我们在设计项目的时候，经常会遇到一些奇奇怪怪的需求，在这些需求中，对于UI的需求更是占了大半部分，这时候我们就避免不了与View打交道，熟悉自定义View的旁友应该会将onMeasure()->onLayout->onDraw的调用过程烂熟于心。因此，为了在撸代码的时候更加的对VIew得心应手，我们有必要对其进行一个深入的了解，下面就从构造方法看起吧。
View的构造方法如下：
public View(Context context) {
mContext = context;
mResources = context != null ? context.getResources() : null;
mViewFlags = SOUND_EFFECTS_ENABLED | HAPTIC_FEEDBACK_ENABLED;
....
sCompatibilityDone = true;
}
}
public View(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
this(context);
final TypedArray a = context.obtainStyledAttributes(
attrs, com.android.internal.R.styleable.View, defStyleAttr, defStyleRes);
if (mDebugViewAttributes) {
saveAttributeData(attrs, a);
}
Drawable background = null;
...
final int N = a.getIndexCount();
for (int i = 0; i < N; i++) {
int attr = a.getIndex(i);
switch (attr) {
//读取attr的参数
...
}
//设置View的滚动模式
setOverScrollMode(overScrollMode);
// Cache start/end user padding as we cannot fully resolve padding here (we dont have yet
// the resolved layout direction). Those cached values will be used later during padding
// resolution.
mUserPaddingStart = startPadding;
mUserPaddingEnd = endPadding;
//设置背景
if (background != null) {
setBackground(background);
}
// setBackground above will record that padding is currently provided by the background.
// If we have padding specified via xml, record that here instead and use it.
mLeftPaddingDefined = leftPaddingDefined;
mRightPaddingDefined = rightPaddingDefined;
...
}
对于View的构造方法而言，在其中只是做了相关参数的初始化以及传入参数attr的数据的获取的操作，相对来说，并没有什么特别需要注意的地方，那我们直接查看到onMeasure中：
/**
* <p>
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overridden by subclasses to provide accurate and efficient
* measurement of their contents.
* </p>
*
* <p>
* <strong>CONTRACT:</strong> When overriding this method, you
* <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
* measured width and height of this view. Failure to do so will trigger an
* <code>IllegalStateException</code>, thrown by
* {@link #measure(int, int)}. Calling the superclass'
* {@link #onMeasure(int, int)} is a valid use.
* </p>
*
* <p>
* The base class implementation of measure defaults to the background size,
* unless a larger size is allowed by the MeasureSpec. Subclasses should
* override {@link #onMeasure(int, int)} to provide better measurements of
* their content.
* </p>
*
* <p>
* If this method is overridden, it is the subclass's responsibility to make
* sure the measured height and width are at least the view's minimum height
* and width ({@link #getSuggestedMinimumHeight()} and
* {@link #getSuggestedMinimumWidth()}).
* </p>
*
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
在View注释中已经把onMeasure()解释得相当的完善了，我们可以在注释中获取一些信息：
onMeasure()方法中会测量其本身View以及包含内容的宽高
当我们在子类中重写onMeasure()方法时，我们需要调用setMeasuredDimension(int, int)为这个View进行保存宽高
View默认的是background的大小
/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
在上述代码中，我们可以看到在getDefaultSize()方法中通过MeasureSpec来获得测量模式和大小(MeasureSpec代表一个32位的int值，高两位代表了specMode，低30位specSize)。specMode总共有三种()
MeasureSpec.UNSPECIFIED:父容器不对View有任何的影响，要多大就给多大
MeasureSpec.EXACTLY:父容器检测出View所需要的精确大小，这是View的最终大小就是specSize的值，其对应了match_parent模式
MeasureSpec.AT_MOST:父容器制定了可用大小，即specSize，View的大小不能大于这个值，对应wrap_content模式
上述三种模式的说明来自于《android艺术开发探索》。在View的测量中，我们可以看到View的大小的测量跟设置的模式以及父布局是息息相关的，因此我们在进行自定义View的时候需要考虑到这两方面的因素。
我们重新回溯到setMeasuredDimension()方法中：
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;
}
在其中我们看到最终会将计算得到的宽高传给mMeasuredHeight和mMeasuredWidth。
接下来在看看onLayout方法：
/**
* Called from layout when this view should
* assign a size and position to each of its children.
