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Android开发:Android中View的工作原理

来源:长沙it培训|发布时间:2017-05-11|浏览量:

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  Android中的任何一个布局、任何一个控件其实都是直接或间接继承View实现的,当然也包括我们在平时开发中所写的各种炫酷的自定义控件了,所以学习View的工作原理对于我们来说显得格外重要,接下来将一起深入学习Android中View的工作原理。

  ViewRoot和DecorView

  1.ViewRoot对应于ViewRootImpl类,是连接WindowManager和DecorView的纽带,View的三大流程均是通过ViewRoot来完成的。在ActivityThread中,当Activity对象被创建完毕后,会将DecorView添加到Window中,同时会创建ViewRootImpl对象,并将ViewRootImpl对象和DecorView建立关联。

  2.View的绘制流程从ViewRoot的performTraversals开始,经过measure、layout和draw三个过程才可以把一个View绘制出来,其中measure用来测量View的宽高,layout用来确定View在父容器中的放置位置,而draw则负责将View绘制到屏幕上。

  3.performTraversals会依次调用performMeasure、performLayout和performDraw三个方法,这三个方法分别完成顶级View的measure、layout和draw这三大流程。其中performMeasure中会调用measure方法,在measure方法中又会调用onMeasure方法,在onMeasure方法中则会对所有子元素进行measure过程,这样就完成了一次measure过程;子元素会重复父容器的measure过程,如此反复完成了整个View数的遍历。

  measure过程决定了View的宽/高,完成后可通过getMeasuredWidth/getMeasureHeight方法来获取View测量后的宽/高。Layout过程决定了View的四个顶点的坐标和实际View的宽高,完成后可通过getTop、getBotton、getLeft和getRight拿到View的四个定点坐标。Draw过程决定了View的显示,完成后View的内容才能呈现到屏幕上。

  DecorView作为顶级View,一般情况下它内部包含了一个竖直方向的LinearLayout,里面分为两个部分(具体情况和Android版本和主题有关),上面是标题栏,下面是内容栏。在Activity通过setContextView所设置的布局文件其实就是被加载到内容栏之中的。

  //获取内容栏

  ViewGroup content = findViewById(R.android.id.content);

  //获取我们设置的Viewcontext.getChildAt(0);

  DecorView其实是一个FrameLayout,View层的事件都先经过DecorView,然后才传给我们的View。

  MeasureSpec

  1.MeasureSpec很大程度上决定一个View的尺寸规格,测量过程中,系统会将View的layoutParams根据父容器所施加的规则转换成对应的MeasureSpec,再根据这个measureSpec来测量出View的宽/高。

  2.MeasureSpec代表一个32位的int值,高2位为SpecMode,低30位为SpecSize,SpecMode是指测量模式,SpecSize是指在某种测量模式下的规格大小。

  MpecMode有三类;

  1.UNSPECIFIED 父容器不对View进行任何限制,要多大给多大,一般用于系统内部:

  2.EXACTLY 父容器检测到View所需要的精确大小,这时候View的最终大小就是SpecSize所指定的值,对应LayoutParams中的match_parent和具体数值这两种模式。

  3.AT_MOST 父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值,不同View实现不同,对应LayoutParams中的wrap_content。

  当View采用固定宽/高的时候,不管父容器的MeasureSpec的是什么,View的MeasureSpec都是精确模式兵其大小遵循Layoutparams的大小。 当View的宽/高是match_parent时,如果他的父容器的模式是精确模式,那View也是精确模式并且大小是父容器的剩余空间;如果父容器是最大模式,那么View也是最大模式并且起大小不会超过父容器的剩余空间。 当View的宽/高是wrap_content时,不管父容器的模式是精确还是最大化,View的模式总是最大化并且不能超过父容器的剩余空间。

  对于DecorView,它的MeasureSpec由Window的尺寸和其自身的LayoutParams来共同确定,对于普通的View,其MeasureSpec由父容器的MeasureSpec和自身的Layoutparams来共同确定。

  对于 DecorView,在ViewRootImpl源码中的measureHierarchy有如下一段代码:

  .........

  if (baseSize != 0 && desiredWindowWidth > baseSize) {

  childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);

  childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);

  performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

  if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("

  + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");

  if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {

  goodMeasure = true;

  .........

