View繪製——畫在哪？
摘要：
這是Android檢視繪製系列文章的第一篇，系列文章目錄如下：
View繪製——畫多大？
View繪製——畫在哪？
View繪製就好比畫畫，先拋開Android概念，如果要畫...
這是Android檢視繪製系列文章的第一篇，系列文章目錄如下：
View繪製就好比畫畫，先拋開Android概念，如果要畫一張圖，首先會想到哪幾個基本問題：
- 畫多大?
- 畫在哪？
- 畫什麼？
Android繪製系統也是按照這個思路對View進行繪製，上面這些問題的答案分別藏在：
- 測量（measure）
- 定位（layout）
- 繪製（draw）
這一篇將從原始碼的角度分析“定位（layout）”。
如何描述位置
位置都是相對的，比如“我在你的右邊”、“你在廣場的西邊”。為了表明位置，總是需要一個參照物。View
的定位也需要一個參照物，這個參照物是View
的父控制元件。可以在View
的成員變數中找到如下四個描述位置的引數：
public class View implements Drawable.Callback, KeyEvent.Callback,AccessibilityEventSource {
/**
* The distance in pixels from the left edge of this view’s parent
* to the left edge of this view.
* view左邊相對於父親左邊的距離
*/
protected int mLeft;
/**
* The distance in pixels from the left edge of this view‘s parent
* to the right edge of this view.
* view右邊相對於父親左邊的距離
*/
protected int mRight;
/**
* The distance in pixels from the top edge of this view’s parent
* to the top edge of this view.
* view上邊相對於父親上邊的距離
*/
protected int mTop;
/**
* The distance in pixels from the top edge of this view‘s parent
* to the bottom edge of this view.
* view底邊相對於父親上邊的距離
*/
protected int mBottom;
...
}
View
通過上下左右四條線圍城的矩形來確定相對於父控制元件的位置。
確定相對位置
那這四個點在哪裡被賦值？全域性搜尋後，找到下面這個函式：
public class View implements Drawable.Callback, KeyEvent.Callback,AccessibilityEventSource {
/**
* Assign a size and position to this view.
* 賦予當前view尺寸和位置
*
* This is called from layout.
* 這個函式在layout中被呼叫
*
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
* @return true if the new size and position are different than the previous ones
*/
protected boolean setFrame(int left, int top, int right, int bottom) {
...
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
...
}
}
沿著呼叫鏈繼續往上查詢：
public class View implements Drawable.Callback, KeyEvent.Callback,AccessibilityEventSource {
/**
* Assign a size and position to a view and all of its
* descendants
* 將尺寸和位置賦予當前view和所有它的孩子
*
* <p>This is the second phase of the layout mechanism.
* (The first is measuring). In this phase, each parent calls
* layout on all of its children to position them.
* This is typically done using the child measurements
* that were stored in the measure pass().</p>
*
* <p>Derived classes should not override this method.
* Derived classes with children should override
* onLayout. In that method, they should
* call layout on each of their children.</p>
* 子類不應該過載這個方法，而應該過載onLayout()，並且在其中區域性所有孩子
*
* @param l Left position, relative to parent
* @param t Top position, relative to parent
* @param r Right position, relative to parent
* @param b Bottom position, relative to parent
*/
public void layout(int l, int t, int r, int b) {
...
//為View上下左右四條線賦值
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
...
//如果佈局改變了則重新佈局
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
...
}
}
...
/**
* Called from layout when this view should
* assign a size and position to each of its children.
* 當需要賦予所有孩子尺寸和位置的時候，這個函式在layout中被呼叫
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
* 帶有孩子的子類應該過載這個方法並呼叫每個孩子的layout()
* @param changed This is a new size or position for this view
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
}
結合呼叫鏈和程式碼註釋，可以得出結論：
孩子的定位是由父控制元件發起的，父控制元件會在ViewGroup.onLayout()
中遍歷所有的孩子並呼叫它們的View.layout()
以設定孩子相對於自己的位置。
不同的ViewGroup
有不同的方式來佈局孩子，以FrameLayout
為例：
public class FrameLayout extends ViewGroup {
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
//佈局所有孩子
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
//遍歷所有孩子
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//排除不可見孩子
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//獲得孩子在measure過程中確定的寬高
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
//確定孩子左邊相對於父控制元件位置
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
//確定孩子上邊相對於父控制元件位置
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
//呼叫孩子的layout()，確定孩子相對父控制元件位置
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
}
總結
layout()