Android的View绘制是从根节点（Activity是DecorView）开始，他是一个自上而下的过程。View的绘制经历三个过程：Measure、Layout、Draw。基本流程如下图：
performTraversals函数，具体的可以参考一下源代码：
1 private void performTraversals() {
2 final View host = mView;
3 ...
4 host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
5 ...
6 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
7 ...
8 draw(fullRedrawNeeded);
9 }
1、Measure过程
Measure过程是计算视图大小，View中视图measure过程相关的方法主要有三个：
1 public final void measure(int widthMeasureSpec, int heightMeasureSpec) 2 protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) 3 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
measure调用onMeasure，onMeasure调用setMeasureDimension，measure，setMeasureDimension是final类型，view的子类不需要重写，onMeasure在view的子类中重写。
measure函数：
1 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
2 if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
3 widthMeasureSpec != mOldWidthMeasureSpec ||
4 heightMeasureSpec != mOldHeightMeasureSpec) {
5
6 // first clears the measured dimension flag
7 mPrivateFlags &= ~MEASURED_DIMENSION_SET;
8
9 if (ViewDebug.TRACE_HIERARCHY) {
10 ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);
11 }
12
13 // measure ourselves, this should set the measured dimension flag back
14 onMeasure(widthMeasureSpec, heightMeasureSpec);
15
16 // flag not set, setMeasuredDimension() was not invoked, we raise
17 // an exception to warn the developer
18 if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {
19 throw new IllegalStateException("onMeasure() did not set the"
20 + " measured dimension by calling"
21 + " setMeasuredDimension()");
22 }
23
24 mPrivateFlags |= LAYOUT_REQUIRED;
25 }
26
27 mOldWidthMeasureSpec = widthMeasureSpec;
28 mOldHeightMeasureSpec = heightMeasureSpec;
29 }
onMeasure函数：
1 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
2 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
3 getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
4 }
重写onMeasure时，要调用setMeasuredDimension或者super.onMeasure来设置自身的mMeasuredWidth和mMeasuredHeight，否则，就会抛出异常.
setMeasuredDimension函数，用来设置view的大小：
1 protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
2 mMeasuredWidth = measuredWidth;
3 mMeasuredHeight = measuredHeight;
4
5 mPrivateFlags |= MEASURED_DIMENSION_SET;
6 }
再看一下onMeasure的getDefaultSize函数：
1 public static int getDefaultSize(int size, int measureSpec) {
2 int result = size;
3 int specMode = MeasureSpec.getMode(measureSpec);
4 int specSize = MeasureSpec.getSize(measureSpec);
5
6 switch (specMode) {
7 case MeasureSpec.UNSPECIFIED:
8 result = size;
9 break;
10 case MeasureSpec.AT_MOST:
11 case MeasureSpec.EXACTLY:
12 result = specSize;
13 break;
14 }
15 return result;
16 }
这里用引入了MeasureSpec类：
1 public static class MeasureSpec {
2
3 private static final int MODE_SHIFT = 30;
4 private static final int MODE_MASK = 0x3 << MODE_SHIFT;
5 public static final int UNSPECIFIED = 0 << MODE_SHIFT;
6 public static final int EXACTLY = 1 << MODE_SHIFT;
7 public static final int AT_MOST = 2 << MODE_SHIFT;
8
9 public static int makeMeasureSpec(int size, int mode) {
10 return size + mode;
11 }
