1.View的绘制流程分几步,从哪开始?哪个过程结束以后能看到view?
答:从ViewRoot的performTraversals开始,经过measure,layout,draw 三个流程。draw流程结束以后就可以在屏幕上看到view了。
2.view的测量宽高和实际宽高有区别吗?
答:基本上百分之99的情况下都是可以认为没有区别的。有两种情况,有区别。第一种 就是有的时候会因为某些原因 view会多次测量,那第一次测量的宽高 肯定和最后实际的宽高 是不一定相等的,但是在这种情况下
最后一次测量的宽高和实际宽高是一致的。此外,实际宽高是在layout流程里确定的,我们可以在layout流程里 将实际宽高写死 写成硬编码,这样测量的宽高和实际宽高就肯定不一样了,虽然这么做没有意义 而且也不好。
3.view的measureSpec 由谁决定?顶级view呢?
答:由view自己的layoutparams和父容器 一起决定自己的measureSpec。一旦确定了spec,onMeasure中就可以确定view的宽高了。
顶级view就稍微特殊一点,对于decorView的测量在ViewRootImpl的源码里。
1
2
3 //desire的这2个参数就代表屏幕的宽高,
4 childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
5 childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
6 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
7
8
9
10 //decorView的measureSpec就是在这里确定的,其实比普通view的measurespec要简单的多
11 //代码就不分析了 一目了然的东西
12 private static int getRootMeasureSpec(int windowSize, int rootDimension) {
13 int measureSpec;
14 switch (rootDimension) {
15
16 case ViewGroup.LayoutParams.MATCH_PARENT:
17 // Window can't resize. Force root view to be windowSize.
18 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
19 break;
20 case ViewGroup.LayoutParams.WRAP_CONTENT:
21 // Window can resize. Set max size for root view.
22 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
23 break;
24 default:
25 // Window wants to be an exact size. Force root view to be that size.
26 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
27 break;
28 }
29 return measureSpec;
30 }
4.对于普通view来说,他的measure过程中,与父view有关吗?如果有关,这个父view也就是viewgroup扮演了什么角色?
答:看源码:
1 //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。
2 //也就是下面这个measureChildWithMargins方法
3
4 protected void measureChildWithMargins(View child,
5 int parentWidthMeasureSpec, int widthUsed,
6 int parentHeightMeasureSpec, int heightUsed) {
7 //第一步 先取得子view的 layoutParams 参数值
8 final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
9
10 //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
11 //还用到了父view 也就是viewgroup自己的spec的值
12 final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
13 mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
14 + widthUsed, lp.width);
15 final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
16 mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
17 + heightUsed, lp.height);
18
19 child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
20 }
21
22
23
24
25
26
27 //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
28 //的meaurespec 同时还有view自己的params来确定 view自己的measureSpec。
29 //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
30 //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。
31
32 //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
33 //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。
34
35 //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,
36 //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
37
38 //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。
39
40
41 public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
42 int specMode = MeasureSpec.getMode(spec);
43 int specSize = MeasureSpec.getSize(spec);
44
45 int size = Math.max(0, specSize - padding);
46
47 int resultSize = 0;
48 int resultMode = 0;
49
50 switch (specMode) {
51 // Parent has imposed an exact size on us
52 case MeasureSpec.EXACTLY:
53 if (childDimension >= 0) {
54 resultSize = childDimension;
55 resultMode = MeasureSpec.EXACTLY;
56 } else if (childDimension == LayoutParams.MATCH_PARENT) {
57 // Child wants to be our size. So be it.
58 resultSize = size;
59 resultMode = MeasureSpec.EXACTLY;
60 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
61 // Child wants to determine its own size. It can't be
62 // bigger than us.
63 resultSize = size;
64 resultMode = MeasureSpec.AT_MOST;
65 }
66 break;
67
68 // Parent has imposed a maximum size on us
69 case MeasureSpec.AT_MOST:
70 if (childDimension >= 0) {
71 // Child wants a specific size... so be it
72 resultSize = childDimension;
73 resultMode = MeasureSpec.EXACTLY;
74 } else if (childDimension == LayoutParams.MATCH_PARENT) {
75 // Child wants to be our size, but our size is not fixed.
76 // Constrain child to not be bigger than us.
77 resultSize = size;
78 resultMode = MeasureSpec.AT_MOST;
79 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
80 // Child wants to determine its own size. It can't be
81 // bigger than us.
