Android(安卓)View 相关源码分析之三 View的绘制过程
16lz
2021-01-24
Android View 相关源码分析之一 从setContentView说起
Android View 相关源码分析之二 继LayoutInflater来说
现在开始分析View的绘制机制
View的测量 布局 绘制过程
测量之前的事情
View的整个绘制流程是开始于ViewRootImpl类的performTraversals方法(1k行) 根据相关设置来觉得十分要重新执行相关功能
private void performTraversals() { // cache mView since it is used so much below... final View host = mView; ... int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); ... //measure mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); ... //layout mView.layout(0, 0, mView.getMeasuredWidth(), mView.getMeasuredHeight()); ... //draw mView.draw(canvas); ... }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; ... } return measureSpec; }View 绘制流程图如下
View 绘制流程图measure源码分析
结论:
measure的过程就是父View向子View递归调用view.measure方法 (measure中回调onMeasure方法)的过程
measure方法是 final的 只能重载onMeasure方法
最顶层的DocerView的MeasureSpec由ViewRootImpl的getRootMeasureSpec方法提供 LayoutParams的参数为MATCH_PARENT specMode是EXACTLY，specSize为物理屏幕大小
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
View的getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高，要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。
/** * * 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) */ //没舍得删这些注释 感觉重要的事情都说了 为了计算整个View树的实际大小 设置实际的高和宽 每个子View都是根据父视图和自身决定实际宽高的 在onMeasure()方法中进行实际测量.传入widthMeasureSpec和heightMeasureSpec参数来表示了父View的规格 不但传入了模式 还传入了size 而对于DecorView来说 传入的模式一般为EXACTLY模式 size对应屏幕的宽高. 所以说子View的大小是父子View共同决定的public final void measure(int widthMeasureSpec, int heightMeasureSpec) { // measure ourselves, this should set the measured dimension flag back onMeasure(widthMeasureSpec, heightMeasureSpec); }MeasureSpec内部类
MeasureSpec是View的内部类 int型，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
- 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；
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 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()}). *
* * @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)); }getDefaultSize方法相关
public static int getDefaultSize(int size, int measureSpec) { int result = size; //通过measureSpec得到mode和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; } //最小宽度和高度由View的Background尺寸和View的minXXX共同决定 protected int getSuggestedMinimumHeight() { return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight()); } protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }setMeasuredDimension方法 对View的成员变量measuredWidth和measuredHeight变量赋值 也就是说该方法最终决定了View的大小
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); } public boolean isLayoutRequested() { return (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT; } private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) { mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; }至此一次最基础的View的measure过程就完成了 但是由于View可以嵌套 所以measure是递归传递的所以ViewGroup中需要对其子类进行measure过程 measureChildren方法实质为循环调用measureChild方法
而measureChild和measureChildWithMargins的区别是后者将margin和padding也作为了子视图的大小
一下分析measureChildWithMargins方法
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { //获取当前子视图的LayoutParams final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); //设定子View的测量规格 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); //子view的继续调用 child.measure(childWidthMeasureSpec, childHeightMeasureSpec); } //在getChildMeasureSpec中通过父View和本身的模式共同决定当前View的size public static int getChildMeasureSpec(int spec, int padding, int childDimension) { //获取当前父View的mode和size int specMode = MeasureSpec.getMode(spec); int specSize = MeasureSpec.getSize(spec); //获取父View的的剩余大小 int size = Math.max(0, specSize - padding); //定义结果变量 int resultSize = 0; int resultMode = 0; //根据对应的mode做处理 //通过父View和本身的模式共同决定当前View的size 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; } //将size和mode整合为MeasureSpec模式后返回 return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }layout源码分析
View layout整体流程与measure过程基本一样
结论:
- 需要根据ViewGroup本身的情况讨论 LinearLayout下会更看重子View的height和width 来安排对应位置 而RelativeLayout则更加关注子View的left right top bottom值 并且优先级高于width和height 甚至在部分自定义ViewGroup中 measure可能是无用的 直接使用layout方法来设置子View的位置也可以
- ViewGroup需要实现自己的layout逻辑
- layout_XXX中的各个熟悉都是针对子View的父ViewGroup的
- 同样使用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