  我们查看一下getRootMeasureSpec方法的源码:

  private static int getRootMeasureSpec(int windowSize, int rootDimension) {

  int measureSpec;

  switch (rootDimension) {

  case ViewGroup.LayoutParams.MATCH_PARENT:

  // Window can't resize. Force root view to be windowSize.

  measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);

  break;

  case ViewGroup.LayoutParams.WRAP_CONTENT:

  // Window can resize. Set max size for root view.

  measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);

  break;

  default:

  // Window wants to be an exact size. Force root view to be that size.

  measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);

  break;

  }

  return measureSpec;

  }

  从上面的代码中就可以很容理解DecorView的MeasureSpec是如何产生的,rootDimension就是DecorView自身的LayoutParams,然后会根据这个值进行判断

  LayoutParams.MATCH_PARENT:DecorView的MeasureSpec被赋值为精确模式,DecorView的大小就是Window的大小

  ViewGroup.LayoutParams.WRAP_CONTENT:DecorView的MeasureSpec被赋值为最大模式,DecorView的大小不定,但是不能超过Window的大小

  默认情况:DecorView的MeasureSpec被赋值为精确模式,DecorView的大小为自身LayoutParams设置的值,也就是rootDimension

  接着是对于普通的View,也就是布局中的View,它的Measure过程由ViewGroup传递而来,其中有一个方法是measureChildWithMargins

  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);

  child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

  }

  在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子view的MeasureSpec,从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec(parentWidthMeasureSpec)还有自身的LayoutParams(lp.height和lp.width),还有View自己的Margin和Padding有关

  接下来查看getChildMeasureSpec方法源码:

  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {

  int specMode = MeasureSpec.getMode(spec);

  int specSize = MeasureSpec.getSize(spec);

  int size = Math.max(0, specSize - padding);

  int resultSize = 0;

  int resultMode = 0;

  switch (specMode) {

  // Parent has imposed an exact size on us

  case MeasureSpec.EXACTLY:

  if (childDimension >= 0) {

  resultSize = childDimension;

  resultMode = MeasureSpec.EXACTLY;

  } else if (childDimension == LayoutParams.MATCH_PARENT) {

  // Child wants to be our size. So be it.

  resultSize = size;

  resultMode = MeasureSpec.EXACTLY;

  } else if (childDimension == LayoutParams.WRAP_CONTENT) {

  // Child wants to determine its own size. It can't be

  // bigger than us.

  resultSize = size;

  resultMode = MeasureSpec.AT_MOST;

  }

  break;

  // Parent has imposed a maximum size on us

  case MeasureSpec.AT_MOST:

  if (childDimension >= 0) {

  // Child wants a specific size... so be it

  resultSize = childDimension;

  resultMode = MeasureSpec.EXACTLY;

  } else if (childDimension == LayoutParams.MATCH_PARENT) {

  // Child wants to be our size, but our size is not fixed.

  // Constrain child to not be bigger than us.

  resultSize = size;

  resultMode = MeasureSpec.AT_MOST;

  } else if (childDimension == LayoutParams.WRAP_CONTENT) {

  // Child wants to determine its own size. It can't be

  // bigger than us.

  resultSize = size;

  resultMode = MeasureSpec.AT_MOST;

  }

  break;

  // Parent asked to see how big we want to be

  case MeasureSpec.UNSPECIFIED:

  if (childDimension >= 0) {

  // Child wants a specific size... let him have it

  resultSize = childDimension;

  resultMode = MeasureSpec.EXACTLY;

  } else if (childDimension == LayoutParams.MATCH_PARENT) {

  // Child wants to be our size... find out how big it should

  // be

  resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;

  resultMode = MeasureSpec.UNSPECIFIED;

  } else if (childDimension == LayoutParams.WRAP_CONTENT) {

  // Child wants to determine its own size.... find out how

  // big it should be

  resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;

  resultMode = MeasureSpec.UNSPECIFIED;

  }

  break;

  }

  return MeasureSpec.makeMeasureSpec(resultSize, resultMode);

  }

  这里参数中的padding是指父容器的padding,这里是父容器所占用的空间,所以子view能使用的空间要减去这个padding的值。同时这个方法内部其实就是根据父容器的MeasureSpec结合子view的LayoutParams来确定子view的MeasureSpec

  measure的过程

  如果只是一个View,那么通过measure方法就完成了其测量的过程,如果是一个ViewGroup,除了测量自身外,还会调用子孩子的measure方法

  1.View的measure过程

  View的measure过程由其measure方法完成,其中有下面一段内容

  .........

