preface
Based on the 2021-01-05 revised edition, source code implementation ‘androidx. Recyclerview: recyclerview: 1.1.0’
Last article, said recycles reuse RecyclerView, this article, we talk about RecyclerView drawing process.
- RecyclerView ItemDecoration (a)
- RecyclerView Cache (2)
- 【Android advanced 】RecyclerView drawing process (three)
- RecyclerView group list add top effect (4)
onMeasure
Let’s take a look at RecyclerView#onMeasure()
protected void onMeasure(int widthSpec, int heightSpec) {
if (this.mLayout == null) {
this.defaultOnMeasure(widthSpec, heightSpec);
} else {
if (!this.mLayout.isAutoMeasureEnabled()) {
if (this.mHasFixedSize) {
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
return;
}
if (this.mAdapterUpdateDuringMeasure) {
this.startInterceptRequestLayout();
this.onEnterLayoutOrScroll();
this.processAdapterUpdatesAndSetAnimationFlags();
this.onExitLayoutOrScroll();
if (this.mState.mRunPredictiveAnimations) {
this.mState.mInPreLayout = true;
} else {
this.mAdapterHelper.consumeUpdatesInOnePass();
this.mState.mInPreLayout = false;
}
this.mAdapterUpdateDuringMeasure = false;
this.stopInterceptRequestLayout(false);
} else if (this.mState.mRunPredictiveAnimations) {
this.setMeasuredDimension(this.getMeasuredWidth(), this.getMeasuredHeight());
return;
}
if (this.mAdapter ! =null) {
this.mState.mItemCount = this.mAdapter.getItemCount();
} else {
this.mState.mItemCount = 0;
}
this.startInterceptRequestLayout();
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
this.stopInterceptRequestLayout(false);
this.mState.mInPreLayout = false;
} else {
int widthMode = MeasureSpec.getMode(widthSpec);
int heightMode = MeasureSpec.getMode(heightSpec);
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
boolean measureSpecModeIsExactly = widthMode == 1073741824 && heightMode == 1073741824;
if (measureSpecModeIsExactly || this.mAdapter == null) {
return;
}
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
}
this.mLayout.setMeasureSpecs(widthSpec, heightSpec);
this.mState.mIsMeasuring = true;
this.dispatchLayoutStep2();
this.mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
if (this.mLayout.shouldMeasureTwice()) {
this.mLayout.setMeasureSpecs(MeasureSpec.makeMeasureSpec(this.getMeasuredWidth(), 1073741824), MeasureSpec.makeMeasureSpec(this.getMeasuredHeight(), 1073741824));
this.mState.mIsMeasuring = true;
this.dispatchLayoutStep2();
this.mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec); }}}}Copy the codeIf we look at it from the top, first of all, mLayout is a LayoutManager, and if it’s null then defaultOnMeasure is executed
void defaultOnMeasure(int widthSpec, int heightSpec) {
int width = RecyclerView.LayoutManager.chooseSize(widthSpec, this.getPaddingLeft() + this.getPaddingRight(), ViewCompat.getMinimumWidth(this));
int height = RecyclerView.LayoutManager.chooseSize(heightSpec, this.getPaddingTop() + this.getPaddingBottom(), ViewCompat.getMinimumHeight(this));
this.setMeasuredDimension(width, height);
}
Copy the codeAs you can see, without measuring the height of the item, we call setMeasuredDimension to set the width and height
If isAutoMeasureEnabled is true or false, two sets of logic will be used. By checking the source code, it can be found that isAutoMeasureEnabled is mAutoMeasure in LayoutManager. The default value is false. But true in the LinearLayoutManager
IsAutoMeasureEnabled to true
LinearLayoutManager related code
public boolean isAutoMeasureEnabled(a) {
return true;
}
Copy the codeThe main logic of onMeasure is also when isAutoMeasureEnabled is true. Let’s move on
int widthMode = MeasureSpec.getMode(widthSpec);
int heightMode = MeasureSpec.getMode(heightSpec);
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
boolean measureSpecModeIsExactly = widthMode == 1073741824 && heightMode == 1073741824;
if (measureSpecModeIsExactly || this.mAdapter == null) {
return;
}
Copy the codeIf the width and height are absolute values, skip the onMeasure method.
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
}
Copy the codeMLayoutStep default value is state.step_start is 1, about dispatchLayoutStep1 method, in fact, there is no need to analyze too much, because the analysis of the source code is mainly for the understanding of the drawing idea, if too much entanglements in the meaning of each line of code, then will be in great trouble. After that, this.mstate.mlayoutStep = 2; That is the STEP_LAYOUT state.
Next, dispatchLayoutStep2 is where the LayoutManager drawing is actually performed.
private void dispatchLayoutStep2(a) {
this.startInterceptRequestLayout();
this.onEnterLayoutOrScroll();
this.mState.assertLayoutStep(6);
this.mAdapterHelper.consumeUpdatesInOnePass();
this.mState.mItemCount = this.mAdapter.getItemCount();
this.mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;
this.mState.mInPreLayout = false;
this.mLayout.onLayoutChildren(this.mRecycler, this.mState);
this.mState.mStructureChanged = false;
this.mPendingSavedState = null;
this.mState.mRunSimpleAnimations = this.mState.mRunSimpleAnimations && this.mItemAnimator ! =null;
this.mState.mLayoutStep = 4;
this.onExitLayoutOrScroll();
this.stopInterceptRequestLayout(false);
}
Copy the codeAs you can see, RecyclerView gives the drawing of items to LayoutManager (mLayout.onLayoutChildren(this.mrecycler, this.mstate); More on LayoutManager in the next article.