12
13 public static int getMode(int measureSpec) {
14 return (measureSpec & MODE_MASK);
15 }
16
17 public static int getSize(int measureSpec) {
18 return (measureSpec & ~MODE_MASK);
19 }
20 }
MODE_MASK为30为长度的二进制数，前两位标示Mode，后面的标示Size。MeasureSpec有三种模式分别是UNSPECIFIED, EXACTLY和AT_MOST。
EXACTLY表示父视图希望子视图的大小应该是由specSize的值来决定的，系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。
AT_MOST表示子视图最多只能是specSize中指定的大小，开发人员应该尽可能小得去设置这个视图，并且保证不会超过specSize。系统默认会按照这个规则来设置子视图的大小，开发人员当然也可以按照自己的意愿设置成任意的大小。
UNSPECIFIED表示开发人员可以将视图按照自己的意愿设置成任意的大小，没有任何限制。这种情况比较少见，不太会用到。
widthMeasureSpec和heightMeasureSpec决定了Mode和Size的值，widthMeasureSpec和heightMeasureSpec来自父视图，这两个值都是由父视图经过计算后传递给子视图的，说明父视图会在一定程度上决定子视图的大小。但是最外层的根视图，它的widthMeasureSpec和heightMeasureSpec又是从哪里得到的呢？这就需要去分析ViewRoot中的源码了，观察performTraversals()方法可以发现如下代码：
1 childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width); 2 childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
可以看到，这里调用了getRootMeasureSpec()方法去获取widthMeasureSpec和heightMeasureSpec的值，注意方法中传入的参数，其中lp.width和lp.height在创建ViewGroup实例的时候就被赋值了，它们都等于MATCH_PARENT。然后看下getRootMeasureSpec()方法中的代码，如下所示：
1 private int getRootMeasureSpec(int windowSize, int rootDimension) {
2 int measureSpec;
3 switch (rootDimension) {
4 case ViewGroup.LayoutParams.MATCH_PARENT:
5 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
6 break;
7 case ViewGroup.LayoutParams.WRAP_CONTENT:
8 measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
9 break;
10 default:
11 measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
12 break;
13 }
14 return measureSpec;
15 }
可以看到，这里使用了MeasureSpec.makeMeasureSpec()方法来组装一个MeasureSpec，当rootDimension参数等于MATCH_PARENT的时候，MeasureSpec的specMode就等于EXACTLY，当rootDimension等于WRAP_CONTENT的时候，MeasureSpec的specMode就等于AT_MOST。并且MATCH_PARENT和WRAP_CONTENT时的specSize都是等于windowSize的，也就意味着根视图总是会充满全屏的。
Measure是一个复杂的过程，因为一个布局中一般都会包含多个子视图，每个视图都需要经历一次measure过程。ViewGroup中定义了一个measureChildren()方法来去测量子视图的大小，如下所示：
1 protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
2 final int size = mChildrenCount;
3 final View[] children = mChildren;
4 for (int i = 0; i < size; ++i) {
5 final View child = children[i];
6 if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
7 measureChild(child, widthMeasureSpec, heightMeasureSpec);
8 }
9 }
10 }
这里会去遍历当前布局下的所有子视图，然后逐个调用measureChild()方法来测量相应子视图的大小：
1 protected void measureChild(View child, int parentWidthMeasureSpec,
2 int parentHeightMeasureSpec) {
3 final LayoutParams lp = child.getLayoutParams();
4
5 final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
6 mPaddingLeft + mPaddingRight, lp.width);
7 final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
8 mPaddingTop + mPaddingBottom, lp.height);
9
10 child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
11 }
从这里我们可以看到视图的大小是由父视图和子视图共同决定的。子布局里面的android:layout_width和android:layout_height只是期望值，父View大小最终是由DecorView决定。父视图提供尺寸大小的一个能力，子视图最终尺寸与父视图能力、子视图期望的关系如下：
|
父视图能力尺寸 |
子视图期望尺寸 |
子视图最终允许尺寸 |
|
EXACTLY + Size1 |
EXACTLY + Size2 |
EXACTLY + Size2 |
|
EXACTLY + Size1 |
fill_parent/match_parent |
EXACTLY+Size1 |
|
EXACTLY + Size1 |