  int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :

  mMeasureCache.indexOfKey(key);

  if (cacheIndex < 0 || sIgnoreMeasureCache) {

  // measure ourselves, this should set the measured dimension flag back

  onMeasure(widthMeasureSpec, heightMeasureSpec);

  mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;

  } else {

  long value = mMeasureCache.valueAt(cacheIndex);

  // Casting a long to int drops the high 32 bits, no mask needed

  setMeasuredDimensionRaw((int) (value >> 32), (int) value);

  mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;

  }

  .........

  可以知道View的measure方法内,其实调用了自身的onMeasure方法

  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

  setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),

  getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));

  }

  //里面有一个getDefaultSize方法

  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;

  }

  一般我们只需要看MeasureSpec.AT_MOST和MeasureSpec.EXACTLY两种情况,这两种情况返回的result其实都是measureSpec中取得的specSize,这个specSize就是View测量后的大小,这里之所以是View测量后的大小,是因为View的最终大小是在layout阶段确定的,所以要加已区分,一般情况下View测量大小和最终大小是一样的。

  UNSPECIFIED情况下,result的值就是getSuggestedMinimumWidth()方法和getSuggestedMinimumHeight()返回的值,查看这两个方法

  protected int getSuggestedMinimumWidth() {

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

  }

  protected int getSuggestedMinimumHeight() {

  return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());

  }

  从getSuggestedMinimumWidth代码可以看出,如果View没有设置背景,那么宽度就为mMinWidth,这个值对应android:minWidth这个属性所设定的值,如果View设置了背景,则为max(mMinWidth, mBackground.getMinimumWidth())

  public int getMinimumWidth() {

  final int intrinsicWidth = getIntrinsicWidth();

  return intrinsicWidth > 0 ? intrinsicWidth : 0;

  }

  查看mBackground.getMinimumWidth()方法,它其实是Drawable的方法,如果intrinsicHeight也就是原始的宽度不为0,就返回它,如果为0,就返回0。

  从View的getDefaultSize方法可以得出结论:View的宽高由specSize决定,如果我们通过继承View来自定义控件需要重写onMeasure方法,并设置WRAP_CONTENT时的大小,否则在布局中使用WRAP_CONTENT相当于使用MATCH_PARENT

  原因:因为View在布局中使用WRAP_CONTENT就相当于specMode为AT_MOST,而这种情况下,result = specSize,这个specSize的大小为parentSize, parentSize就是父容器目前可用的大小,也就是父容器当前剩余空间的大小,那这时候和在布局中使用MATCH_PARENT效果是一样的

  所以在AT_MOST模式下,我们一般都会给View设定默认的内部宽高,并在WRAP_CONTENT时设置此宽高即可。

  可以通过查看TextView、ImageView的源码,可以得知在WRAP_CONTENT下,onMeasure方法均做了特殊的处理,下面是TextView的onMeasure中的一段内容

  if (widthMode == MeasureSpec.AT_MOST) {

  width = Math.min(widthSize, width);

  }

  2.ViewGroup的measure流程

  ViewGroup是一个抽象类,它没有重写View的onMeasure方法,而是自己提供了一个measureChildren方法

  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);

  }

  }

  }

  里面会对子元素进行遍历,然后调用measureChild方法去测量每一个子元素的宽高

  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);

  }

  在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子孩子的MeasureSpec,从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec(parentWidthMeasureSpec和parentHeightMeasureSpec)还有自身的LayoutParams(lp.height和lp.width),还有View自己的Margin和Padding有关,最后就是调用子view的measure方法

  ViewGroup并没有去定义测量的具体过程,这是因为ViewGroup是一个抽象类,其onMeasure方法需要各个子类去实现,因为每个ViewGroup的实现类,例如LinearLayout,RelativeLayout等的布局方式都是不同的,所以不可能一概而论的来写onMeasure方法。

  接下来分析LinearLayout的onMeasure方法:

  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

  if (mOrientation == VERTICAL) {

  measureVertical(widthMeasureSpec, heightMeasureSpec);