82 resultSize = size;
83 resultMode = MeasureSpec.AT_MOST;
84 }
85 break;
86
87 // Parent asked to see how big we want to be
88 case MeasureSpec.UNSPECIFIED:
89 if (childDimension >= 0) {
90 // Child wants a specific size... let him have it
91 resultSize = childDimension;
92 resultMode = MeasureSpec.EXACTLY;
93 } else if (childDimension == LayoutParams.MATCH_PARENT) {
94 // Child wants to be our size... find out how big it should
95 // be
96 resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
97 resultMode = MeasureSpec.UNSPECIFIED;
98 } else if (childDimension == LayoutParams.WRAP_CONTENT) {
99 // Child wants to determine its own size.... find out how
100 // big it should be
101 resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
102 resultMode = MeasureSpec.UNSPECIFIED;
103 }
104 break;
105 }
106 return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
107 }
View Code
5.view的meaure和onMeasure有什么关系?
答:看源码:
1 //view的measure是final 方法 我们子类无法修改的。
2 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
3 boolean optical = isLayoutModeOptical(this);
4 if (optical != isLayoutModeOptical(mParent)) {
5 Insets insets = getOpticalInsets();
6 int oWidth = insets.left + insets.right;
7 int oHeight = insets.top + insets.bottom;
8 widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
9 heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
10 }
11
12 // Suppress sign extension for the low bytes
13 long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
14 if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
15
16 if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
17 widthMeasureSpec != mOldWidthMeasureSpec ||
18 heightMeasureSpec != mOldHeightMeasureSpec) {
19
20 // first clears the measured dimension flag
21 mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
22
23 resolveRtlPropertiesIfNeeded();
24
25 int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
26 mMeasureCache.indexOfKey(key);
27 if (cacheIndex < 0 || sIgnoreMeasureCache) {
28 // measure ourselves, this should set the measured dimension flag back
29 onMeasure(widthMeasureSpec, heightMeasureSpec);
30 mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
31 } else {
32 long value = mMeasureCache.valueAt(cacheIndex);
33 // Casting a long to int drops the high 32 bits, no mask needed
34 setMeasuredDimensionRaw((int) (value >> 32), (int) value);
35 mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
36 }
37
38 // flag not set, setMeasuredDimension() was not invoked, we raise
39 // an exception to warn the developer
40 if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
41 throw new IllegalStateException("View with id " + getId() + ": "
42 + getClass().getName() + "#onMeasure() did not set the"
43 + " measured dimension by calling"
44 + " setMeasuredDimension()");
45 }
46
47 mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
48 }
49
50 mOldWidthMeasureSpec = widthMeasureSpec;
51 mOldHeightMeasureSpec = heightMeasureSpec;
52
53 mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
54 (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
55 }
56
57 //不过可以看到的是在measure方法里调用了onMeasure方法
58 //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!
59 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
60 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
61 getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
62 }
View Code
6.简要分析view的measure流程?
答:先回顾问题4,viewgroup 算出子view的spec以后 会调用子view的measure方法,而子view的measure方法 我们问题5也看过了实际上是调用的onMeasure方法
所以我们只要分析好onMeasure方法即可,注意onMeasure方法的参数 正是他的父view算出来的那2个spec的值(这里view的measure方法会把这个spec里的specSize值做略微的修改 这个部分 不做分析 因为measure方法修改specSize的部分很简单)。
1 //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
2 //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
3 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
4 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
5 getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
6 }
7
8
9 //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
10 //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
11 public static int getDefaultSize(int size, int measureSpec) {
12 int result = size;
13 int specMode = MeasureSpec.getMode(measureSpec);
14 int specSize = MeasureSpec.getSize(measureSpec);
15
16 switch (specMode) {
17 case MeasureSpec.UNSPECIFIED:
18 result = size;
19 break;
20 case MeasureSpec.AT_MOST:
21 case MeasureSpec.EXACTLY:
22 result = specSize;
23 break;
24 }
25 return result;
26 }
27
28 //跟view的背景相关 这里不多做分析了
29 protected int getSuggestedMinimumWidth() {
30 return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
31 }
View Code
7.自定义view中 如果onMeasure方法 没有对wrap_content 做处理 会发生什么?为什么?怎么解决?
答:如果没有对wrap_content做处理 ,那即使你在xml里设置为wrap_content.其效果也和match_parent相同。看问题4的分析。我们可以知道view自己的layout为wrap,那mode就是at_most(不管父亲view是什么specmode).
这种模式下宽高就是等于specSize(getDefaultSize函数分析可知),而这里的specSize显然就是parentSize的大小。也就是父容器剩余的大小。那不就和我们直接设置成match_parent是一样的效果了么?
解决方式就是在onMeasure里 针对wrap 来做特殊处理 比如指定一个默认的宽高,当发现是wrap_content 就设置这个默认宽高即可。
8.ViewGroup有onMeasure方法吗?为什么?
