Android-自定义View前传-View的三大流程-Layout

青春须早为，岂能长少年。这篇文章主要讲述Android-自定义View前传-View的三大流程-Layout相关的知识，希望能为你提供帮助。
android-自定义View前传-View的三大流程-Layout 参考

《Android开发艺术探索》
https://github.com/hongyangAndroid/FlowLayout

写在前头
在之前的文章中，我们学习了Android View的 Measure的流程，本篇文章来学习一下View的 Layout 的过程。学完了这一篇文章后，我们可以尝试自己去自定义一个自己的Layout。
Overview

我对于Layout过程的理解：Layout的过程就是给Child安家的过程

Layout的过程主要是放在 ViewGroup 中的，ViewGroup不仅需要定位自己，还需要定位Child。
View和ViewGroup
Layout流程的起点也是在 ViewRootImpl 中的 performTraversals 方法中。

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; if (host == null) { return; }try { //首先调用了host的layout方法 host = mView = DecorView host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); //.... }

我们接着来看View的layout方法

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; //调用 setOpticalFrame或者 setFrame 来确定自己的位置 boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {、 //调用onLayout onLayout(changed, l, t, r, b); //..... }protected boolean setFrame(int left, int top, int right, int bottom) { boolean changed = false; if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) { changed = true; // Remember our drawn bit int 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 position invalidate(sizeChanged); //确定4个点 //这四个点一旦确定了那么View在ViewGroup中的位置也就确定了 mLeft = left; mTop = top; mRight = right; mBottom = bottom; mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom); //......

View的 layout 方法主要是做了:

通过 setFrame 确定了自己的位置，一篇Left,Top,Right,Bottom这几个值确定了,那么View的位置也就确定了。
紧接着调用了onLayout 方法。

LinearLayout的onLayout
View是不需要实现onLayout方法的，只用ViewGroup才需要实现。由于各种ViewGroup的布局方式的不同，无法统一，所以ViewGroup也并没有实现onLayout. 而是将onLayout的过程放到了子类中。我们还是通过 LinearLayout 来学习。

@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); } }

onLayout根据 Orientation 属性来调用 layoutVertical 或者 layoutHorizontal

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; // Space available for child //获取Child可用的空间 int childSpace = width - paddingLeft - mPaddingRight; //Child的Group final int count = getVirtualChildCount(); final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK; final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK; //根据Gravity确定初始的childTop 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; } //LayoutView //遍历 for (int i = 0; i < count; i++) { final View child = getVirtualChildAt(i); if (child == null) { childTop += measureNullChild(i); } else if (child.getVisibility() != GONE) {//过滤Child的Visibility是GONE的情况 //获取Child的大小 final int childWidth = child.getMeasuredWidth(); final int childHeight = child.getMeasuredHeight(); //获取LP 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); //处理Child的Gravity switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) { //水平居中 case Gravity.CENTER_HORIZONTAL: childLeft = paddingLeft + ((childSpace - childWidth) / 2) + lp.leftMargin - lp.rightMargin; break; //居右 case Gravity.RIGHT: childLeft = childRight - childWidth - lp.rightMargin; break; case Gravity.LEFT: default: childLeft = paddingLeft + lp.leftMargin; break; } //childTop 加上 Divider if (hasDividerBeforeChildAt(i)) { childTop += mDividerHeight; } //加上Margin childTop += lp.topMargin; //调用Child的layout方法 setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight); childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child); i += getChildrenSkipCount(child, i); } } }private void setChildFrame(View child, int left, int top, int width, int height) { child.layout(left, top, left + width, top + height); }

上面的代码还是比较清晰的，首先是有一个childTop变量，来确定child与ViewGroup顶部的距离，通过不断的遍历Child然后不断增加childTop的值，这样就实现了LinearLayout的垂直布局的效果。当然其中也有处理LinearLayout和Child的Gravity的过程。
FlowLayout
通过学习LinearLayout的Layout的过程，发现其实Layout的过程就是确定View的Left,Top,Right,Bottom 4个值的过程，学习了 Measure 和 Layout 的过程以后，我们就已经可以着手做一个自己的Layout了，这里我选的是模仿 hongyang大神的FlowLayout。效果图如下:

文章图片

package layoutimport android.annotation.SuppressLint import android.content.Context import android.util.AttributeSet import android.view.View import android.view.ViewGroup/** * Created by ShyCoder on 2019/1/16. */ class MyFlowLayout(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : ViewGroup(context, attrs, defStyleAttr) { constructor(context: Context?, attributeSet: AttributeSet?) : this(context, attributeSet, 0)constructor(context: Context?) : this(context, null)init {}/** * 存贮所有的View根据行来存贮 * */ private val mAllViews = mutableListOf< List< View> > ()/** * 存贮每一行View的高度 * */ private val mHeightList = mutableListOf< Int> ()override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) { //从MeasureSpec获取Mode和Size val widthMode = MeasureSpec.getMode(widthMeasureSpec) val widthSize = MeasureSpec.getMode(widthMeasureSpec) val heightMode = MeasureSpec.getMode(heightMeasureSpec) val heightSize = MeasureSpec.getMode(heightMeasureSpec)//计算Wrap_content的情况 var totalHeight = 0//总高度 var totalWidth = 0//总宽度var lineWidth = 0//当前行的宽度 var lineHeight = 0//当前行的高度val viewCount = childCountfor (i in 0.until(viewCount)) { val child = getChildAt(i) //如果是GONE状态不用测量 if (child.visibility == View.GONE) { continue } //测量Child measureChild(child, widthMeasureSpec, heightMeasureSpec) val lp = child.layoutParams as MarginLayoutParams//计算child的width = 测量后宽度+左右的两个Margin val childWidth = child.measuredWidth + lp.leftMargin + lp.rightMargin //计算child的Height = 测量后高度 + 上下两个Margin val childHeight = child.measuredHeight + lp.bottomMargin + lp.topMargin//需要换行时候的处理方式 //如果已有的行宽+当前child的宽度> FlowLayout的宽度(减去左右的Padding) if (childWidth + lineWidth > widthSize - this.paddingLeft + this.paddingRight) { totalWidth = Math.max(totalWidth, lineWidth) lineWidth = childWidth totalHeight += lineHeight lineHeight = childHeight } else {//如果不需要换行增加行宽 lineWidth += childWidth //获取最大高度 lineHeight = Math.max(lineHeight, childHeight) } //最后一个View if (i == viewCount - 1) { totalWidth = Math.max(totalWidth, lineWidth) totalHeight += lineHeight } } this.setMeasuredDimension( //如果MeasureSpec的Mode是EXACTLY 的话，测量后大小等会传进来的测量大小， //否则则是我们自己计算的大小 if (widthMode == MeasureSpec.EXACTLY) widthSize else totalWidth, if (heightMode == MeasureSpec.EXACTLY) heightSize else totalHeight ) }override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) { this.mAllViews.clear() this.mHeightList.clear()val viewCount = this.childCountvar lineHeight = 0 var lineWidth = 0var lineViews = mutableListOf< View> ()for (i in 0.until(viewCount)) { val child = this.getChildAt(i) if (child.visibility == View.GONE) { continue } val lp = child.layoutParams as MarginLayoutParamsval childWidth = child.measuredWidth + lp.leftMargin + lp.rightMargin val childHeight = child.measuredHeight + lp.topMargin + lp.bottomMargin//行上无法继续放置View if (childWidth + lineWidth > this.width - this.paddingLeft - this.paddingRight) { //添加line height mHeightList.add(lineHeight) lineWidth = 0 //添加正行的View到集合中 this.mAllViews.add(lineViews) lineViews = mutableListOf()} //区当前行的View的最大高度 lineHeight = StrictMath.max(lineHeight, childHeight) lineWidth += childWidth //向行上添加View lineViews.add(child) }//进行Layout var left = this.paddingLeft var top = this.paddingTopfor (i in 0.until(this.mAllViews.size)) { val lineViews = mAllViews[i] val lineHeight = mHeightList[i] left = this.paddingLeftfor (j in 0.until(lineViews.size)) { val child = lineViews[j] val lp = child.layoutParams as MarginLayoutParams//view的四个边 val l = left + lp.leftMargin val t = top + lp.topMargin val r = l + child.measuredWidth val b = t + child.measuredHeight//调用Child的Layout方法 child.layout(l, t, r, b)left += child.measuredWidth + lp.leftMargin + lp.rightMargin } top += lineHeight } } }

在onMeasure对wrap_content的情况进行了处理，计算出所需要的大小。
在onLayout方法中，首先计算出每一行的高度并存储和获取每一行的View的List进行存储，当这些都计算完之后，进行layout操作。
在layout操作的时候，首先是在同一行上的View的top是统一的，每当这一行的View处理完成之后，就执行换行的操作，即增加view的大小。
写在最后
【Android-自定义View前传-View的三大流程-Layout】本篇文章就到此结束了，Layout的过程还是相对简单的，在下一篇文章呢，我们将会学习View的最后一个流程 Draw 流程。