  } else {

  measureHorizontal(widthMeasureSpec, heightMeasureSpec);

  }

  }

  查看measureVertical方法

  // See how tall everyone is. Also remember max width.

  for (int i = 0; i < count; ++i) {

  final View child = getVirtualChildAt(i);

  if (child == null) {

  mTotalLength += measureNullChild(i);

  continue;

  }

  if (child.getVisibility() == View.GONE) {

  i += getChildrenSkipCount(child, i);

  continue;

  }

  if (hasDividerBeforeChildAt(i)) {

  mTotalLength += mDividerHeight;

  }

  LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

  totalWeight += lp.weight;

  if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {

  // Optimization: don't bother measuring children who are going to use

  // leftover space. These views will get measured again down below if

  // there is any leftover space.

  final int totalLength = mTotalLength;

  mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);

  skippedMeasure = true;

  } else {

  int oldHeight = Integer.MIN_VALUE;

  if (lp.height == 0 && lp.weight > 0) {

  // heightMode is either UNSPECIFIED or AT_MOST, and this

  // child wanted to stretch to fill available space.

  // Translate that to WRAP_CONTENT so that it does not end up

  // with a height of 0

  oldHeight = 0;

  lp.height = LayoutParams.WRAP_CONTENT;

  }

  // Determine how big this child would like to be. If this or

  // previous children have given a weight, then we allow it to

  // use all available space (and we will shrink things later

  // if needed).

  measureChildBeforeLayout(

  child, i, widthMeasureSpec, 0, heightMeasureSpec,

  totalWeight == 0 ? mTotalLength : 0);

  if (oldHeight != Integer.MIN_VALUE) {

  lp.height = oldHeight;

  }

  final int childHeight = child.getMeasuredHeight();

  final int totalLength = mTotalLength;

  mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +

  lp.bottomMargin + getNextLocationOffset(child));

  if (useLargestChild) {

  largestChildHeight = Math.max(childHeight, largestChildHeight);

  }

  }

  /**

  * If applicable, compute the additional offset to the child's baseline

  * we'll need later when asked {@link #getBaseline}.

  */

  if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {

  mBaselineChildTop = mTotalLength;

  }

  // if we are trying to use a child index for our baseline, the above

  // book keeping only works if there are no children above it with

  // weight. fail fast to aid the developer.

  if (i < baselineChildIndex && lp.weight > 0) {

  throw new RuntimeException("A child of LinearLayout with index "

  + "less than mBaselineAlignedChildIndex has weight > 0, which "

  + "won't work. Either remove the weight, or don't set "

  + "mBaselineAlignedChildIndex.");

  }

  boolean matchWidthLocally = false;

  if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {

  // The width of the linear layout will scale, and at least one

  // child said it wanted to match our width. Set a flag

  // indicating that we need to remeasure at least that view when

  // we know our width.

  matchWidth = true;

  matchWidthLocally = true;

  }

  final int margin = lp.leftMargin + lp.rightMargin;

  final int measuredWidth = child.getMeasuredWidth() + margin;

  maxWidth = Math.max(maxWidth, measuredWidth);

  childState = combineMeasuredStates(childState, child.getMeasuredState());

  allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;

  if (lp.weight > 0) {

  /*

  * Widths of weighted Views are bogus if we end up

  * remeasuring, so keep them separate.

  */

  weightedMaxWidth = Math.max(weightedMaxWidth,

  matchWidthLocally ? margin : measuredWidth);

  } else {

  alternativeMaxWidth = Math.max(alternativeMaxWidth,

  matchWidthLocally ? margin : measuredWidth);

  }

  i += getChildrenSkipCount(child, i);

  }

  遍历子元素,调用他们的measureChildBeforeLayout方法,这个方法内会测量子孩子的宽高,并且有一个mTotalLength来记录LinearLayout 在竖直方向的初步高度,每测量一次子元素,mTotalLength都会增加,增加部分包括子元素的高度以及子元素竖直方向的margin

  void measureChildBeforeLayout(View child, int childIndex,

  int widthMeasureSpec, int totalWidth, int heightMeasureSpec,

  int totalHeight) {

  measureChildWithMargins(child, widthMeasureSpec, totalWidth,

  heightMeasureSpec, totalHeight);