答:没有,这个方法是交给子类自己实现的。不同的viewgroup子类 肯定布局都不一样,那onMeasure索性就全部交给他们自己实现好了。
9.为什么在activity的生命周期里无法获得测量宽高?有什么方法可以解决这个问题吗?
答:因为measure的过程和activity的生命周期 没有任何关系。你无法确定在哪个生命周期执行完毕以后 view的measure过程一定走完。可以尝试如下几种方法 获取view的测量宽高。
1 //重写activity的这个方法
2 public void onWindowFocusChanged(boolean hasFocus) {
3 super.onWindowFocusChanged(hasFocus);
4 if (hasFocus) {
5 int width = tv.getMeasuredWidth();
6 int height = tv.getMeasuredHeight();
7 Log.v("burning", "width==" + width);
8 Log.v("burning", "height==" + height);
9
10 }
11 }
View Code
或者重写这个方法
1 @Override
2 protected void onStart() {
3 super.onStart();
4 tv.post(new Runnable() {
5 @Override
6 public void run() {
7 int width = tv.getMeasuredWidth();
8 int height = tv.getMeasuredHeight();
9 }
10 });
11 }
View Code
再或者:
1 @Override
2 protected void onStart() {
3 super.onStart();
4 ViewTreeObserver observer = tv.getViewTreeObserver();
5 observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
6 @Override
7 public void onGlobalLayout() {
8 int width = tv.getMeasuredWidth();
9 int height = tv.getMeasuredHeight();
10 tv.getViewTreeObserver().removeOnGlobalLayoutListener(this);
11 }
12 });
13 }
View Code
10.layout和onLayout方法有什么区别?
答:layout是确定本身view的位置 而onLayout是确定所有子元素的位置。layout里面 就是通过serFrame方法设设定本身view的 四个顶点的位置。这4个位置以确定 自己view的位置就固定了
然后就调用onLayout来确定子元素的位置。view和viewgroup的onlayout方法都没有写。都留给我们自己给子元素布局
11.draw方法 大概有几个步骤?
答: 一共是4个步骤, 绘制背景---------绘制自己--------绘制chrildren----绘制装饰。
12.setWillNotDraw方法有什么用?
答:这个方法在view里。
1 /**
2 * If this view doesn't do any drawing on its own, set this flag to
3 * allow further optimizations. By default, this flag is not set on
4 * View, but could be set on some View subclasses such as ViewGroup.
5 *
6 * Typically, if you override {@link #onDraw(android.graphics.Canvas)}
7 * you should clear this flag.
8 *
9 * @param willNotDraw whether or not this View draw on its own
10 */
11 public void setWillNotDraw(boolean willNotDraw) {
12 setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
13 }
View Code
用于设置标志位的 也就是说 如果你的自定义view 不需要draw的话,就可以设置这个方法为true。这样系统知道你这个view 不需要draw 可以优化执行速度。viewgroup 一般都默认设置这个为true,因为viewgroup多数都是只负责布局
不负责draw的。而view 这个标志位 默认一般都是关闭的。
13.自定义view 有哪些需要注意的点?
答:主要是要处理wrap_content 和padding。否则xml 那边设置这2个属性就根本没用了。还有不要在view中使用handler 因为人家已经提供了post方法。如果是继承自viewGroup,那在onMeasure和onLayout里面 也要考虑
padding和layout的影响。也就是说specSize 要算一下 。最后就是如果view的动画或者线程需要停止,可以考虑在onDetachedFromWindow里面来做。
针对上述的几点,给出几个简单的自定义view 供大家理解。
给出一个圆形的view 范例:
1 package com.example.administrator.motioneventtest;
2
3 import android.content.Context;
4 import android.graphics.Canvas;
5 import android.graphics.Color;
6 import android.graphics.Paint;
7 import android.util.AttributeSet;
8 import android.view.View;
9
10 /**
11 * Created by Administrator on 2016/2/4.