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
* @param changed This is a new size or position for this view
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
onLayout()是一个空实现的方法，从注释当中我们可以看到此方法更适用于ViewGroup，只有View是一个ViewGroup才需要考虑到layout的位置的问题。
往下走，onDraw():
/**
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
protected void onDraw(Canvas canvas) {
}
同样是空实现，这里自定义View通过重写该方法来进行绘制的操作。
ViewRootIpml
分析完上面的三个方法，我们更多地是看到一些赋值和空实现的方法，那么到底View的怎么开始绘制的过程的呢？这时候涉及到ViewRootIpml这个类了，它是连接View和WindowManager的桥梁，具体的作用是什么呢，我们从构建开始说起，一步步往下学习，首先我们找到其初始化的地方：
//WindowManagerGlobal
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
ViewRootImpl root;
View panelParentView = null;
...
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
try {
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
...
}
我们在WindowManager的实现类WindowManagerGlobal找到了其ViewRootImpl初始化的地方(关于WindowManager在以后会进行深入探讨，现在主要知道它是window窗口的管理类即可)，从addView()方法中我们推测其是将View添加到window的一个过程(addView这个方法在ActivityThread的handleResumeActivity()中实现)，然后又调用了ViewRootImpl的setView()方法：
//ViewRootImpl
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
...
requestLayout();
...
}
}
}
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
private void performTraversals() {
...
performMeasure()；
...
performLayout();
...
performDraw();
}
在这里我们可以看到当第一次进来的时候会设置mView的值，在其中调用了requestLayout()进行布局，然后依次是scheduleTraversals()->doTraversal()->performTraversals()的过程，在performTraversals()方法中调用了performMeasure(),performLayout(),performDraw()方法：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
//当前mView是由WindowsManagerGlobal.addView传递过来，在ViewRootImpl中
//通过setView()获得到的。
final View host = mView;
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
mInLayout = false;
}
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
try {
mWindowSession.finishDrawing(mWindow);
} catch (RemoteException e) {
}
}
}
private void draw(boolean fullRedrawNeeded) {
....
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}
/**
* @return true if drawing was successful, false if an error occurred
*/
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
return true;
}
在上面我们最终可以看到，measure->layout->draw的过程最终回调到了View中的对应的是三个方法,所以我们重新回到View中研究一下这些方法,从measure()看起：
/**
* <p>
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
* </p>
*
* <p>
* The actual measurement work of a view is performed in
* {@link #onMeasure(int, int)}, called by this method. Therefore, only
* {@link #onMeasure(int, int)} can and must be overridden by subclasses.
* </p>
*
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
*
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
//是否采用视觉边界
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
···
onMeasure(widthMeasureSpec, heightMeasureSpec);
···
}
从注释中我们可以了解到，其实该方法是计算View树的宽高的最终方法，宽高由父布局和子布局共同决定，且该方法不允许子类进行相关操作，因为其是final的，子类想要设置宽高需要通过onMeasure()方法来进行。
接下来我们再看下layout()方法：
/**
* Assign a size and position to a view and all of its
* descendants
*
* <p>This is the second phase of the layout mechanism.
* (The first is measuring). In this phase, each parent calls
* layout on all of its children to position them.
* This is typically done using the child measurements
* that were stored in the measure pass().</p>
*
* <p>Derived classes should not override this method.
* Derived classes with children should override
* onLayout. In that method, they should
* call layout on each of their children.</p>
*
* @param l Left position, relative to parent
* @param t Top position, relative to parent
* @param r Right position, relative to parent
* @param b Bottom position, relative to parent
*/
@SuppressWarnings({"unchecked"})
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);
//setFrame会判断是否需要重新布局，即如果我们在完成一次View展示后，重新设置View的大小，那么
//setFrame返回tru
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;
}
确认过大小后，我们需要定位View最终位置，在layout中会调用到onLayout()来获得我们需要展示的childrenView的效果。
draw()方法：
/**
* Manually render this view (and all of its children) to the given Canvas.
* The view must have already done a full layout before this function is
* called. When implementing a view, implement
* {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
* If you do need to override this method, call the superclass version.
*
* @param canvas The Canvas to which the View is rendered.