wrap_content |
AT_MOST+Size1 |
|
AT_MOST+Size1 |
EXACTLY + Size2 |
EXACTLY+Size2 |
|
AT_MOST+Size1 |
fill_parent/match_parent |
AT_MOST+Size1 |
|
AT_MOST+Size1 |
wrap_content |
AT_MOST+Size1 |
|
UNSPECIFIED+Size1 |
EXACTLY + Size2 |
EXACTLY + Size2 |
|
UNSPECIFIED+Size1 |
fill_parent/match_parent |
UNSPECIFIED+0 |
|
UNSPECIFIED+Size1 |
wrap_content |
UNSPECIFIED+0 |
关于视图的measure过程可以阅读以下LinearLayout源码，这样可以更清楚的了解过程。
2、Layout过程
measure过程确定视图的大小，而layout过程确定视图的位置。loyout是从view的layout方法开始的：
1 public void layout(int l, int t, int r, int b) {
2 int oldL = mLeft;
3 int oldT = mTop;
4 int oldB = mBottom;
5 int oldR = mRight;
6 boolean changed = setFrame(l, t, r, b);
7 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
8 onLayout(changed, l, t, r, b);
9 mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
10
11 ListenerInfo li = mListenerInfo;
12 if (li != null && li.mOnLayoutChangeListeners != null) {
13 ArrayList<OnLayoutChangeListener> listenersCopy =
14 (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
15 int numListeners = listenersCopy.size();
16 for (int i = 0; i < numListeners; ++i) {
17 listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
18 }
19 }
20 }
21 mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
22 }
函数中参数l、t、r、b是指view的左、上、右、底的位置，这几个参数是父视图传入的，而根视图中参数是由performTraversals()方法传入的。
1 host.layout(0, 0, host.mMeasuredWidth, host.mMeasuredHeight);
layout中调用了onLayout方法，在view中onLayout方法是一个空函数，他需要其子类实现。
1 protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
2 }
我们关注一下LinearLayout：
1 @Override
2 protected void onLayout(boolean changed, int l, int t, int r, int b) {
3 if (mOrientation == VERTICAL) {
4 layoutVertical();
5 } else {
6 layoutHorizontal();
7 }
8 }
1 void layoutVertical() {
2 final int paddingLeft = mPaddingLeft;
3
4 int childTop;
5 int childLeft;
6
7 // Where right end of child should go
8 final int width = mRight - mLeft;
9 int childRight = width - mPaddingRight;
10
11 // Space available for child
12 int childSpace = width - paddingLeft - mPaddingRight;
13
14 final int count = getVirtualChildCount();
15
16 final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
17 final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
18
19 switch (majorGravity) {
20 case Gravity.BOTTOM:
21 // mTotalLength contains the padding already
22 childTop = mPaddingTop + mBottom - mTop - mTotalLength;
23 break;
24
25 // mTotalLength contains the padding already
26 case Gravity.CENTER_VERTICAL:
27 childTop = mPaddingTop + (mBottom - mTop - mTotalLength) / 2;
28 break;
29
30 case Gravity.TOP:
31 default:
32 childTop = mPaddingTop;
33 break;
34 }
35
36 for (int i = 0; i < count; i++) {
37 final View child = getVirtualChildAt(i);
38 if (child == null) {
39 childTop += measureNullChild(i);
40 } else if (child.getVisibility() != GONE) {
41 final int childWidth = child.getMeasuredWidth();
42 final int childHeight = child.getMeasuredHeight();
43
44 final LinearLayout.LayoutParams lp =
45 (LinearLayout.LayoutParams) child.getLayoutParams();
46
47 int gravity = lp.gravity;
48 if (gravity < 0) {