12 */
13 public class CircleView extends View {
14
15 private int mColor = Color.RED;
16 private Paint mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
17
18 private void init() {
19 mPaint.setColor(mColor);
20 }
21
22 @Override
23 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
24 super.onMeasure(widthMeasureSpec, heightMeasureSpec);
25 int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
26 int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
27 int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
28 int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
29
30 //处理为wrap_content时的情况
31 if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
32 setMeasuredDimension(200, 200);
33 } else if (widthSpecMode == MeasureSpec.AT_MOST) {
34 setMeasuredDimension(200, heightSpecSize);
35 } else if (heightSpecMode == MeasureSpec.AT_MOST) {
36 setMeasuredDimension(widthSpecSize, 200);
37 }
38
39 }
40
41 @Override
42 protected void onDraw(Canvas canvas) {
43 super.onDraw(canvas);
44 //处理padding的情况
45 final int paddingLeft = getPaddingLeft();
46 final int paddingRight = getPaddingRight();
47 final int paddingTop = getPaddingTop();
48 final int paddingBottom = getPaddingBottom();
49
50
51 int width = getWidth() - paddingLeft - paddingRight;
52 int height = getHeight() - paddingTop - paddingBottom;
53 int radius = Math.min(width, height) / 2;
54 canvas.drawCircle(paddingLeft + width / 2, paddingTop + height / 2, radius, mPaint);
55 }
56
57 public CircleView(Context context, AttributeSet attrs, int defStyleAttr) {
58 super(context, attrs, defStyleAttr);
59 init();
60 }
61
62 public CircleView(Context context) {
63 super(context);
64 init();
65
66 }
67
68 public CircleView(Context context, AttributeSet attrs) {
69 super(context, attrs);
70 init();
71 }
72 }
View Code
然后下面再给出一个范例,稍微复杂一点是自定义viewgroup了(主要是加强对onMeasure和onLayout的理解), 需求如下:
一个水平的viewgroup,内部的子元素 为了简单 我们假定他们的宽高都是一样的。来写一个这样的简单的viewgroup。
1 package com.example.administrator.motioneventtest;
2
3 import android.content.Context;
4 import android.util.AttributeSet;
5 import android.util.Log;
6 import android.view.View;
7 import android.view.ViewGroup;
8
9 /**
10 * Created by Administrator on 2016/2/4.
11 */
12 //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
13 //就留给读者自己完成了
14 public class CustomHorizontalLayout extends ViewGroup {
15
16 //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
17 final int minHeight = 0;
18 final int minWidth = 0;
19
20
21 public CustomHorizontalLayout(Context context) {
22 super(context);
23 }
24
25 public CustomHorizontalLayout(Context context, AttributeSet attrs) {
26 super(context, attrs);
27 }
28
29 public CustomHorizontalLayout(Context context, AttributeSet attrs, int defStyleAttr) {
30 super(context, attrs, defStyleAttr);
31 }
32
33 @Override
34 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
35 super.onMeasure(widthMeasureSpec, heightMeasureSpec);
36 int measureWidth = 0;
37 int measureHeight = 0;
38 final int childCount = getChildCount();
39 measureChildren(widthMeasureSpec, heightMeasureSpec);
40 int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
41 int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
42 int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
43 int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
44 final View childView = getChildAt(0);
45 final int paddingLeft = getPaddingLeft();
46 final int paddingRight = getPaddingRight();
47 final int paddingTop = getPaddingTop();
48 final int paddingBottom = getPaddingBottom();
49 //没有子控件 时 我们的宽高要作特殊处理
50 if (childCount == 0) {
51 //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
52 //这里我们最小设置为0
53 if (widthSpecMode == MeasureSpec.AT_MOST || heightSpecMode == MeasureSpec.AT_MOST) {
54 setMeasuredDimension(minWidth, minHeight);
55 } else {
56 //否则根据我们的layout属性来
57 setMeasuredDimension(getLayoutParams().width, getLayoutParams().height);
58 }
59
60 } else if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
61 measureWidth = childView.getMeasuredWidth() * childCount;
62 measureHeight = childView.getMeasuredHeight();
63 setMeasuredDimension(paddingLeft + measureWidth + paddingRight, paddingTop + measureHeight + paddingBottom);
64 } else if (heightSpecMode == MeasureSpec.AT_MOST) {
65 measureHeight = childView.getMeasuredHeight();
66 setMeasuredDimension(paddingLeft + paddingRight + widthSpecSize, paddingTop + paddingBottom + measureHeight);
67 } else if (widthSpecMode == MeasureSpec.AT_MOST) {
68 measureWidth = childView.getMeasuredWidth() * childCount;
69 setMeasuredDimension(paddingLeft + paddingRight + measureWidth, paddingTop + paddingBottom + heightSpecSize);
70 }
71 }
72
73 @Override
74 protected void onLayout(boolean changed, int l, int t, int r, int b) {
75 final int paddingLeft = getPaddingLeft();
76 final int paddingRight = getPaddingRight();
77 final int paddingTop = getPaddingTop();
78 final int paddingBottom = getPaddingBottom();
79 //左边初始位置为0
80 int childLeft = 0 + paddingLeft;
81 final int childCount = getChildCount();
82 for (int i = 0; i < childCount; i++) {
83 final View childView = getChildAt(i);
84 if (childView.getVisibility() != View.GONE) {
85 final int childWidth = childView.getMeasuredWidth();
86 childView.layout(childLeft, 0 + paddingTop, childLeft + childWidth, paddingTop + childView.getMeasuredHeight());
87 childLeft += childWidth;
88 }
89 }
90 }
91 }
View Code