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自定义控件——onMeasure(转)
转自:http://blog.csdn.net/pi9nc/article/details/18764863
可以说重载onMeasure(),onLayout(),onDraw()三个函数构建了自定义View的外观形象。再加上onTouchEvent()等重载视图的行为,可以构建任何我们需要的可感知到的自定义View。
本节我们探索自定义View中onMeasure()起到了什么样的作用,题外要插的一句是,Activity框架,View框架中大量的on函数基本上都应用到了Template模式,掌握这一模式对于理解这些框架大有裨益。
我们知道,不管是自定义View还是系统提供的TextView这些,它们都必须放置在LinearLayout等一些ViewGroup中,因此理论上我们可以很好的理解onMeasure(),onLayout(),onDraw()这三个函数:1.View本身大小多少,这由onMeasure()决定;2.View在ViewGroup中的位置如何,这由onLayout()决定;3.绘制View,onDraw()定义了如何绘制这个View。
首先我们看看TextView.java中的onMeasure()源码:
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int widthMode = MeasureSpec.getMode(widthMeasureSpec); int heightMode = MeasureSpec.getMode(heightMeasureSpec); int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int width; int height; ... if (widthMode == MeasureSpec.EXACTLY) { // Parent has told us how big to be. So be it. width = widthSize; } else { if (mLayout != null && mEllipsize == null) { des = desired(mLayout); } ... setMeasuredDimension(width, height);
首先我们要理解的是widthMeasureSpec, heightMeasureSpec这两个参数是从哪里来的?onMeasure()函数由包含这个View的具体的ViewGroup调用,因此值也是从这个ViewGroup中传入的。这里我直接给出答案:子类View的这两个参数,由ViewGroup中的layout_width,layout_height和padding以及View自身的layout_margin共同决定。权值weight也是尤其需要考虑的因素,有它的存在情况可能会稍微复杂点。
了解了这两个参数的来源,还要知道这两个值的作用。我们只取heightMeasureSpec作说明。这个值由高32位和低16位组成,高32位保存的值叫specMode,可以通过如代码中所示的MeasureSpec.getMode()获取;低16位为specSize,同样可以由MeasureSpec.getSize()获取。那么specMode和specSize的作用有是什么呢?要想知道这一点,我们需要知道代码中的最后一行,所有的View的onMeasure()的最后一行都会调用setMeasureDimension()函数的作用——这个函数调用中传进去的值是View最终的视图大小。也就是说onMeasure()中之前所作的所有工作都是为了最后这一句话服务的。
我们知道在ViewGroup中,给View分配的空间大小并不是确定的,有可能随着具体的变化而变化,而这个变化的条件就是传到specMode中决定的,specMode一共有三种可能:
MeasureSpec.EXACTLY:父视图希望子视图的大小应该是specSize中指定的。
MeasureSpec.AT_MOST:子视图的大小最多是specSize中指定的值,也就是说不建议子视图的大小超过specSize中给定的值。
MeasureSpec.UNSPECIFIED:我们可以随意指定视图的大小。
由TextView中源码也可以知道这个值的设计意义是为了根据ViewGroup中具体能够提供的空间大小来指定子View的视图大小。
通过以上这些分析,可以知道视图最终的大小由父视图,子视图以及程序员根据需要决定,良好的设计一般会根据子视图的measureSpec设置合适的布局大小。
讲到上述这些内容,可能已经了解了如何去使用onMeasure来设置我们的视图的大小,但还有一个疑惑的地方,EXACTLY,AT_MOST,UNSPECIFIED和layout_是如何对应的呢?什么情况下对应什么值呢?
我们通过如下例子,稍作了解:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" > <LinearLayout android:layout_width="200dp" android:layout_height="wrap_content" android:paddingTop="20dp" android:layout_marginTop="30dp" android:background="@android:color/darker_gray" > <com.sean.myview.MyView android:layout_width="match_parent" android:layout_height="wrap_content" android:paddingTop="10dp" android:layout_marginTop="15dp" android:background="@android:color/holo_red_light" /> </LinearLayout> </LinearLayout>
效果图如下:
package com.sean.myview; import android.content.Context; import android.util.AttributeSet; import android.util.Log; import android.view.View; public class MyView extends View { public MyView(Context context, AttributeSet attrs) { super(context, attrs); // TODO Auto-generated constructor stub } @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // TODO Auto-generated method stub Log.d("MyView","------", new Throwable()); int speSize = MeasureSpec.getSize(heightMeasureSpec); int speMode = MeasureSpec.getMode(heightMeasureSpec); Log.d("MyView", "---speSize = " + speSize + ""); Log.d("MyView", "---speMode = " + speMode + ""); if(speMode == MeasureSpec.AT_MOST){ Log.d("MyView", "---AT_MOST---"); } if(speMode == MeasureSpec.EXACTLY){ Log.d("MyView", "---EXACTLY---"); } if(speMode == MeasureSpec.UNSPECIFIED){ Log.d("MyView", "---UNSPECIFIED---"); } setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec), speSize); } }
当前情况下打印出的log如下:
widthMeasureSpecD/MyView ( 3506): java.lang.ThrowableD/MyView ( 3506): at com.sean.myview.MyView.onMeasure(MyView.java:18)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)D/MyView ( 3506): at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1411)D/MyView ( 3506): at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1059)D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)D/MyView ( 3506): at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1411)D/MyView ( 3506): at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1059)D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:590)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)D/MyView ( 3506): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.widget.LinearLayout.measureVertical(LinearLayout.java:850)D/MyView ( 3506): at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4942)D/MyView ( 3506): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)D/MyView ( 3506): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2193)D/MyView ( 3506): at android.view.View.measure(View.java:15775)D/MyView ( 3506): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2212)D/MyView ( 3506): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1291)D/MyView ( 3506): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1486)D/MyView ( 3506): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1181)D/MyView ( 3506): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4942)D/MyView ( 3506): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:776)D/MyView ( 3506): at android.view.Choreographer.doCallbacks(Choreographer.java:579)D/MyView ( 3506): at android.view.Choreographer.doFrame(Choreographer.java:548)D/MyView ( 3506): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:762)D/MyView ( 3506): at android.os.Handler.handleCallback(Handler.java:800)D/MyView ( 3506): at android.os.Handler.dispatchMessage(Handler.java:100)D/MyView ( 3506): at android.os.Looper.loop(Looper.java:194)D/MyView ( 3506): at android.app.ActivityThread.main(ActivityThread.java:5391)D/MyView ( 3506): at java.lang.reflect.Method.invokeNative(Native Method)D/MyView ( 3506): at java.lang.reflect.Method.invoke(Method.java:525)D/MyView ( 3506): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)D/MyView ( 3506): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)D/MyView ( 3506): at dalvik.system.NativeStart.main(Native Method)D/MyView ( 3506): ---speSize = 940D/MyView ( 3506): ---speMode = -2147483648D/MyView ( 3506): ---AT_MOST---
查看onMeasure()的调用堆栈,然后查看源码,可以知道heightMeasureSpec和widthMeasureSpec的值在ViewRootImpl.java中初始化,而初始化又参考了view和LinearLayout中的属性参数的设置最终得到了heightMeasureSpec和widthMeasureSpec的值。
而specMode怎么对应呢?我先给出设置对应值的地方,以下代码皆出自ViewRootImpl.java
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);//这里是赋值的代码
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; }
从这里我们基本上可以看出了MATCH_PARENT对应于EXACTLY,WRAP_CONTENT对应于AT_MOST,其他情况也对应于EXACTLY,它和MATCH_PARENT的区别在于size值不一样。现在我们需要知道这个rootDimension即lp.height对应于什么。
private void performTraversals() { // cache mView since it is used so much below... ... WindowManager.LayoutParams lp = mWindowAttributes;
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) { synchronized (this) { if (mView == null) { mView = view; mViewLayoutDirectionInitial = mView.getRawLayoutDirection(); mFallbackEventHandler.setView(view); mWindowAttributes.copyFrom(attrs);
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) { synchronized (this) { if (mView == null) { mView = view; mViewLayoutDirectionInitial = mView.getRawLayoutDirection(); mFallbackEventHandler.setView(view); mWindowAttributes.copyFrom(attrs);
这个View就是ViewGroup中的各个子类视图,这里我们用实验说明,更改MyView中的android:layout_height的值为"match_parent"。log中输出变为了
D/MyView ( 4249): ---speSize = 940D/MyView ( 4249): ---speMode = 1073741824D/MyView ( 4249): ---EXACTLY---
而更改LinearLayout中的这个值对这里的specMode是没有影响的。
下面我们再来通过实验来验证specSize的值由那些属性决定:
<LinearLayout android:layout_width="200dp" android:layout_height="300dp" android:paddingTop="20dp" android:layout_marginTop="30dp" android:background="@android:color/darker_gray" > <com.sean.myview.MyView android:layout_width="match_parent" android:layout_height="match_parent" android:paddingTop="10dp" android:layout_marginTop="15dp" android:background="@android:color/holo_red_light" /> </LinearLayout>
D/MyView ( 4959): ---speSize = 530D/MyView ( 4959): ---speMode = 1073741824D/MyView ( 4959): ---EXACTLY---
先说明一点,xml中用的单位是dp,log中得到的单位是px,我所使用的机子屏幕密度为2.0,只需要进行简单的换算即可px = 2.0 * dp
我们可以通过控制变量法,逐一改变代码中的LinearLayout和MyView中的相关属性值,看看是哪些影响了specSize,这里我直接给出答案:
530 = 300 * 2.0 - 20 * 2.0 - 15 * 2.0
影响specSize height的因素为:父视图的layout_height和paddingTop以及自身的layout_marginTop。但是我们不要忘记有weight时的影响。
onMeasure实例分析
上面这个两个视图是Android API中没有给出来的但在来电接听和闹钟被使用到的一个widget视图——GlowPadView.java
我们通过源码来看看这个View的大小是怎么通过onMeasure来控制的。
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { final int minimumWidth = getSuggestedMinimumWidth(); final int minimumHeight = getSuggestedMinimumHeight(); int computedWidth = resolveMeasured(widthMeasureSpec, minimumWidth); int computedHeight = resolveMeasured(heightMeasureSpec, minimumHeight); ... setMeasuredDimension(computedWidth, computedHeight); }
@Override protected int getSuggestedMinimumWidth() { // View should be large enough to contain the background + handle and // target drawable on either edge. return (int) (Math.max(mOuterRing.getWidth(), 2 * mOuterRadius) + mMaxTargetWidth); }
private int resolveMeasured(int measureSpec, int desired) { int result = 0; int specSize = MeasureSpec.getSize(measureSpec); switch (MeasureSpec.getMode(measureSpec)) { case MeasureSpec.UNSPECIFIED: result = desired; break; case MeasureSpec.AT_MOST: result = Math.min(specSize, desired); break; case MeasureSpec.EXACTLY: default: result = specSize; } return result; }
二是match_parent,对应只有一个父视图的值,这也是符合要求的。
一个好的设计应该要考虑到各种情况下的使用,而一个可复用的框架设计更是如此,设计之前需要预想到各种可能的应用情况。