android应用层view绘制流程之measure,layout,draw三步曲

Android 应用层 View 绘制流程之 measure,layout,draw 三步曲 View 的 measure 过程 三个流程均是从 ViewRootImpl 的 performTraversals 方法开始的，如下所示： private void performTraversals() int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); mView.layout(0, 0, mView.getMeasuredWidth(), mView.getMeasuredHeight(); mView.draw(canvas); 首先看下 getRootMeasureSpec 方法,如下所示： /* * Figures out the measure spec for the root view in a window based on its * layout params. * * param windowSize * The available width or height of the window * * param rootDimension * The layout params for one dimension (width or height) of the * window. * * return The measure spec to use to measure the root view. */ private static int getRootMeasureSpec(int windowSize, int rootDimension) int measureSpec; switch (rootDimension) case ViewGroup.LayoutParams.MATCH_PARENT: / Window cant 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; 从上面的注释可以看出这个 getRootMeasureSpec 是为了根据根视图的 LayoutParams 计算根 视图的 MeasureSpec，这个根视图就是上篇博客讲的 DecorView。 关于 MeasureSpec 关于 MeasureSpec 来做一个简单的说明：通过 MeasureSpec.makeMeasureSpec 来得到一个 32 位的整数，高两位代码测量模式 mode,低 30 位代表测量大小 size，如下所示： public static int makeMeasureSpec(IntRange(from = 0, to = (1 * This is called to find out how big a view should be. The parent * supplies constraint information in the width and height parameters. * * * * 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. * * * * 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) . onMeasure(widthMeasureSpec, heightMeasureSpec); . 通过注释可以看出，这个方法是用来计算当前 View 应该为多大，也就是实际的宽高。 widthMeasureSpec 和 heightMeasureSpec 是由父 View 传入的约束信息，代表了父 View 给 当前 View 的测量规格，当前 View 的宽高是由父 View 和自身一起决定的。measure 方法 是 final 的，不可重载，实际的测量过程是在 onMeasure 方法里面完成了，因此子类必须且 只能重载 onMeasure 方法来实现自身的测量逻辑。 接下来看 onMeasure 方法： /* * * 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. * * * * CONTRACT: When overriding this method, you * must call link #setMeasuredDimension(int, int) to store the * measured width and height of this view. Failure to do so will trigger an * IllegalStateException, thrown by * link #measure(int, int). Calling the superclass * link #onMeasure(int, int) is a valid use. * * * * 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. * * * * If this method is overridden, it is the subclasss responsibility to make * sure the measured height and width are at least the views minimum height * and width (link #getSuggestedMinimumHeight() and * link #getSuggestedMinimumWidth(). * * * param widthMeasureSpec horizontal space requirements as imposed by the parent. * The requirements are encoded with * link android.view.View.MeasureSpec. * param heightMeasureSpec vertical space requirements as imposed by the parent. * The requirements are encoded with * link android.view.View.MeasureSpec. * * see #getMeasuredWidth() * see #getMeasuredHeight() * see #setMeasuredDimension(int, int) * see #getSuggestedMinimumHeight() * see #getSuggestedMinimumWidth() * see android.view.View.MeasureSpec#getMode(int) * see android.view.View.MeasureSpec#getSize(int) */ protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec); 注释已经写的非常明白了，子类必须复写 onMeasure 方法，且最终通过调用 setMeasuredDimension 方法来存储当前 View 测量得到的宽和高。这个宽和高是通过 getDefaultSize 方法得来的，如下所示： /* * 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; 可以看出，如果 specMode 等于 AT_MOST 或者 EXACTLY 就返回 specSize，也就是父类 指定的 specSize，否则返回通过 getSuggestedMinimumWidth 和 getSuggestedMinimumHeight 得到的 size，从名字可以看出是建议的最小宽度和高度，代码如下所示： protected int getSuggestedMinimumHeight() return (mBackground = null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight(); protected int getSuggestedMinimumWidth() return (mBackground = null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth(); 可以看出，建议的最小宽度和高度是由 view 的 background 以及其 mMinWidth、mMinHeight 共同决定的。 setMeasuredDimension 方法如下所示： 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; 可以看出这个方法就是给 mMeasuredHeight 和 mMeasuredWidth 进行赋值。进行了赋值之 后调用 View 的 getMeasuredWidth 和 getMeasuredHeight 方法才能得到其正确的测量宽高！ ViewGroup 的 measure 过程 上面提到 View 的 measure 方法传入的 widthMeasureSpec 和 heightMeasureSpec 是由父 View 传入的约束信息，那么这些信息是何时传入的呢？由于 View 是嵌套的，因此 measure 过程 也是递归传递的，子 View 的 measure 是由父类调用的，然后子 View 根据传入的父类约束 来设置自身的测量规格。 