrs);} else if (TAG_INCLUDE.equals(name)) {if (parser.getDepth() == 0) {throw new InflateException("<include /> cannot be the root element");}parseInclude(parser, context, parent, attrs);} else if (TAG_MERGE.equals(name)) {throw new InflateException("<merge /> must be the root element");} else {final View view = createViewFromTag(parent, name, context, attrs);final ViewGroup viewGroup = (ViewGroup) parent;final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);rInflateChildren(parser, view, attrs, true);viewGroup.addView(view, params);}}if (finishInflate) {parent.onFinishInflate();}
}

完成后，会有个判断

if (finishInflate) {parent.onFinishInflate();
}

这时候通知父控件执行onFinishInflate方法，而此时，也紧紧是将所有的子控件实例化到内存中，也就是可以通过getChildAt（）来获取相应的子控件实例了。

这时候还没执行onMeasure呢，而通过layoutInflate.inflate出来的view或viewGroup往往是要添加到已有的父控件，比如使用setContentView方式获取我们的xml文件，其中做了几个步骤：

首先去xml中解析出我们的布局view，然后在把这个view添加到activity的顶级视图中（即DecorView的子布局content中），添加的过程必然使用到了addView的操作，而该操作就会触发requestLayout和invalidate这两个方法，这两个方法又必然会触发ViewParent（即ViewRootImpl）这个视图管理者的一系列操作，这一系列操作由performTraversalse开始，顺序去调用performMeasure -> view.measure -> onMeasure，performLayout -> view.layout -> onLayout，performDraw -> draw -> drawSoftWare -> view.draw -> onDraw（实现自身的绘制） -> dispatchDraw （实现子view的绘制，调用drawChild）。

所以我们的onMeasure及后续的方法应该是在onFinishInflate之后调用的。

而onSizeChanged方法一般是视图大小发生变化的时候回调了，那么具体看源码是在layout的过程中出发的，在layout方法中会调用setFrame方法，在setFrame方法中又调用了sizeChange，在该方法里面回调了onSizeChanged，然后才回去回调onLayout过程。

layout过程：

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);if (shouldDrawRoundScrollbar()) {if(mRoundScrollbarRenderer == null) {mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);}} else {mRoundScrollbarRenderer = null;}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;}

setFrame过程：

protected boolean setFrame(int left, int top, int right, int bottom) {boolean changed = false;if (DBG) {Log.d("View", this + " View.setFrame(" + left + "," + top + ","+ right + "," + bottom + ")");}if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {changed = true;// Remember our drawn bitint drawn = mPrivateFlags & PFLAG_DRAWN;int oldWidth = mRight - mLeft;int oldHeight = mBottom - mTop;int newWidth = right - left;int newHeight = bottom - top;boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);// Invalidate our old positioninvalidate(sizeChanged);mLeft = left;mTop = top;mRight = right;mBottom = bottom;mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);mPrivateFlags |= PFLAG_HAS_BOUNDS;if (sizeChanged) {sizeChange(newWidth, newHeight, oldWidth, oldHeight);}if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {// If we are visible, force the DRAWN bit to on so that// this invalidate will go through (at least to our parent).// This is because someone may have invalidated this view// before this call to setFrame came in, thereby clearing// the DRAWN bit.mPrivateFlags |= PFLAG_DRAWN;invalidate(sizeChanged);// parent display list may need to be recreated based on a change in the bounds// of any childinvalidateParentCaches();}// Reset drawn bit to original value (invalidate turns it off)mPrivateFlags |= drawn;mBackgroundSizeChanged = true;if (mForegroundInfo != null) {mForegroundInfo.mBoundsChanged = true;}notifySubtreeAccessibilityStateChangedIfNeeded();}return changed;}

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