提示:本文的源码均取自Android 7.0(API 24)
自定义View是Android进阶路线上必须攻克的难题,而在这之前就应该先对View的工作原理有一个系统的理解。本系列将分为4篇博客进行讲解,本文将主要对MeasureSpec、DecorView、ViewRootImpl等基础知识进行讲解。相关内容如下:
- Android View原理解析之基础知识(MeasureSpec、DecorView、ViewRootImpl)
- Android View原理解析之测量流程(measure)
- Android View原理解析之布局流程(layout)
- Android View原理解析之绘制流程(draw)
既然要探究View的工作原理,阅读源码自然是最好的手段了。其实在AndroidStudio中就可以很方便地查阅到Android系统的相关源码,但是在实际阅读过程中会发现很多类和方法都是被隐藏的(@hide注解
),严重影响学习热情。
Github上已经有开发者上传了去除@hide注解的源码jar包,我们只需要下载相应版本的Android.jar文件,并且替换本地SDK目录下相应版本的jar文件,就可以很清爽地阅读源码了。需要替换的路径为:
Github的地址如下:
https://github.com/anggrayudi/android-hidden-api
该解决方案的原文地址如下:
https://www.jianshu.com/p/7c1aca9f001b
注意:这种方式建议只在研究时使用,否则在开发过程中可能会在无意中引用@hide标记的API。
MeasureSpec可以翻译为测量规格,主要用于View的测量过程,封装着View的大小和测量模式(size和mode)。根据Android官方的注释,MeasureSpec表达的是父容器对子View的一种布局要求,可以简单理解为对子View大小的限制。不过MeasureSpec并非是由父容器独立决定的。实际上,父容器会通过自身的MeasureSpec结合子View的LayoutParams
,共同生成子View的MeasureSpec,这一点在后续文章中会详细讲到。
关于LayoutParams的知识可以参考这篇博客:
Android LayoutParams详解
为了减少测量过程中创建对象的开销,在实际使用中MeasureSpec以int(32位数)的形式存在,其高2位用于保存mode,其余位用于保存size。系统提供的MeasureSpec类是View的静态内部类,主要提供几种实用的静态方法,用于对int形式的MeasureSpec进行打包和解包,关键源码如下:
/**
* A MeasureSpec encapsulates the layout requirements passed from parent to child.
* Each MeasureSpec represents a requirement for either the width or the height.
* A MeasureSpec is comprised of a size and a mode. There are three possible modes.
*/
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
// 测量模式:父容器对子View的大小不作任何限制
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
// 测量模式:父容器已经为View指定了一个精确的大小
public static final int EXACTLY = 1 << MODE_SHIFT;
// 测量模式:父容器为View指定了一个最大的大小
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* 根据传入的size和mode创建一个MeasureSpec(打包)
*/
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
/**
* 根据传入的MeasureSpec获取mode(解包)
*/
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
/**
* 根据传入的MeasureSpec获取size(解包)
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
MeasureSpec在View的测量流程中有着很重要的作用,这里只是简单介绍一下,在后续的文章中会结合具体源码进一步讲解。
回想我们在Activity中经常使用的方法setContentView
,不知道大家有没有想过我们设置的View被添加到了哪里。由于在Android中只有ViewGroup的派生类才可以添加子View,那么可以自然地想到,整个视图树(ViewTree)的根节点必定是一个ViewGroup,而DecorView就是ViewTree最顶级的容器。
实际上,DecorView只不过是FrameLayout的一个子类。PhoneWindow
负责将其实例化,并根据当前Activity的风格特性(theme、feature、flag)为其添加不同的子布局。但是无论使用什么子布局,子布局中都会存在一个id为content
的FrameLayout。我们平时在Activity中使用setContentView
方法设置的View,就会被添加到这个名为content的FrameLayout中。这一过程发生在PhoneWindow#generateLayout
中,关键代码如下:
protected ViewGroup generateLayout(DecorView decor) {
..........
// Inflate the window decor.
// ① 根据Feature使用不同的布局资源文件
int layoutResource;
int features = getLocalFeatures();
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
setCloseOnSwipeEnabled(true);
} else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleIconsDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_title_icons;
}
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0
&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {
layoutResource = R.layout.screen_progress;
} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogCustomTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else {
layoutResource = R.layout.screen_custom_title;
}
removeFeature(FEATURE_ACTION_BAR);
} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title;
}
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
layoutResource = R.layout.screen_simple;
}
mDecor.startChanging();
// ② 在这个方法中解析布局资源文件(mDecor即DecorView)
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
// ③ 这里的contentParent就是指id为content的那个FrameLayout
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
..........
mDecor.finishChanging();
return contentParent;
}
可以看到,在代码①的位置根据不同的Feature为layoutResource赋值,其实就是在选择DecorView需要使用的子布局资源文件。在代码②的位置调用了mDecor的onResourcesLoaded
方法,这里的mDecor指的就是DecorView。在这里方法中会根据①中获取的布局资源进行解析,并将解析获得的View添加到DecorView中。
在代码③的位置根据ID_ANDROID_CONTENT
获取了名为contentParent
的ViewGroup,这其实就是setContentView
方法设置的View的父容器。在Activity中我们也可以通过android.R.id.content
获得这个ViewGroup。
上面说了这么多,其实就是想说明一件事,那就是DecorView是整个视图树的根容器。后续文章要讲到测量、布局、绘制流程,就是从DecorView开始向下传递的。
ViewRootImpl实现了ViewParent
接口(定义了View的父容器应该承担的职责),处于视图层级的顶点,实现了View和WindowManager之间必需的协议。抛开这些复杂定义,我们要知道的是ViewRootImpl将负责开启整个视图树的测量-布局-绘制流程。这一过程体现在ViewRootImpl的performTraversals
方法中,这个方法将依次调用performMeasure
、performLayout
和performDraw
方法完成上述流程,关键代码如下:
private void performTraversals() {
// mView是与ViewRootImpl绑定的DecorView
final View host = mView;
if (host == null || !mAdded)
return;
mIsInTraversal = true;
boolean newSurface = false;
WindowManager.LayoutParams lp = mWindowAttributes;
// 这里是屏幕的宽度和高度
int desiredWindowWidth;
int desiredWindowHeight;
........