继承自 ViewGroup 的视图均需要实现 onMeasure 方法，在这个方法里面对其子 View 进行 测量，同时也对自身进行测量，比如 LinearLayout 的 onMeasure 方法如下： Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) if (mOrientation = VERTICAL) measureVertical(widthMeasureSpec, heightMeasureSpec); else measureHorizontal(widthMeasureSpec, heightMeasureSpec); 根据布局的方向分别调用 measureHorizontal 和 measureVertical 方法。 在 ViewGroup 中定义了 measureChildren, measureChild, measureChildWithMargins 方法来对 子视图进行测量。measureChildren 内部循环调用了 measureChild。 measureChild 和 measureChildWithMargins 的区别在于 measureChildWithMargins 把 child 的 margin 也考虑在内。下面来对 measureChildWithMargins 方法来分析： /* * Ask one of the children of this view to measure itself, taking into * account both the MeasureSpec requirements for this view and its padding * and margins. The child must have MarginLayoutParams The heavy lifting is * done in getChildMeasureSpec. * * param child The child to measure * param parentWidthMeasureSpec The width requirements for this view * param widthUsed Extra space that has been used up by the parent * horizontally (possibly by other children of the parent) * param parentHeightMeasureSpec The height requirements for this view * param heightUsed Extra space that has been used up by the parent * vertically (possibly by other children of the parent) */ protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); /子视图的测量规格是由父视图的测量测量规格以及子视图的 LayoutParams 来共同 决定的 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); /调用子视图的 measure 方法来设置子视图的测量规格 child.measure(childWidthMeasureSpec, childHeightMeasureSpec); 从以上代码可以看出：子视图的测量规格是由父视图的测量测量规格以及子视图的 LayoutParams 来共同决定的，因此关键函数是 getChildMeasureSpec 函数，如下所示： /* * Does the hard part of measureChildren: figuring out the MeasureSpec to * pass to a particular child. This method figures out the right MeasureSpec * for one dimension (height or width) of one child view. * * The goal is to combine information from our MeasureSpec with the * LayoutParams of the child to get the best possible results. For example, * if the this view knows its size (because its MeasureSpec has a mode of * EXACTLY), and the child has indicated in its LayoutParams that it wants * to be the same size as the parent, the parent should ask the child to * layout given an exact size. * * param spec The requirements for this view * param padding The padding of this view for the current dimension and * margins, if applicable * param childDimension How big the child wants to be in the current * dimension * return a MeasureSpec integer for the child */ public static int getChildMeasureSpec(int spec, int padding, int childDimension) int specMode = MeasureSpec.getMode(spec);/得到父视图的 mode int specSize = MeasureSpec.getSize(spec);/得到父视图的 size /得到 Parent 视图剩余的大小 int size = Math.max(0, specSize - padding); int resultSize = 0; int resultMode = 0; /根据 Parent 视图的 specMode 来进行分支判断 switch (specMode) / Parent has imposed an exact size on us case MeasureSpec.EXACTLY:/父类是精确模式 if (childDimension = 0) /子视图是精确模式，直接设置了精确的大小（在 xml 当中设置了 layout_width=“xxx“或者在代码中设置了具体的数值),子视图的 size 就是精确值,子视图的 mode 就是 EXACTLY resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; else if (childDimension = LayoutParams.MATCH_PARENT) /如果子视图的 layout_width 或者 layout_height 为 MATCH_PARENT,也就是 为父视图的大小，那么子视图的 size 就是 Parent 视图剩余的大小，且 mode 与父类相同， 也为 EXACTLY / Child wants to be our size. So be it. resultSize = size; resultMode = MeasureSpec.EXACTLY; else if (childDimension = LayoutParams.WRAP_CONTENT) /如果子视图的 layout_width 或者 layout_height 为 WRAP_CONTENT,也就是 不超过父视图的大小，那么子视图的 size 为 size，且 mode 为 AT_MOST。 / Child wants to determine its own size. It cant 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) /子视图是精确模式，直接设置了精确的大小（在 xml 当中设置了 layout_width=“xxx“或者在代码中设置了具体的数值),子视图的 size 就是精确值,子视图的 mode 就是 EXACTLY / Child wants a specific size. so be it resultSize = childDimension; resultMode = MeasureSpec.EXACTLY; else if (childDimension = LayoutParams.MATCH_PARENT) /如果子视图的 layout_width 或者 layout_height 为 MATCH_PARENT,也就是 为父视图的大小，那么子视图的 size 就是 Parent 视图剩余的大小，且 mode 与父类相同， 也是 AT_MOST。 / 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) /如果子视图的 layout_width 或者 layout_height 为 WRAP_CONTENT,也就 是不超过父视图的大小，那么子视图的 size 为 size，且 mode 为 AT_MOST。 / Child wants to determine its own size. It cant 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; / 将 resultSize 和 resultMode 进行组装为 32 为整数返回 /noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); 可以看到，getChildMeasureSpec 就是根据父视图的 specSize 和 specMode 以及 child 视图的 LayoutParams 来确定子视图的 resultSize 和 resultMode，然后把 resultSize 和 resultMode 进 行组装成 32 位的整数，作为 child.measure 的参数来对子视图进行测量。 有一个需要特别注意的地方： 当 childDimension = LayoutParams.WRAP_CONTENT 的时候，其 specSize 和 specMode 分 别为父视图的 size 和 MeasureSpec.AT_MOST。 再回到上面的 View 测量过程当中的 getDefaultSize 方法，如下所示。我们发现当 View 的 specMode 为 AT_MOST 的时候，其 size 默认就是 parent 视图的 size! 因此，在我们自定义 View 的时候，需要考虑当 specMode 为 AT_MOST 的时候（也就是在 xml 布局当中设置为 WRAP_CONTENT 的时候）给当前 View 的宽高设置一个具体的值， 大家可以去看看比如 TextView 的源代码，均对 WRAP_CONTENT 的情况进行了特殊的处 理！ public static int getDefaultSize(int size, int measureSpec) int result = size; int specMode = MeasureSpec.getMode(measureSpec); int specSize = MeasureSpec.getSize(measureSpec); case MeasureSpec.AT_MOST: case MeasureSpec.EXACTLY: result = specSize; break; return result; 以上就是 View 和 ViewGroup 的 measure 过程,在 ViewGroup 的实现视图当中递归调用子视 图的的 measure 方法来实现整个 View 树的测量。在自定义 View 的时候，当我们需要对 View 的尺寸进行更改的时候，需要实现 onMeasure 方法，在里面根据父视图给的 specSize 和 specMode 来设置当前 View 的 specMode 和 specSize,需要注意的是当父视图给的 specMode=AT_MOST 的时候，需要给当前 View 的宽高设置一个具体的值。 View 的 layout 过程 讲完了 View 的 measure 过程，接下来就是 layout 过程。那么这个 layout 过程是干什么的呢？ 在 measure 过程当中设置了 view 的宽高，那么设置了宽高之后，具体 view 是显示在屏幕 的哪个位置呢？这个就是 layout 过程干的事。 layout 跟 measure 一样，也是递归结构，来看下 View 的 layout 方法： /* * Assign a size and position to a view and all of its * descendants * * This is the second phase of the layout mechanism. * (The first is measuring). In this phase, each parent calls * layout on all of its ldren to position them. * This is typically done using the child measurements * that were stored in the measure pass(). * * 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. * * 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 mPrivateFlags3 int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; /setFrame 方法把参数分别赋值给 mLeft、mTop 、 mRight 和 mBottom 这几个变量 /判断布局是否发生改变 boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed | (mPrivateFlags 在 layout 方法里面首先通过 setFrame 来设置自身的位置，然后调用了 onLayout 方法，是不 是跟 measure 方法里面调用 onMeasure 方法类似！来看下 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) 发现 onLayout 是一个空方法，通过注释可以看出：具有子视图的子类需要重写这个 onLayout 方法并且调用其每一个子视图的 layout 方法。 这就完全明白了：也就是说直接或者间接继承自 ViewGroup 的视图需要重写 onLayout 方 法，然后调用其每个子视图的 layout 方法来设置子视图的位置！我们可以查看 LinearLayout，其肯定是实现了 onLayout 方法，在这个方法里面来一一设置子视图的位置！ LinearLayout 的 onLayout 方法如下所示： 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); 来看下 layoutVertical 方法： /* * Position the children during a layout pass if the orientation of this * LinearLayout is set to link #VERTICAL. * * see #getOrientation() * see #setOrientation(int) * see #onLayout(boolean, int, int, int, int) * param left * param top * param right * param bottom */ void layoutVertical(int left, int top, int right, int bottom) final int paddingLeft = mPaddingLeft; int childTop; int childLeft; / Where right end of child should go final int width = right - left; int childRight = width - mPaddingRight; /child 可以使用的空间 / Space available for child int childSpace = width - paddingLeft - mPaddingRight; /得到 child 的个数 final int count = getVirtualChildCount(); final int majorGravity = mGravity final int minorGravity = mGravity /根据 majorGravity 计算 childTop 的位置 switch (majorGravity) case Gravity.BOTTOM: / mTotalLength contains the padding already childTop = mPaddingTop + bottom - top - mTotalLength; break; / mTotalLength contains the padding already case Gravity.CENTER_VERTICAL: childTop = mPaddingTop + (bottom - top - mTotalLength) / 2; break; case Gravity.TOP: default: childTop = mPaddingTop; break; / 开始进行遍历 child 视图 for (int i = 0; i = 0 if (transientChild.mViewFlags transientIndex+; if (transientIndex = transientCount) transientIndex = -1; final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder); final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex); if (child.mViewFlags . 从上述方法看出主要是遍历 child,然后调用 child 的 drawChi