13问13答全面学习Android View绘制

 //desire的这2个参数就代表屏幕的宽高， childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); //decorView的measureSpec就是在这里确定的，其实比普通view的measurespec要简单的多 //代码就不分析了 一目了然的东西 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; } 

 //对于普通view的measure来说 是由这个view的 父view ，也就是viewgroup来触发的。 //也就是下面这个measureChildWithMargins方法 protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { //第一步 先取得子view的 layoutParams 参数值 final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); //然后开始计算子view的spec的值，注意这里看到 计算的时候除了要用子view的 layoutparams参数以外 //还用到了父view 也就是viewgroup自己的spec的值 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); } //这个算view的spec的方法 看上去一大串 但是真的逻辑非常简单 就是根据父亲viewgroup //的meaurespec 同时还有view自己的params来确定 view自己的measureSpec。 //注意这里的参数是padding,这个值的含义是 父容器已占用的控件的大小 所以view的Specsize //的值 你们可以看到 是要减去这个padding的值的。总大小-已经用的 =可用的。 很好理解。 //然后就是下面的switch逻辑 要自己梳理清楚。其实也不难，主要是下面几条原则 //如果view采用固定宽高，也就是写死的数值那种。那就不管父亲的spec的值了，view的spec 就肯定是exactly 并且大小遵循layout参数里设置的大小。 //如果view的宽高是match_parent ，那么就要看父容器viewgroup的 spec的值了，如果父view的spec是exactly模式， //那view也肯定是exactly,并且大小就是父容器剩下的空间。如果父容器是at_most模式，那view也是at_most 并且不会超过剩余空间大小 //如果view的宽高是wrap_content, 那就不管父容器的spec了，view的spec一定是at_most 并且不会超过父view 剩余空间的大小。 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); } 

 //view的measure是final 方法 我们子类无法修改的。 public final void measure(int widthMeasureSpec, int heightMeasureSpec) { boolean optical = isLayoutModeOptical(this); 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); } // Suppress sign extension for the low bytes long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL; if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2); if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT || widthMeasureSpec != mOldWidthMeasureSpec || heightMeasureSpec != mOldHeightMeasureSpec) { // first clears the measured dimension flag mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET; resolveRtlPropertiesIfNeeded(); int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 : mMeasureCache.indexOfKey(key); if (cacheIndex <0 || signoremeasurecache) {>> 32), (int) value); mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } // flag not set, setMeasuredDimension() was not invoked, we raise // an exception to warn the developer if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) { throw new IllegalStateException("View with id " + getId() + ": " + getClass().getName() + "#onMeasure() did not set the" + " measured dimension by calling" + " setMeasuredDimension()"); } mPrivateFlags |= PFLAG_LAYOUT_REQUIRED; } mOldWidthMeasureSpec = widthMeasureSpec; mOldHeightMeasureSpec = heightMeasureSpec; mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 | (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension } //不过可以看到的是在measure方法里调用了onMeasure方法 //所以就能知道 我们在自定义view的时候一定是重写这个方法！ protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } 

 //可以看出来这个就是setMeasuredDimension方法的调用 这个方法看名字就知道就是确定view的测量宽高的 //所以我们分析的重点就是看这个getDefaultSize 方法 是怎么确定view的测量宽高的 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } //这个方法特别简单 基本可以认为就是近似的返回spec中的specSize，除非你的specMode是UNSPECIFIED //UNSPECIFIED 这个一般都是系统内部测量才用的到，这种时候返回size 也就是getSuggestedMinimumWidth的返回值 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; } //跟view的背景相关 这里不多做分析了 protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); } 

 //重写activity的这个方法 public void onWindowFocusChanged(boolean hasFocus) { super.onWindowFocusChanged(hasFocus); if (hasFocus) { int width = tv.getMeasuredWidth(); int height = tv.getMeasuredHeight(); Log.v("burning", "width==" + width); Log.v("burning", "height==" + height); } } 

 @Override protected void onStart() { super.onStart(); tv.post(new Runnable() { @Override public void run() { int width = tv.getMeasuredWidth(); int height = tv.getMeasuredHeight(); } }); } 

 @Override protected void onStart() { super.onStart(); ViewTreeObserver observer = tv.getViewTreeObserver(); observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() { @Override public void onGlobalLayout() { int width = tv.getMeasuredWidth(); int height = tv.getMeasuredHeight(); tv.getViewTreeObserver().removeOnGlobalLayoutListener(this); } }); } 