*/
@CallSuper
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
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
//递归draw
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;
}
看代码注释就可以很轻松知道draw中的步骤，这里就不详细解释了。
看到这里就对我们对View从哪里开始，并且如何一步步从测量绘制完成有了一个稍微深入的认识，画一个图先总结一下吧：
View绘制过程[1]
但是我们只看到了绘制完成的部分，并没有看到View是怎么展示在我们面前的，想要知道View如何被调用再到绘制完成显示出来，这就牵扯到atcivity的启动的过程了，这里不对这方面进行讲解，详细的过程可以大概看下：
http://www.jianshu.com/p/93c66b3f08d6
我写的这篇文章(写的比较乱，有时间整理一下=￣ω￣=)，本身我们就知道activity启动完成显示到手机上会执行到onResume()这个方法，这个方法可硬对应在ActivityThread中的handleResumeActivity():
final void handleResumeActivity(...){
....
else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// Get rid of anything left hanging around.
cleanUpPendingRemoveWindows(r);
// The window is now visible if it has been added, we are not
// simply finishing, and we are not starting another activity.
if (!r.activity.mFinished && willBeVisible
&& r.activity.mDecor != null && !r.hideForNow) {
if (r.newConfig != null) {
r.tmpConfig.setTo(r.newConfig);
if (r.overrideConfig != null) {
r.tmpConfig.updateFrom(r.overrideConfig);
}
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity "
+ r.activityInfo.name + " with newConfig " + r.tmpConfig);
performConfigurationChanged(r.activity, r.tmpConfig);
freeTextLayoutCachesIfNeeded(r.activity.mCurrentConfig.diff(r.tmpConfig));
r.newConfig = null;
}
if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward="
+ isForward);
WindowManager.LayoutParams l = r.window.getAttributes();
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
l.softInputMode = (l.softInputMode
& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
| forwardBit;
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
r.activity.mVisibleFromServer = true;
mNumVisibleActivities++;
if (r.activity.mVisibleFromClient) {
r.activity.makeVisible();
}
}
if (!r.onlyLocalRequest) {
r.nextIdle = mNewActivities;
mNewActivities = r;
if (localLOGV) Slog.v(
TAG, "Scheduling idle handler for " + r);
Looper.myQueue().addIdleHandler(new Idler());
}
r.onlyLocalRequest = false;
// Tell the activity manager we have resumed.
if (reallyResume) {
try {
ActivityManagerNative.getDefault().activityResumed(token);
} catch (RemoteException ex) {
}
}
} else {
// If an exception was thrown when trying to resume, then
// just end this activity.
try {
ActivityManagerNative.getDefault()
.finishActivity(token, Activity.RESULT_CANCELED, null, false);
} catch (RemoteException ex) {
}
}
}
代码中有句 r.activity.makeVisible()就是展示的关键的代码，他会让DecorView变为Visibile状态，最终展示出来。
ViewGrop
上面通过一系列的讲解只是从最简单的角度上而言——当页面中只有一个View的时候，但是绝大部分情况下是不可能的，往往项目中VIew树的层级是很深的(这里建议使用RelativeLayout作为初始布局容器)，那就涉及到ViewGroup了，其实ViewGroup也很简单，只不过多了一些递归的过程而已。
这里选择FrameLayout来进行大概的讲述吧(因为好多文章用了LinearLayout，RelativeLayout我又嫌麻烦。。)，从onMeasure()开始看：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//获取子View的数量
int count = getChildCount();
//当前布局宽高否是match_parent
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//GONE的布局不会进行测量
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//测量时加上margin参数
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
//子布局中是否由含match_parent的布局，有则添加到mMatchParentChildren中
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
//计算图片宽度，取图片宽高和最大宽高两者之间最大值
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
//这里对于子布局是match_parent的进行重新测量得到准确值
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
//调用子布局的measure()方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
//调用子布局的measure()方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
从onMeasure()可以看到主要的方法就是child.measure()的调用，他会调用到View的measure()，measure()又会重新调用onMeasure()由此形成依次递归直到获取到正确的测量结果。
onLayout()方法：
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
//调用子View的layout
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
可以看到layout也是一个递归调用的过程，这里就不详细讲述了，而由于ViewGroup没有onDraw()方法，这里就不需要讲了。
附上一张View的生命周期:
View的生命周期
终于写完了，脑袋都要爆炸了，View形成的过程大概就是这样的，谢谢阅读，如果觉得哪写的不对或者不好的地方欢迎留言交流，希望能够一起进步~
