TextView用作计时时的一处优化点

问题

开发中，经常会用到TextView作为计时用，通过handle或者其他线程，每隔1s刷新一下时间。看一下简单的代码实现：

根布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".tmp.TempActivity">

    <com.sollian.sourcestudy.tmp.MyRecyclerView
        android:id="@+id/list"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>

MyRecyclerView

public class MyRecyclerView extends RecyclerView {

    public MyRecyclerView(@NonNull Context context) {
        super(context);
    }

    public MyRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyRecyclerView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthSpec, int heightSpec) {
        super.onMeasure(widthSpec, heightSpec);
        Log.d("MyRecyclerView", "onMeasure");
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        Log.d("MyRecyclerView", "onLayout");
    }
}

很简单，在每次测量、布局时，打个log。

item

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:padding="10dp">

    <TextView
        android:id="@+id/time"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="0" />
</LinearLayout>

在item中通过一个TextView来计数。在代码中，通过handle每隔1s使TextView的计数加1。此时看一下log输出：

2019-01-28 15:04:55.481 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:55.535 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:55.536 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onLayout
2019-01-28 15:04:55.657 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:55.657 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onLayout
2019-01-28 15:04:56.656 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:56.658 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onLayout
2019-01-28 15:04:57.656 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:57.658 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onLayout
2019-01-28 15:04:58.655 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:04:58.656 8025-8025/com.sollian.sourcestudy D/MyRecyclerView: onLayout
...

可以看到，TextView没刷新一次，都会引起RecyclerView重新测量、布局。

现在修改一下，将TextView的宽度固定为100dp。再看日志：

2019-01-28 15:05:48.257 8134-8134/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:05:48.294 8134-8134/com.sollian.sourcestudy D/MyRecyclerView: onMeasure
2019-01-28 15:05:48.295 8134-8134/com.sollian.sourcestudy D/MyRecyclerView: onLayout

可以看到，现在不会再引起RecyclerView的频繁测量、布局了。

原因

为什么会造成这种差异呢？我们来追一下setText方法的源码。
api28 TextView源码：

    private void setText(CharSequence text, BufferType type,
                         boolean notifyBefore, int oldlen) {
        ...
        if (mLayout != null) {
            checkForRelayout();
        }
        ...
    }

    private void checkForRelayout() {
        // If we have a fixed width, we can just swap in a new text layout
        // if the text height stays the same or if the view height is fixed.

        if ((mLayoutParams.width != LayoutParams.WRAP_CONTENT
                || (mMaxWidthMode == mMinWidthMode && mMaxWidth == mMinWidth))
                && (mHint == null || mHintLayout != null)
                && (mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight() > 0)) {
            // Static width, so try making a new text layout.

            int oldht = mLayout.getHeight();
            int want = mLayout.getWidth();
            int hintWant = mHintLayout == null ? 0 : mHintLayout.getWidth();

            /*
             * No need to bring the text into view, since the size is not
             * changing (unless we do the requestLayout(), in which case it
             * will happen at measure).
             */
            makeNewLayout(want, hintWant, UNKNOWN_BORING, UNKNOWN_BORING,
                          mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight(),
                          false);

            if (mEllipsize != TextUtils.TruncateAt.MARQUEE) {
                // In a fixed-height view, so use our new text layout.
                if (mLayoutParams.height != LayoutParams.WRAP_CONTENT
                        && mLayoutParams.height != LayoutParams.MATCH_PARENT) {
                    autoSizeText();
                    invalidate();
                    return;
                }

                // Dynamic height, but height has stayed the same,
                // so use our new text layout.
                if (mLayout.getHeight() == oldht
                        && (mHintLayout == null || mHintLayout.getHeight() == oldht)) {
                    autoSizeText();
                    invalidate();
                    return;
                }
            }

            // We lose: the height has changed and we have a dynamic height.
            // Request a new view layout using our new text layout.
            requestLayout();
            invalidate();
        } else {
            // Dynamic width, so we have no choice but to request a new
            // view layout with a new text layout.
            nullLayouts();
            requestLayout();
            invalidate();
        }
    }

注意看checkForRelayout方法，在宽度固定，并且mEllipsize != TextUtils.TruncateAt.MARQUEE时，只调用invalidate()进行重绘；否则会先调用requestLayout()进行重新布局，然后再调用invalidate()重绘。

结论

在需要频繁刷新TextView时，尽量不要使用wrapt_content来设置宽度，避免频繁引起测量和布局。