49 gravity = minorGravity;
50 }
51 final int layoutDirection = getLayoutDirection();
52 final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
53 switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
54 case Gravity.CENTER_HORIZONTAL:
55 childLeft = paddingLeft + ((childSpace - childWidth) / 2)
56 + lp.leftMargin - lp.rightMargin;
57 break;
58
59 case Gravity.RIGHT:
60 childLeft = childRight - childWidth - lp.rightMargin;
61 break;
62
63 case Gravity.LEFT:
64 default:
65 childLeft = paddingLeft + lp.leftMargin;
66 break;
67 }
68
69 if (hasDividerBeforeChildAt(i)) {
70 childTop += mDividerHeight;
71 }
72
73 childTop += lp.topMargin;
74 setChildFrame(child, childLeft, childTop + getLocationOffset(child),
75 childWidth, childHeight);
76 childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
77
78 i += getChildrenSkipCount(child, i);
79 }
80 }
81 }
layout设置了view的位置，还设置了子视图位置，layoutHorizontal()方法中调用了setChildFrame方法：
1 private void setChildFrame(View child, int left, int top, int width, int height) {
2 child.layout(left, top, left + width, top + height);
3 }
4
从上面看出，layout也是一个自上而下的过程，先设置父视图位置，在循环子视图，父视图位置一定程度上决定了子视图位置。
3、Draw过程
draw过程调用顺序在measure()和layout()之后，同样的，performTraversals()发起的draw过程最终会调用到mView的draw()函数，这里的mView对于Activity来说就是PhoneWindow.DecorView。看一下view类的draw方法：
1 public void draw(Canvas canvas) {
2 final int privateFlags = mPrivateFlags;
3 final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
4 (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
5 mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
6
7 /*
8 * Draw traversal performs several drawing steps which must be executed
9 * in the appropriate order:
10 *
11 * 1. Draw the background
12 * 2. If necessary, save the canvas‘ layers to prepare for fading
13 * 3. Draw view‘s content
14 * 4. Draw children
15 * 5. If necessary, draw the fading edges and restore layers
16 * 6. Draw decorations (scrollbars for instance)
17 */
18
19 // Step 1, draw the background, if needed
20 int saveCount;
21
22 if (!dirtyOpaque) {
23 final Drawable background = mBackground;
24 if (background != null) {
25 final int scrollX = mScrollX;
26 final int scrollY = mScrollY;
27
28 if (mBackgroundSizeChanged) {
29 background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
30 mBackgroundSizeChanged = false;
31 }
32
33 if ((scrollX | scrollY) == 0) {
34 background.draw(canvas);
35 } else {
36 canvas.translate(scrollX, scrollY);
37 background.draw(canvas);
38 canvas.translate(-scrollX, -scrollY);
39 }
40 }
41 }
42
43 // skip step 2 & 5 if possible (common case)
44 final int viewFlags = mViewFlags;
45 boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
46 boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
47 if (!verticalEdges && !horizontalEdges) {
48 // Step 3, draw the content
49 if (!dirtyOpaque) onDraw(canvas);
50
51 // Step 4, draw the children
52 dispatchDraw(canvas);
53
54 // Step 6, draw decorations (scrollbars)
55 onDrawScrollBars(canvas);
56
57 // we‘re done...
58 return;
59 }
60
61 /*
62 * Here we do the full fledged routine...
