图解Android:View的绘制机制与源码解析
前言
Android的视图是如何绘制的?深入了解一下UI的绘制原理无论对我们APP的性能优化还是对我们的自定义view都有很大的帮助。下文将和大家一道探究一下Android的viewTree的绘制原理,希望对大家的开发和学习有所帮助。
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Activity的视图结构
先看一下activity的视图结构图
Activity的视图结构- 每个activity都有一个Window(实际是phonewindow)
- Phonewindow含有一个DecorView,这是我们window的topview
- DecorView是继承自Framelayout,换言之其为整个ViewTree的根节点viewGroup
再看一下Phonewindow的类图
Activity的视图结构接下来我们来看一下单个Activity的viewTree的结构,我选择了两版sdk来查看
1)Android4.4系统的activity:
Activity的视图结构
2)Android6.0系统的activity:
Activity的视图结构
ViewTree的绘制
id为“content”的ContentFrameLayout是我们的布局文件加载显示的区域,更确切地说是我们activity的setcontentView()方法设置的视图显示的区域。下面我么就看看ContentFrameLayout中整个viewTree是如何绘制出来的。
在《图解Android:事件传递机制》中我们说过Android中的任何一个布局、任何一个控件包括我们自定义的控件其实都是直接或间接继承自View实现的,所以说这些View应该都具有相同的绘制流程与机制才能显示到屏幕上(可能每个控件的具体绘制逻辑有差异, 但是主流程都是一样的)。每一个View的绘制过程都必须经历三个最主要的过程,也就是measure()、layout()和draw()。
先看一下类图:
那么,整个Android的UI绘制机制是从哪里开始的即入口在哪里呢?答案就是ViewRootImpl类的performTraversals()方法。ViewRootImpl这个类是一个隐藏类,所以如果你是使用Eclipse开发的话可能看不到这个文件(AndroidStudio可以),没关系,根据路径(androidSDK\android-sdk-windows\sources\android-23\android\view\)去找到ViewRootImpl.Java文件,然后用文本阅读工具直接打开就好。
看一下官方对ViewRootImpl的介绍:
/*** The top of a view hierarchy, implementing the needed protocol between View* and the WindowManager. This is for the most part an internal implementation* detail of {@link WindowManagerGlobal}.** {@hide}*/
上面这段注释啥意思呢?说白了就是ViewRootImpl是一个window中的viewTree的入口,实现了window对viewTree管理的必需逻辑。
ViewRootImpl类performTraversals()代码,源代码长的恐怖,这里给大家过滤一下
private void performTraversals() { ...... //lp.width和lp.height在创建ViewGroup实例时值为MATCH_PARENT int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); ...... performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ...... performLayout(lp, desiredWindowWidth, desiredWindowHeight); ...... performDraw(); ...... } //执行rootView的测量 private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { //ViewGroup的measure()方法 mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } //执行layout操作 private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { mLayoutRequested = false; mScrollMayChange = true; mInLayout = true; final View host = mView; ...... try { //viewRoot先进行layout host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mInLayout = false; //需要layout的子view的数量 int numViewsRequestingLayout = mLayoutRequesters.size(); if (numViewsRequestingLayout > 0) { //需要layout的子view ArrayList validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, false); if (validLayoutRequesters != null) { //如果view中有调用requestLayout()方法,则说明界面需要刷新 mHandlingLayoutInLayoutRequest = true; int numValidRequests = validLayoutRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = validLayoutRequesters.get(i); view.requestLayout(); } //整个viewTree重新measure measureHierarchy(host, lp, mView.getContext().getResources(), desiredWindowWidth, desiredWindowHeight); mInLayout = true; //整个viewTree重新layout host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); mHandlingLayoutInLayoutRequest = false; // 再次检查是否有view需要刷新 validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true); if (validLayoutRequesters != null) { final ArrayList finalRequesters = validLayoutRequesters; // Post请求,在下一帧的显示的时候去执行刷新 getRunQueue().post(new Runnable() { @Override public void run() { int numValidRequests = finalRequesters.size(); for (int i = 0; i < numValidRequests; ++i) { final View view = finalRequesters.get(i); view.requestLayout(); } } }); } } } } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } mInLayout = false; } private void performDraw() { ...... try { draw(fullRedrawNeeded); } finally { mIsDrawing = false; Trace.traceEnd(Trace.TRACE_TAG_VIEW); } ...... } private void draw(boolean fullRedrawNeeded) { ...... if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) { //使用硬件渲染,比如GPU mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this); } else { //使用软件渲染 if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) { return; } } ...... } /** * @return true if drawing was successful, false if an error occurred */ private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty) { // Draw with software renderer. final Canvas canvas; ...... try { canvas.translate(-xoff, -yoff); if (mTranslator != null) { mTranslator.translateCanvas(canvas); } canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0); attachInfo.mSetIgnoreDirtyState = false; mView.draw(canvas); drawAccessibilityFocusedDrawableIfNeeded(canvas); } finally { if (!attachInfo.mSetIgnoreDirtyState) { attachInfo.mIgnoreDirtyState = false; } } ...... return true; }
