View繪制的三部曲， 測量，布局，繪畫
今天我們分析測量過程
view的測量是從ViewRootImpl發(fā)起的，View需要重繪，都是發(fā)送請(qǐng)求給ViewRootImpl，然后他組織重繪
在重繪的過程中，有一步就是測量，通過代碼來分析測量過程
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp, final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) { int childWidthMeasureSpec; int childHeightMeasureSpec; boolean windowSizeMayChange = false; if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG, "Measuring " + host + " in display " + desiredWindowWidth + "x" + desiredWindowHeight + "...");
boolean goodMeasure = false; if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) { // On large screens, we don't want to allow dialogs to just // stretch to fill the entire width of the screen to display // one line of text. First try doing the layout at a smaller // size to see if it will fit.
final DisplayMetrics packageMetrics = res.getDisplayMetrics();
res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true); int baseSize = 0; if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
baseSize = (int)mTmpValue.getDimension(packageMetrics);
} if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize); if (baseSize != 0 && desiredWindowWidth > baseSize) { //獲取測量的規(guī)格，是一個(gè)32位的二進(jìn)制數(shù)值，前兩位標(biāo)識(shí)mode，后30位表示view的長/寬
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); //向DecorView發(fā)起重繪 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")"); if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
goodMeasure = true;
} else { // Didn't fit in that size... try expanding a bit.
baseSize = (baseSize+desiredWindowWidth)/2; if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
+ baseSize);
childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
+ host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")"); if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) { if (DEBUG_DIALOG) Log.v(TAG, "Good!");
goodMeasure = true;
}
}
}
} if (!goodMeasure) {
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
windowSizeMayChange = true;
}
} if (DBG) {
System.out.println("======================================");
System.out.println("performTraversals -- after measure");
host.debug();
} return windowSizeMayChange;
}
這個(gè)函數(shù)通過getRootMeasureSpec方法，獲取測量規(guī)格，然后調(diào)用performMeasure方法開始分發(fā)給整個(gè)的view樹。
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;
}
通過MeasureSpec的makeMeasureSpec方法來生成測量規(guī)格，先判斷出布局是 match_parent 或者是 wrap_content，或者是確定的數(shù)值
然后把windowSize傳遞下去。
public static int makeMeasureSpec(int size, int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode;
} else { return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
API大于17的都會(huì)走else判斷，這塊我分析一下計(jì)算結(jié)果。有助于理解后邊的運(yùn)算
makeMeasureSpec的運(yùn)算結(jié)果是一個(gè)32位的二進(jìn)制數(shù)值，前2位表示測量的規(guī)格 EXACTLY/AT_most 后30位表示 windowSize，舉個(gè)運(yùn)算例子
size=320,mode=EXACTLY,換算成二進(jìn)制就是下邊的兩串值
size=0000 0000 0000 0000 0000 0001 0100 0000
mode=0100 0000 0000 0000 0000 0000 0000 0000
mask=1100 0000 0000 0000 0000 0000 0000 0000
最后用與運(yùn)算整合size和mode
0000 0000 0000 0000 0000 0001 0100 0000 |
0100 0000 0000 0000 0000 0000 0000 0000 =
0100 0000 0000 0000 0000 0001 0100 0000
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
這個(gè)mView就是我們?cè)趙indow類中組合出來的DecorView，這個(gè)方法調(diào)用了view的measure方法，measure會(huì)調(diào)用OnMeasure方法，然后就實(shí)現(xiàn)了整個(gè)view樹的測量工作
public final void measure(int widthMeasureSpec, int heightMeasureSpec) { boolean optical = isLayoutModeOptical(this); if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets(); int oWidth = insets.left + insets.right; int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
} // 這塊又把寬的測量規(guī)格和高的測量規(guī)格拼接在了一起，作為緩存中的key
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL; if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2); if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) { // first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded(); //判斷是否強(qiáng)制測量，如果強(qiáng)制就重新調(diào)用onMeasure，整個(gè)view樹重新測量，否則就從緩存中得到上次的測量規(guī)格， //因?yàn)镈ecorView是FrameLayout的子類,所以onMeasure就是調(diào)用FrameLayout的onMeasure方法
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
mMeasureCache.indexOfKey(key); if (cacheIndex < 0 || sIgnoreMeasureCache) { // measure ourselves, this should set the measured dimension flag back onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else { long value = mMeasureCache.valueAt(cacheIndex); // Casting a long to int drops the high 32 bits, no mask needed //調(diào)用完這個(gè)方法之后，getMeasuredWidth和getMeasuredHeight就可以取到值了
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} // flag not set, setMeasuredDimension() was not invoked, we raise // an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) { throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec; //在按照固定的格式，把本次的測量規(guī)格put到集合中
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
這個(gè)方法做的功能大概就是這樣，先判斷是否有緩存，沒緩存就測量一下整個(gè)樹，然后按照固定的格式在存入緩存中
從這里開始，我們的整個(gè)測量過程就開始跑起來了
http://www.cnblogs.com/kezhuang/p/7119859.html
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