63 * (this is an uncommon case where speed matters less,
64 * this is why we repeat some of the tests that have been
65 * done above)
66 */
67
68 boolean drawTop = false;
69 boolean drawBottom = false;
70 boolean drawLeft = false;
71 boolean drawRight = false;
72
73 float topFadeStrength = 0.0f;
74 float bottomFadeStrength = 0.0f;
75 float leftFadeStrength = 0.0f;
76 float rightFadeStrength = 0.0f;
77
78 // Step 2, save the canvas‘ layers
79 int paddingLeft = mPaddingLeft;
80
81 final boolean offsetRequired = isPaddingOffsetRequired();
82 if (offsetRequired) {
83 paddingLeft += getLeftPaddingOffset();
84 }
85
86 int left = mScrollX + paddingLeft;
87 int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
88 int top = mScrollY + getFadeTop(offsetRequired);
89 int bottom = top + getFadeHeight(offsetRequired);
90
91 if (offsetRequired) {
92 right += getRightPaddingOffset();
93 bottom += getBottomPaddingOffset();
94 }
95
96 final ScrollabilityCache scrollabilityCache = mScrollCache;
97 final float fadeHeight = scrollabilityCache.fadingEdgeLength;
98 int length = (int) fadeHeight;
99
100 // clip the fade length if top and bottom fades overlap
101 // overlapping fades produce odd-looking artifacts
102 if (verticalEdges && (top + length > bottom - length)) {
103 length = (bottom - top) / 2;
104 }
105
106 // also clip horizontal fades if necessary
107 if (horizontalEdges && (left + length > right - length)) {
108 length = (right - left) / 2;
109 }
110
111 if (verticalEdges) {
112 topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
113 drawTop = topFadeStrength * fadeHeight > 1.0f;
114 bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
115 drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
116 }
117
118 if (horizontalEdges) {
119 leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
120 drawLeft = leftFadeStrength * fadeHeight > 1.0f;
121 rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
122 drawRight = rightFadeStrength * fadeHeight > 1.0f;
123 }
124
125 saveCount = canvas.getSaveCount();
126
127 int solidColor = getSolidColor();
128 if (solidColor == 0) {
129 final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
130
131 if (drawTop) {
132 canvas.saveLayer(left, top, right, top + length, null, flags);
133 }
134
135 if (drawBottom) {
136 canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
137 }
138
139 if (drawLeft) {
140 canvas.saveLayer(left, top, left + length, bottom, null, flags);
141 }
142
143 if (drawRight) {
144 canvas.saveLayer(right - length, top, right, bottom, null, flags);
145 }
146 } else {
147 scrollabilityCache.setFadeColor(solidColor);
148 }
149
150 // Step 3, draw the content
151 if (!dirtyOpaque) onDraw(canvas);
152
153 // Step 4, draw the children
154 dispatchDraw(canvas);
155
156 // Step 5, draw the fade effect and restore layers
157 final Paint p = scrollabilityCache.paint;
158 final Matrix matrix = scrollabilityCache.matrix;
159 final Shader fade = scrollabilityCache.shader;
160
161 if (drawTop) {
162 matrix.setScale(1, fadeHeight * topFadeStrength);
163 matrix.postTranslate(left, top);
164 fade.setLocalMatrix(matrix);
165 canvas.drawRect(left, top, right, top + length, p);
166 }
167
168 if (drawBottom) {
169 matrix.setScale(1, fadeHeight * bottomFadeStrength);
170 matrix.postRotate(180);
171 matrix.postTranslate(left, bottom);
172 fade.setLocalMatrix(matrix);
173 canvas.drawRect(left, bottom - length, right, bottom, p);
174 }
175
176 if (drawLeft) {
177 matrix.setScale(1, fadeHeight * leftFadeStrength);
178 matrix.postRotate(-90);
179 matrix.postTranslate(left, top);
180 fade.setLocalMatrix(matrix);
181 canvas.drawRect(left, top, left + length, bottom, p);
182 }
183
184 if (drawRight) {
185 matrix.setScale(1, fadeHeight * rightFadeStrength);
186 matrix.postRotate(90);
187 matrix.postTranslate(right, top);
188 fade.setLocalMatrix(matrix);
189 canvas.drawRect(right - length, top, right, bottom, p);
190 }
191
192 canvas.restoreToCount(saveCount);
193
194 // Step 6, draw decorations (scrollbars)
195 onDrawScrollBars(canvas);
196 }
draw方法分成了6个步骤：