if (mFirst || windowShouldResize || insetsChanged ||
viewVisibilityChanged || params != null || mForceNextWindowRelayout) {
........
if (!mStopped || mReportNextDraw) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
(relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
|| mHeight != host.getMeasuredHeight() || contentInsetsChanged ||updatedConfiguration) {
// ① 获取测量需要的MeasureSpec
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// ② 开启测量流程
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
// ③ 测量流程结束后就可以获得DecorView的宽高了
int width = host.getMeasuredWidth();
int height = host.getMeasuredHeight();
boolean measureAgain = false;
if (lp.horizontalWeight > 0.0f) {
width += (int) ((mWidth - width) * lp.horizontalWeight);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (lp.verticalWeight > 0.0f) {
height += (int) ((mHeight - height) * lp.verticalWeight);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (measureAgain) {
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
layoutRequested = true;
}
}
} else {
maybeHandleWindowMove(frame);
}
final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
if (didLayout) {
// ④ 开启布局流程
performLayout(lp, mWidth, mHeight);
// 计算透明区域
........
}
........
// Remember if we must report the next draw.
if ((relayoutResult & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
reportNextDraw();
}
boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;
if (!cancelDraw && !newSurface) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
// ⑤ 开启绘制流程
performDraw();
} else {
........
}
mIsInTraversal = false;
}
可以看到,首先在代码①的位置通过getRootMeasureSpec
方法获得了测量DecorView需要的MeasureSpec,这也是整个视图树(ViewTree)最初的MeasureSpec,关键代码如下:
/**
* Figures out the measure spec for the root view in a window based on it's
* 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 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;
}
在这个方法中将根据DecorView的布局参数生成不同的MeasureSpec,windowSize即屏幕的宽/高,dimension则是DecorView中LayoutParams的width/height。可以看到在布局参数为MATCH_PARENT或设置了具体宽/高(比如20dp这种形式)的情况下,生成的MeasureSpec都是使用EXACTLY模式(精确模式),否则使用AT_MOST
模式。
随后,在代码②的位置开启了测量流程,代码④的位置开启了布局流程,代码⑤的位置开启了绘制流程。
在performMeasure中将调用DecorView的measure方法进行测量:
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
try {
// 开始测量(mView就是DecorView)
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
......
}
}
在performLayout中将调用DecorView的layout方法进行布局,并传入测量完成后获得的宽高:
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mInLayout = true;
final View host = mView;
if (host == null) {
return;
}
try {
// 开始布局(host就是DecorView)
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
} finally {
......
}
mInLayout = false;
}
在performDraw方法中将调用ViewRootImpl的draw方法,之后又会调用drawSoftware
方法,最终将调用DecorView的draw方法开启绘制流程:
/**
* ViewRootImpl#performDraw
*/
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
} else if (mView == null) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded;
mFullRedrawNeeded = false;
mIsDrawing = true;
try {
// 开始绘制
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
}
......
}
/**
* ViewRootImpl#draw
*/
private void draw(boolean fullRedrawNeeded) {
......
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
/**
* ViewRootImpl#drawSoftware
*/
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;
// 调用DecorView的draw方法开始绘制
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
} finally {
......
}
return true;
}
上面讲了ViewRootImpl的作用,这里再提一下ViewRootImpl的生成过程:
首先是ActivityThread
类的handleResumeActivity
方法,在这里会调用WindowManager
的addView方法添加DecorView,关键代码如下:
ActivityThread#handleResumeActivity
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
ActivityClientRecord r = mActivities.get(token);
r = performResumeActivity(token, clearHide, reason);
.......
if (r != null) {
final Activity a = r.activity;
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
// 调用WindowManager的addView方法添加DecorView
wm.addView(decor, l);
} else {
.......
}
}
}
}
.......
}
上述代码中的wm是一个WindowManagerImpl
对象(实现WindowManager),它的addView方法如下:
WindowManagerImpl#addView
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
mGlobal是一个WindowManagerGlobal
对象,这里调用了它的addView方法,关键代码如下:
WindowManagerGlobal#addView
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
.......
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
.......
// ① 实例化一个ViewRootImpl
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
// do this last because it fires off messages to start doing things
try {
// ② 建立ViewRootImpl和DecorView的联系(view就是DecorView)
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
.......
}
}
}
在代码①的位置实例化了一个ViewRootImpl对象,然后在代码②的位置通过setView
方法建立了ViewRootImpl和DecorView的联系,这里的view其实就是DecorView。
本文简单讲解了在学习View的工作原理中需要知道的基础知识。如果看完后觉得不是很懂也没有关系,现在先对它们有个大致印象,然后结合后续的文章一起理解,效果可能会更好。
https://blog.csdn.net/a553181867/article/details/51477040
https://blog.csdn.net/singwhatiwanna/article/details/50775201