After this, this.mstate.mLayOutStep = 4; That is the STEP_ANIMATIONS state.
IsAutoMeasureEnabled to false
if (mHasFixedSize) {
// No need to remeasure
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
return;
}
// custom onMeasure
if (mAdapterUpdateDuringMeasure) {
startInterceptRequestLayout();
onEnterLayoutOrScroll();
processAdapterUpdatesAndSetAnimationFlags();
onExitLayoutOrScroll();
if (mState.mRunPredictiveAnimations) {
mState.mInPreLayout = true;
} else {
// consume remaining updates to provide a consistent state with the layout pass.
mAdapterHelper.consumeUpdatesInOnePass();
mState.mInPreLayout = false;
}
mAdapterUpdateDuringMeasure = false;
stopInterceptRequestLayout(false);
} else if (mState.mRunPredictiveAnimations) {
// If mAdapterUpdateDuringMeasure is false and mRunPredictiveAnimations is true:
// this means there is already an onMeasure() call performed to handle the pending
// adapter change, two onMeasure() calls can happen if RV is a child of LinearLayout
// with layout_width=MATCH_PARENT. RV cannot call LM.onMeasure() second time
// because getViewForPosition() will crash when LM uses a child to measure.
setMeasuredDimension(getMeasuredWidth(), getMeasuredHeight());
return;
}
if(mAdapter ! =null) {
mState.mItemCount = mAdapter.getItemCount();
} else {
mState.mItemCount = 0;
}
startInterceptRequestLayout();
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
stopInterceptRequestLayout(false);
mState.mInPreLayout = false; // clear
Copy the codeFirst, there is no remeasurement when mHasFixedSize is true, which is set by the setHasFixedSize method
/**
* RecyclerView can perform several optimizations if it can know in advance that RecyclerView's
* size is not affected by the adapter contents. RecyclerView can still change its size based
* on other factors (e.g. its parent's size) but this size calculation cannot depend on the
* size of its children or contents of its adapter (except the number of items in the adapter).
* <p>
* If your use of RecyclerView falls into this category, set this to {@code true}. It will allow
* RecyclerView to avoid invalidating the whole layout when its adapter contents change.
*
* @param hasFixedSize true if adapter changes cannot affect the size of the RecyclerView.
*/
public void setHasFixedSize(boolean hasFixedSize) {
mHasFixedSize = hasFixedSize;
}
Copy the codeWhen the change of Item in Adapter does not affect the width and height of RecyclerView, you can set it to True to avoid recalculating the size of RecyclerView.
This is also one of the memory optimization methods of RecyclerView
When mAdapterUpdateDuringMeasure is true, you need to do to measure, and mAdapterUpdateDuringMeasure is set to true in triggerUpdateProcessor method
void triggerUpdateProcessor(a) {
if (POST_UPDATES_ON_ANIMATION && mHasFixedSize && mIsAttached) {
ViewCompat.postOnAnimation(RecyclerView.this, mUpdateChildViewsRunnable);
} else {
mAdapterUpdateDuringMeasure = true; requestLayout(); }}Copy the codeThe triggerUpdateProcessor method is called by the following method,
onItemRangeChangedonItemRangeInsertedonItemRangeRemovedonItemRangeMoved
RequestLayout () is not required according to the value of mHasFixedSize when the Adapter’s add, delete, change, insert methods are called.
As mentioned earlier, the main logic of onMeasure is true when isAutoMeasureEnabled, so why is the default value false in LayoutManager?
If isAutoMeasureEnabled is false, can item be drawn normally? Let’s try it out
We override the isAutoMeasureEnabled method to return false
class MyLinLayoutManager extends LinearLayoutManager {
public MyLinLayoutManager(Context context) {
super(context);
}
@Override
public boolean isAutoMeasureEnabled(a) {
return false; }}Copy the codeThen set it to RecyclerView, and when it runs, it will find that the item can be displayed normally. Why? So this is the onLayout method
onLayout
protected void onLayout(boolean changed, int l, int t, int r, int b) {
TraceCompat.beginSection("RV OnLayout");
this.dispatchLayout();
TraceCompat.endSection();
this.mFirstLayoutComplete = true;
}
Copy the codeHere it’s a little easier. Let’s look at the dispatchLayout method
void dispatchLayout(a) {
if (this.mAdapter == null) {
Log.e("RecyclerView"."No adapter attached; skipping layout");
} else if (this.mLayout == null) {
Log.e("RecyclerView"."No layout manager attached; skipping layout");
} else {
this.mState.mIsMeasuring = false;
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
this.mLayout.setExactMeasureSpecsFrom(this);
this.dispatchLayoutStep2();
} else if (!this.mAdapterHelper.hasUpdates() && this.mLayout.getWidth() == this.getWidth() && this.mLayout.getHeight() == this.getHeight()) {
this.mLayout.setExactMeasureSpecsFrom(this);
} else {
this.mLayout.setExactMeasureSpecsFrom(this);
this.dispatchLayoutStep2();
}
this.dispatchLayoutStep3(); }}Copy the codeIt can be seen that the main logic of onMeasure is re-executed here, which also explains why onMeasure directly skipped execution when we set a fixed width and height for RecyclerView before, but the sub-view can still be displayed.