  }

  里面调用了child.measure方法,也就是子孩子的measure方法

  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);

  child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

  }

  当子元素测量完毕后,LinearLayout会测量自身的大小,对于竖直的LinearLayout,它在水平方向上的测量过程,遵循View的测量过程,在竖直方向上,如果采用的是match_parent或者具体的数值,那么它的测量过程和View的一致,即高度为specSize;如果它的布局中高度采用wrap_content,那么高度是子元素所占用的高度总和,但这个和不能超过父容器的剩余空间,当然还要考虑padding,竖直方向的结论可以从下面代码得知:

  public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {

  final int specMode = MeasureSpec.getMode(measureSpec);

  final int specSize = MeasureSpec.getSize(measureSpec);

  final int result;

  switch (specMode) {

  case MeasureSpec.AT_MOST:

  if (specSize < size) {

  result = specSize | MEASURED_STATE_TOO_SMALL;

  } else {

  result = size;

  }

  break;

  case MeasureSpec.EXACTLY:

  result = specSize;

  break;

  case MeasureSpec.UNSPECIFIED:

  default:

  result = size;

  }

  return result | (childMeasuredState & MEASURED_STATE_MASK);

  }

  有时候onMeasure中拿到的测量宽高可能是不准确的,比较好的习惯是在onLayout中去获取View的测量宽高和最终宽高

  在Activity中,在onCreate,onStart,onResume中均无法正确获得View的宽高信息,这是因为measure和Activity的生命周期是不同步的,所以很可能View没有测量完毕,获得的宽高是0.

  measure总结

  1.measure过程主要就是从顶层父View向子View递归调用view.measure方法(measure中又回调onMeasure方法)的过程。具体measure核心主要有如下几点:

  2.MeasureSpec(View的内部类)测量规格为int型,值由高2位规格模式specMode和低30位具体尺寸specSize组成。其中specMode只有三种值:

  MeasureSpec.EXACTLY //确定模式,父View希望子View的大小是确定的,由specSize决定;

  MeasureSpec.AT_MOST //最多模式,父View希望子View的大小最多是specSize指定的值;

  MeasureSpec.UNSPECIFIED //未指定模式,父View完全依据子View的设计值来决定;

  3.View的measure方法是final的,不允许重载,View子类只能重载onMeasure来完成自己的测量逻辑。

  4.最顶层DecorView测量时的MeasureSpec是由ViewRootImpl中getRootMeasureSpec方法确定的(LayoutParams宽高参数均为MATCH_PARENT,specMode是EXACTLY,specSize为物理屏幕大小)。

  5.ViewGroup类提供了measureChild,measureChild和measureChildWithMargins方法,简化了父子View的尺寸计算。

  6.只要是ViewGroup的子类就必须要求LayoutParams继承子MarginLayoutParams,否则无法使用layout_margin参数。

  7.View的布局大小由父View和子View共同决定。

  8.使用View的getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高,必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。

  layout的过程

  ViewGroup的位置确定后,它在onLayout中会遍历所有的子元素并调用子元素layout方法,子元素layout方法中又会调用onLayout方法,View的layout方法确定自身的位置,而onLayout方法方法确定子孩子的位置

  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 listenersCopy =

  (ArrayList)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;

  }

  layout方法的大致流程如下:首先会通过setFrame方法来确定mLeft;mTop;mBottom;

  mRight;只要这四个点一旦确定,那么View在父容器中的位置就确定了,接着会调用onLayout方法,该方法目的是父容器来确定子元素的位置,无论是View还是ViewGroup都没有实现onLayout方法,我们查看LinearLayout的onLayout方法

  @Override

  protected void onLayout(boolean changed, int l, int t, int r, int b) {

  if (mOrientation == VERTICAL) {

  layoutVertical(l, t, r, b);

  } else {

  layoutHorizontal(l, t, r, b);

  }

  }

  查看layoutVertical中关键代码

  for (int i = 0; i < count; i++) {

  final View child = getVirtualChildAt(i);

  if (child == null) {

  childTop += measureNullChild(i);

  } else if (child.getVisibility() != GONE) {

  final int childWidth = child.getMeasuredWidth();

  final int childHeight = child.getMeasuredHeight();

  final LinearLayout.LayoutParams lp =

  (LinearLayout.LayoutParams) child.getLayoutParams();

  int gravity = lp.gravity;

  if (gravity < 0) {

  gravity = minorGravity;