 /** * If this view doesn't do any drawing on its own, set this flag to * allow further optimizations. By default, this flag is not set on * View, but could be set on some View subclasses such as ViewGroup. * * Typically, if you override {@link #onDraw(android.graphics.Canvas)} * you should clear this flag. * * @param willNotDraw whether or not this View draw on its own */ public void setWillNotDraw(boolean willNotDraw) { setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK); } 

 package com.example.administrator.motioneventtest; import android.content.Context; import android.graphics.Canvas; import android.graphics.Color; import android.graphics.Paint; import android.util.AttributeSet; import android.view.View; /** * Created by Administrator on 2016/2/4. */ public class CircleView extends View { private int mColor = Color.RED; private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG); private void init() { mPaint.setColor(mColor); } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec); int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec); int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec); int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec); //处理为wrap_content时的情况 if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) { setMeasuredDimension(200, 200); } else if (widthSpecMode == MeasureSpec.AT_MOST) { setMeasuredDimension(200, heightSpecSize); } else if (heightSpecMode == MeasureSpec.AT_MOST) { setMeasuredDimension(widthSpecSize, 200); } } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); //处理padding的情况 final int paddingLeft = getPaddingLeft(); final int paddingRight = getPaddingRight(); final int paddingTop = getPaddingTop(); final int paddingBottom = getPaddingBottom(); int width = getWidth() - paddingLeft - paddingRight; int height = getHeight() - paddingTop - paddingBottom; int radius = Math.min(width, height) / 2; canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, mPaint); } public CircleView(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); init(); } public CircleView(Context context) { super(context); init(); } public CircleView(Context context, AttributeSet attrs) { super(context, attrs); init(); } } 

 package com.example.administrator.motioneventtest; import android.content.Context; import android.util.AttributeSet; import android.util.Log; import android.view.View; import android.view.ViewGroup; /** * Created by Administrator on 2016/2/4. */ //这里我们只处理了padding的状态 没有处理margin的状态，子view的margin 对measure和layout的影响 //就留给读者自己完成了 public class CustomHorizontalLayout extends ViewGroup { //设置默认的控件最小是多少 这里不提供自定义属性了 写死在代码里 你们可以自行拓展 final int minHeight = 0; final int minWidth = 0; public CustomHorizontalLayout(Context context) { super(context); } public CustomHorizontalLayout(Context context, AttributeSet attrs) { super(context, attrs); } public CustomHorizontalLayout(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { super.onMeasure(widthMeasureSpec, heightMeasureSpec); int measureWidth = 0; int measureHeight = 0; final int childCount = getChildCount(); measureChildren(widthMeasureSpec, heightMeasureSpec); int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec); int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec); int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec); int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec); final View childView = getChildAt(0); final int paddingLeft = getPaddingLeft(); final int paddingRight = getPaddingRight(); final int paddingTop = getPaddingTop(); final int paddingBottom = getPaddingBottom(); //没有子控件 时 我们的宽高要作特殊处理 if (childCount == 0) { //当没有子控件时，如果长宽有一个为wrap 那么就让这个控件以最小的形式展现 //这里我们最小设置为0 if (widthSpecMode == MeasureSpec.AT_MOST || heightSpecMode == MeasureSpec.AT_MOST) { setMeasuredDimension(minWidth, minHeight); } else { //否则根据我们的layout属性来 setMeasuredDimension(getLayoutParams().width, getLayoutParams().height); } } else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) { measureWidth = childView.getMeasuredWidth() * childCount; measureHeight = childView.getMeasuredHeight(); setMeasuredDimension(paddingLeft + measureWidth + paddingRight, paddingTop + measureHeight + paddingBottom); } else if (heightSpecMode == MeasureSpec.AT_MOST) { measureHeight = childView.getMeasuredHeight(); setMeasuredDimension(paddingLeft + paddingRight + widthSpecSize, paddingTop + paddingBottom + measureHeight); } else if (widthSpecMode == MeasureSpec.AT_MOST) { measureWidth = childView.getMeasuredWidth() * childCount; setMeasuredDimension(paddingLeft + paddingRight + measureWidth, paddingTop + paddingBottom + heightSpecSize); } } @Override protected void onLayout(boolean changed, int l, int t, int r, int b) { final int paddingLeft = getPaddingLeft(); final int paddingRight = getPaddingRight(); final int paddingTop = getPaddingTop(); final int paddingBottom = getPaddingBottom(); //左边初始位置为0 int childLeft = 0 + paddingLeft; final int childCount = getChildCount(); for (int i = 0; i