measure相关
View类的UI绘制相关函数
//final说明该函数不允许被子类override,不需要关注细节 public final void measure(int widthMeasureSpec, int heightMeasureSpec) { ...... //widthMeasureSpec,heightMeasureSpec是由parent决定的 onMeasure(widthMeasureSpec, heightMeasureSpec); ...... } /** * * @param widthMeasureSpec horizontal space requirements as imposed by the parent. * * @param heightMeasureSpec vertical space requirements as imposed by the parent. * * @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) { //View类的默认实现,如果自定义view的话,需要我们自己override //child的宽高有来自parent的widthMeasureSpec、heightMeasureSpec和子的MeasureSpecMode共同决定 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }
- 注意onMeasure(int widthMeasureSpec, int heightMeasureSpec)入参的含义:
- widthMeasureSpec和heightMeasureSpec是parent暴露给child的尺寸
- widthMeasureSpec和heightMeasureSpec是32位的数值,其中高16位为MeasureSpecMode,低16位为MeasureSpecSize
- MeasureSpecMode有三种取值:
- MeasureSpec.EXACTLY:child为精准尺寸(layout_with=mach_parent、24dp的情况)
- MeasureSpec.AT_MOST:child为最大尺寸(layout_with=wrap_content的情况)
- MeasureSpec.UNSPECIFIED:child未指定尺寸
- child的尺寸有parent穿过来的widthMeasureSpec、heightMeasureSpec和子的MeasureSpecMode共同决定
layout相关
View的layout相关代码
//非final类型,子类可以重载public void layout(int l, int t, int r, int b) { ...... 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); ...... } ...... } //View的onlayout函数默认为空(如果自定义view中需要,可重载) protected void onLayout(boolean changed, int left, int top, int right, int bottom) { }
ViewGroup的layout相关代码
/** * {@inheritDoc} */ @Override public final void layout(int l, int t, int r, int b) { if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) { if (mTransition != null) { mTransition.layoutChange(this); } //view的layout方法 super.layout(l, t, r, b); } else { mLayoutCalledWhileSuppressed = true; } } /** * 抽象方法,子类必须实现(因为内部必然存在多个view控件,需要layout) */ @Override protected abstract void onLayout(boolean changed, int l, int t, int r, int b);
/** * ViewGroup的onLayout()方法都需要子类去实现 * 所以我们来看一下LinearLayout的实现 */ @Override protected void onLayout(boolean changed, int l, int t, int r, int b) { if (mOrientation == VERTICAL) { layoutVertical(); } else { layoutHorizontal(); } } //以垂直方向的布局为例 void layoutVertical() { ...... final int count = getVirtualChildCount(); ...... //遍历child 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(); ...... //递归child调用layout setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight); ...... } } } private void setChildFrame(View child, int left, int top, int width, int height) { child.layout(left, top, left + width, top + height); }
注意:
- 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。
- 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。
- 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。
- 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相关
View的draw相关代码
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); ...... }
注意:
- View的onDraw()方法为空,需要用户自己实现
- 关于draw,官方的注释已经很清楚,我们需要注意的是第四步:递归调用完成viewTree的绘制
- dispatchdraw()为空,需要在子类去实现
ViewGroup的draw相关代码
/** * 遍历各种类型的情况的child,并draw */ @Override protected void dispatchDraw(Canvas canvas) { boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode); final int childrenCount = mChildrenCount; final View[] children = mChildren; ...... for (int i = 0; i < childrenCount; i++) { ...... if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) { more |= drawChild(canvas, child, drawingTime); } } while (transientIndex >= 0) { ...... if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE || transientChild.getAnimation() != null) { more |= drawChild(canvas, transientChild, drawingTime); } ...... } if (mDisappearingChildren != null) { ...... for (int i = disappearingCount; i >= 0; i--) { final View child = disappearingChildren.get(i); more |= drawChild(canvas, child, drawingTime); } } ...... } protected boolean drawChild(Canvas canvas, View child, long drawingTime) { return child.draw(canvas, this, drawingTime); }
关于invalidate方法
invalidate系列方法请求重绘View树(也就是draw方法),如果View大小没有发生变化就不会调用layout过程,并且只绘制那 些“需要重绘的”View,也就是哪个View(View只绘制该View,ViewGroup绘制整个ViewGroup)请求invalidate系 列方法,就绘制该View。
有以下几种触发invalidate方法的情况:
- 直接调用invalidate方法:会绘制调用者本身。
- 触发setSelection方法:会绘制调用者本身。
- 触发setVisibility方法:当View可视状态在INVISIBLE转换VISIBLE时会间接调用invalidate方法,继而绘制该View。当View的可视状态在 INVISIBLE\VISIBLE 转换为GONE状态时会间接调用requestLayout和invalidate方法,同时由于View树大小发生了变化,所以会请求measure过 程以及draw过程,同样只绘制需要“重新绘制”的视图。
- 触发setEnabled方法:不会重新绘制任何View包括该调用者本身。
- 触发requestFocus方法:只绘制“需要重绘”的View。
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