  }

  final int layoutDirection = getLayoutDirection();

  final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);

  switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {

  case Gravity.CENTER_HORIZONTAL:

  childLeft = paddingLeft + ((childSpace - childWidth) / 2)

  + lp.leftMargin - lp.rightMargin;

  break;

  case Gravity.RIGHT:

  childLeft = childRight - childWidth - lp.rightMargin;

  break;

  case Gravity.LEFT:

  default:

  childLeft = paddingLeft + lp.leftMargin;

  break;

  }

  if (hasDividerBeforeChildAt(i)) {

  childTop += mDividerHeight;

  }

  childTop += lp.topMargin;

  setChildFrame(child, childLeft, childTop + getLocationOffset(child),

  childWidth, childHeight);

  childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

  i += getChildrenSkipCount(child, i);

  }

  }

  这个方法会遍历所有的子元素并调用setChildFrame方法来为子元素指定对应的位置,其中childTop的数值会不断的增大,这意味着后面的子元素还位于靠下的位置,刚好符合竖直的LinearLayout的特性,setChildFrame方法中不过是调用了子元素的Layout方法而已

  private void setChildFrame(View child, int left, int top, int width, int height) {

  child.layout(left, top, left + width, top + height);

  }

  同时,会发现setChildFrame中的width和height实际上就是子元素的测量宽高

  final int childWidth = child.getMeasuredWidth();

  final int childHeight = child.getMeasuredHeight();

  View的layout方法中会通过setFrame方法去设置子元素四个顶点的位置,这样子元素的位置就可以确定

  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 position

  invalidate(sizeChanged);

  mLeft = left;

  mTop = top;

  mRight = right;

  mBottom = bottom;

  mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

  接下来是View的getWidth和getHeight方法,结合里面的实现,可以发现他们分别返回的就是View测量的宽度和高度

  @ViewDebug.ExportedProperty(category = "layout")

  public final int getWidth() {

  return mRight - mLeft;

  }

  /**

  * Return the height of your view.

  *

  * @return The height of your view, in pixels.

  */

  @ViewDebug.ExportedProperty(category = "layout")

  public final int getHeight() {

  return mBottom - mTop;

  }

  layout总结

  1.layout也是从顶层父View向子View的递归调用view.layout方法的过程,即父View根据上一步measure子View所得到的布局大小和布局参数,将子View放在合适的位置上。

  2.View.layout方法可被重载,ViewGroup.layout为final的不可重载,ViewGroup.onLayout为abstract的,子类必须重载实现自己的位置逻辑。

  3.measure操作完成后得到的是对每个View经测量过的measuredWidth和measuredHeight,layout操作完成之后得到的是对每个View进行位置分配后的mLeft、mTop、mRight、mBottom,这些值都是相对于父View来说的。

  4.凡是layout_XXX的布局属性基本都针对的是包含子View的ViewGroup的,当对一个没有父容器的View设置相关layout_XXX属性是没有任何意义的。

  5.使用View的getWidth()和getHeight()方法来获取View测量的宽高,必须保证这两个方法在onLayout流程之后被调用才能返回有效值。

  draw的过程

  View的绘制过程遵循以下几步:

  1)绘制背景background.draw(canvas)

  2)绘制自己(onDraw)

  3)绘制 children(dispatchDraw)

  4)绘制装饰(onDrawScrollBars)

  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

  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;

  }

  /*

  * Here we do the full fledged routine...

  * (this is an uncommon case where speed matters less,

  * this is why we repeat some of the tests that have been

  * done above)

  */

  boolean drawTop = false;

  boolean drawBottom = false;

  boolean drawLeft = false;

  boolean drawRight = false;

  float topFadeStrength = 0.0f;

  float bottomFadeStrength = 0.0f;

  float leftFadeStrength = 0.0f;

  float rightFadeStrength = 0.0f;

  // Step 2, save the canvas' layers

  int paddingLeft = mPaddingLeft;

  final boolean offsetRequired = isPaddingOffsetRequired();

  if (offsetRequired) {

  paddingLeft += getLeftPaddingOffset();