/** * 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 children 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 */ //同样注解写的很好了 分派给他和他的所有的子视图大小和位置 @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; } //调用setFrame方法把参数分别赋值于 int oldL = mLeft; int oldT = mTop; int oldB = mBottom; int oldR = mRight; //判断view的位置是否发生过变化 , 确定是否对当前view重新layout 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); 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; } onLyayout方法
View中 protected void onLayout(boolean changed, int left, int top, int right, int bottom) { } ViewGroup中 protected abstract void onLayout(boolean changed, int l, int t, int r, int b);均是空方法 后面会就LinearLayout和RelativeLayout源码进行分析
draw源码分析
View的draw流程图如下
View draw流程.png结论:
- View需要在子类中实现onDraw的过程
- 在ViewGroup中 会调用其子View的方法 顺序与子view的添加顺序一致
draw的源码也很长 但是官方也给出给出了draw的过程
public void draw(Canvas canvas) { ... /* * 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 ... if (!dirtyOpaque) { drawBackground(canvas); } // skip step 2 & 5 if possible (common case) ... // Step 2, save the canvas' layers ... if (drawTop) { canvas.saveLayer(left, top, right, top + length, null, flags); } ... // 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 ... 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); } ... // Step 6, draw decorations (scrollbars) onDrawScrollBars(canvas); ... }Step 1, draw the background, if needed
// Step 1, draw the background, if needed //如果需要的话绘制背景 if (!dirtyOpaque) { drawBackground(canvas); } private void drawBackground(Canvas canvas) { //通过xml中属性background或者代码中setBackGroundColor\setBackgroundResource等方法赋值的背景drawable final Drawable background = mBackground; if (background == null) { return; } //根据layout中确定的view位置来设置背景的绘制区域 setBackgroundBounds(); // 如果需要的话使用显示列表 //canvas.isHardwareAccelerated() 硬件加速判定 //硬件加速时会将图层缓存到GPU上 而不是重绘View的每一层 if (canvas.isHardwareAccelerated() && mAttachInfo != null && mAttachInfo.mHardwareRenderer != null) { mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode); final RenderNode renderNode = mBackgroundRenderNode; if (renderNode != null && renderNode.isValid()) { setBackgroundRenderNodeProperties(renderNode); ((DisplayListCanvas) canvas).drawRenderNode(renderNode); return; } } final int scrollX = mScrollX; final int scrollY = mScrollY; //调用Drawable的draw方法来完成背景的绘制工作 if ((scrollX | scrollY) == 0) { background.draw(canvas); } else { canvas.translate(scrollX, scrollY); background.draw(canvas); canvas.translate(-scrollX, -scrollY); } } void setBackgroundBounds() { if (mBackgroundSizeChanged && mBackground != null) { mBackground.setBounds(0, 0, mRight - mLeft, mBottom - mTop); mBackgroundSizeChanged = false; rebuildOutline(); } }Step 2, save the canvas' layers
// Step 2, save the canvas' layers //保存绘制图层 if (drawTop) { canvas.saveLayer(left, top, right, top + length, null, flags); }Step 3, draw the content
// Step 3, draw the content //对View的内容进行绘制 if (!dirtyOpaque) onDraw(canvas); /** * Implement this to do your drawing. * * @param canvas the canvas on which the background will be drawn */ //onDraw也是空方法需要子类根据自身去实现相应的 protected void onDraw(Canvas canvas) { }Step 4, draw the children
// Step 4, draw the children //绘制其子View dispatchDraw(canvas); /** * Called by draw to draw the child views. This may be overridden * by derived classes to gain control just before its children are drawn * (but after its own view has been drawn). * @param canvas the canvas on which to draw the view */ protected void dispatchDraw(Canvas canvas) { //dispatchDraw同样空方法 与onDraw不同的是dispatchDraw在ViewGroup中被重写 }ViewGroup
//dispatchDraw方法中根据子View的不同情况 包括但不只包括该View是否显示 是否有进入或消失动画等进行了部分的调整 protected void dispatchDraw(Canvas canvas) { ... more |= drawChild(canvas, transientChild, drawingTime); ... } protected boolean drawChild(Canvas canvas, View child, long drawingTime) { return child.draw(canvas, this, drawingTime); }Step 5, draw the fade effect and restore layers
// Step 5, draw the fade effect and restore layers //绘制过度效果和恢复图层 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); }Step 6, draw decorations (scrollbars)
// Step 6, draw decorations (scrollbars) //对滚动条进行绘制 onDrawScrollBars(canvas);至此 View的绘制过程全部分析完了
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