1 /* 2 * Draw traversal performs several drawing steps which must be executed 3 * in the appropriate order: 4 * 5 * 1. Draw the background 6 * 2. If necessary, save the canvas‘ layers to prepare for fading 7 * 3. Draw view‘s content 8 * 4. Draw children 9 * 5. If necessary, draw the fading edges and restore layers 10 * 6. Draw decorations (scrollbars for instance) 11 */
第三部, Draw view‘s content步骤调用了onDraw方法，子类中实现onDraw方法。
第四步，Draw children步骤使用的dispatchDraw方法，这个方法在ViewGroup中有实现。
View或ViewGroup的子类不用再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作。
1 /**
2 * {@inheritDoc}
3 */
4 @Override
5 protected void dispatchDraw(Canvas canvas) {
6 final int count = mChildrenCount;
7 final View[] children = mChildren;
8 int flags = mGroupFlags;
9
10 if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
11 final boolean cache = (mGroupFlags & FLAG_ANIMATION_CACHE) == FLAG_ANIMATION_CACHE;
12
13 final boolean buildCache = !isHardwareAccelerated();
14 for (int i = 0; i < count; i++) {
15 final View child = children[i];
16 if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
17 final LayoutParams params = child.getLayoutParams();
18 attachLayoutAnimationParameters(child, params, i, count);
19 bindLayoutAnimation(child);
20 if (cache) {
21 child.setDrawingCacheEnabled(true);
22 if (buildCache) {
23 child.buildDrawingCache(true);
24 }
25 }
26 }
27 }
28
29 final LayoutAnimationController controller = mLayoutAnimationController;
30 if (controller.willOverlap()) {
31 mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
32 }
33
34 controller.start();
35
36 mGroupFlags &= ~FLAG_RUN_ANIMATION;
37 mGroupFlags &= ~FLAG_ANIMATION_DONE;
38
39 if (cache) {
40 mGroupFlags |= FLAG_CHILDREN_DRAWN_WITH_CACHE;
41 }
42
43 if (mAnimationListener != null) {
44 mAnimationListener.onAnimationStart(controller.getAnimation());
45 }
46 }
47
48 int saveCount = 0;
49 final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
50 if (clipToPadding) {
51 saveCount = canvas.save();
52 canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
53 mScrollX + mRight - mLeft - mPaddingRight,
54 mScrollY + mBottom - mTop - mPaddingBottom);
55
56 }
57
58 // We will draw our child‘s animation, let‘s reset the flag
59 mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
60 mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
61
62 boolean more = false;
63 final long drawingTime = getDrawingTime();
64
65 if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
66 for (int i = 0; i < count; i++) {
67 final View child = children[i];
68 if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
69 more |= drawChild(canvas, child, drawingTime);
70 }
71 }
72 } else {
73 for (int i = 0; i < count; i++) {
74 final View child = children[getChildDrawingOrder(count, i)];
75 if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
76 more |= drawChild(canvas, child, drawingTime);
77 }
78 }
79 }
80
81 // Draw any disappearing views that have animations
82 if (mDisappearingChildren != null) {
83 final ArrayList<View> disappearingChildren = mDisappearingChildren;
84 final int disappearingCount = disappearingChildren.size() - 1;
85 // Go backwards -- we may delete as animations finish
86 for (int i = disappearingCount; i >= 0; i--) {
87 final View child = disappearingChildren.get(i);
88 more |= drawChild(canvas, child, drawingTime);
89 }
90 }
91
92 if (debugDraw()) {
93 onDebugDraw(canvas);
94 }
95
96 if (clipToPadding) {
97 canvas.restoreToCount(saveCount);
98 }
99
100 // mGroupFlags might have been updated by drawChild()
101 flags = mGroupFlags;
102
103 if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
104 invalidate(true);
105 }
106
107 if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
108 mLayoutAnimationController.isDone() && !more) {
109 // We want to erase the drawing cache and notify the listener after the
110 // next frame is drawn because one extra invalidate() is caused by
111 // drawChild() after the animation is over
112 mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
113 final Runnable end = new Runnable() {
114 public void run() {
115 notifyAnimationListener();
116 }
117 };
118 post(end);
119 }
120 }
上面基本介绍完了View的绘制流程。更多的细节需要在日常学习中总结。
转自：http://blog.csdn.net/xyz_lmn/article/details/20385049
时间： 2024-08-09 19:52:37