  }

  int left = mScrollX + paddingLeft;

  int right = left + mRight - mLeft - mPaddingRight - paddingLeft;

  int top = mScrollY + getFadeTop(offsetRequired);

  int bottom = top + getFadeHeight(offsetRequired);

  if (offsetRequired) {

  right += getRightPaddingOffset();

  bottom += getBottomPaddingOffset();

  }

  final ScrollabilityCache scrollabilityCache = mScrollCache;

  final float fadeHeight = scrollabilityCache.fadingEdgeLength;

  int length = (int) fadeHeight;

  // clip the fade length if top and bottom fades overlap

  // overlapping fades produce odd-looking artifacts

  if (verticalEdges && (top + length > bottom - length)) {

  length = (bottom - top) / 2;

  }

  // also clip horizontal fades if necessary

  if (horizontalEdges && (left + length > right - length)) {

  length = (right - left) / 2;

  }

  if (verticalEdges) {

  topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));

  drawTop = topFadeStrength * fadeHeight > 1.0f;

  bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));

  drawBottom = bottomFadeStrength * fadeHeight > 1.0f;

  }

  if (horizontalEdges) {

  leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));

  drawLeft = leftFadeStrength * fadeHeight > 1.0f;

  rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));

  drawRight = rightFadeStrength * fadeHeight > 1.0f;

  }

  saveCount = canvas.getSaveCount();

  int solidColor = getSolidColor();

  if (solidColor == 0) {

  final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

  if (drawTop) {

  canvas.saveLayer(left, top, right, top + length, null, flags);

  }

  if (drawBottom) {

  canvas.saveLayer(left, bottom - length, right, bottom, null, flags);

  }

  if (drawLeft) {

  canvas.saveLayer(left, top, left + length, bottom, null, flags);

  }

  if (drawRight) {

  canvas.saveLayer(right - length, top, right, bottom, null, flags);

  }

  } else {

  scrollabilityCache.setFadeColor(solidColor);

  }

  // Step 3, draw the content

  if (!dirtyOpaque) onDraw(canvas);

  // Step 4, draw the children

  dispatchDraw(canvas);

  // Step 5, draw the fade effect and restore layers

  final Paint p = scrollabilityCache.paint;

  final Matrix matrix = scrollabilityCache.matrix;

  final Shader fade = scrollabilityCache.shader;

  if (drawTop) {

  matrix.setScale(1, fadeHeight * topFadeStrength);

  matrix.postTranslate(left, top);

  fade.setLocalMatrix(matrix);

  p.setShader(fade);

  canvas.drawRect(left, top, right, top + length, p);

  }

  if (drawBottom) {

  matrix.setScale(1, fadeHeight * bottomFadeStrength);

  matrix.postRotate(180);

  matrix.postTranslate(left, bottom);

  fade.setLocalMatrix(matrix);

  p.setShader(fade);

  canvas.drawRect(left, bottom - length, right, bottom, p);

  }

  if (drawLeft) {

  matrix.setScale(1, fadeHeight * leftFadeStrength);

  matrix.postRotate(-90);

  matrix.postTranslate(left, top);

  fade.setLocalMatrix(matrix);

  p.setShader(fade);

  canvas.drawRect(left, top, left + length, bottom, p);

  }

  if (drawRight) {

  matrix.setScale(1, fadeHeight * rightFadeStrength);

  matrix.postRotate(90);

  matrix.postTranslate(right, top);

  fade.setLocalMatrix(matrix);

  p.setShader(fade);

  canvas.drawRect(right - length, top, right, bottom, p);

  }

  canvas.restoreToCount(saveCount);

  // 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);

  }

  View的绘制过程的传递是通过dispatchDraw实现的,dispatchdraw会遍历调用所有子元素的draw方法。如此draw事件就一层一层的传递下去。

  draw总结

  1.如果该View是一个ViewGroup,则需要递归绘制其所包含的所有子View。

  2.View默认不会绘制任何内容,真正的绘制都需要自己在子类中实现。

  3.View的绘制是借助onDraw方法传入的Canvas类来进行的。

  4.在获取画布剪切区(每个View的draw中传入的Canvas)时会自动处理掉padding,子View获取Canvas不用关注这些逻辑,只用关心如何绘制即可。

  5.默认情况下子View的ViewGroup.drawChild绘制顺序和子View被添加的顺序一致,但是你也可以重载ViewGroup.getChildDrawingOrder()方法提供不同顺序。

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