Usage and principle of precomputedtextcompat

One 、 The official introduction

        Text presentation is very complicated , The features covered are ： Multiple fonts 、 Row spacing 、 Space between words 、 The text direction 、 Line breaks 、 Character connection, etc . To measure and lay out a given text ,TextView A lot of work has to be done , For example, read the font file 、 Find glyphs 、 Determine the shape 、 Measure the bounding box and cache the text in the internal text cache . what's more , All this work is UI Thread , This may lead to app Frame count decrease .

        We find that the time spent on text measurement takes up... Of the text settings 90%. To solve this problem , stay Android P in , And as Jetpack Part of , We have launched a new API: PrecomputedText. The API Earlier in API 14 You can pass PrecomputedTextCompat visit .

       PrecomputedTextCompat Can make app You can perform the most time-consuming part of the text layout in advance, even in the background thread , To cache layout results , And return valuable measurement data . then , Can be in TextView Set the result in . such , Only about 10% Your work is left to TextView perform .

Two 、 Usage method

• compileSdkVersion No less than 28,appcompat-v7 28.0.0 or androidx appcompat 1.0.0 And above
• Use AppCompatTextView To replace TextView
• Use setTextFuture  Replace  setText  Method

       Java Example ：

Future<PrecomputedTextCompat> future = PrecomputedTextCompat.getTextFuture(“text”,             
        textView.getTextMetricsParamsCompat(), null);

textView.setTextFuture(future);

       Kotlin Example ：

fun AppCompatTextView.setTextFuture(charSequence: CharSequence){
    this.setTextFuture(PrecomputedTextCompat.getTextFuture(
            charSequence,
            TextViewCompat.getTextMetricsParams(this),
            null
    ))
}

textView.setTextFuture(“text”)

3、 ... and 、 Realization principle

       PrecomputedTextCompat Put time-consuming measurements into FutureTask Asynchronous execution ：

@UiThread
public static Future<PrecomputedTextCompat> getTextFuture(
        @NonNull final CharSequence charSequence, @NonNull PrecomputedTextCompat.Params params,
        @Nullable Executor executor) {
    PrecomputedTextFutureTask task = new PrecomputedTextFutureTask(params, charSequence);
    if (executor == null) {
        synchronized (sLock) {
            if (sExecutor == null) {
                sExecutor = Executors.newFixedThreadPool(1);
            }
            executor = sExecutor;
        }
    }
    executor.execute(task);
    return task;
}

       AppCompatTextView stay onMeasure Method call consumeTextFutureAndSetBlocking() when ,future.get() Blocking the thread to get the measured result . Final setText To TextView Displayed on the .

public void setTextFuture(@Nullable Future<PrecomputedTextCompat> future) {
    mPrecomputedTextFuture = future;
    if (future != null) {
        requestLayout();
    }
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    consumeTextFutureAndSetBlocking();
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

private void consumeTextFutureAndSetBlocking() {
    if (mPrecomputedTextFuture != null) {
        try {
            Future<PrecomputedTextCompat> future = mPrecomputedTextFuture;
            mPrecomputedTextFuture = null;
            TextViewCompat.setPrecomputedText(this, future.get());
        } catch (InterruptedException | ExecutionException e) {
            // ignore
        }
    }
}

1. Android 9.0 above , Use PrecomputedText Realization .
2. Android 5.0~9.0, Use StaticLayout Realization .
3. Android 5.0 following , Don't deal with it .

/**
 * A helper class for computing text layout in background
 */
private static class PrecomputedTextFutureTask extends FutureTask<PrecomputedTextCompat> {
    private static class PrecomputedTextCallback implements Callable<PrecomputedTextCompat> {
        private PrecomputedTextCompat.Params mParams;
        private CharSequence mText;

        PrecomputedTextCallback(@NonNull final PrecomputedTextCompat.Params params,
                @NonNull final CharSequence cs) {
            mParams = params;
            mText = cs;
        }

        @Override
        public PrecomputedTextCompat call() throws Exception {
            return PrecomputedTextCompat.create(mText, mParams);
        }
    }

    PrecomputedTextFutureTask(@NonNull final PrecomputedTextCompat.Params params,
            @NonNull final CharSequence text) {
        super(new PrecomputedTextCallback(params, text));
    }
}

/**
 * Create a new {@link PrecomputedText} which will pre-compute text measurement and glyph
 * positioning information.
 * <p>
 * This can be expensive, so computing this on a background thread before your text will be
 * presented can save work on the UI thread.
 * </p>
 *
 * Note that any {@link android.text.NoCopySpan} attached to the text won't be passed to the
 * created PrecomputedText.
 *
 * @param text the text to be measured
 * @param params parameters that define how text will be precomputed
 * @return A {@link PrecomputedText}
 */
public static PrecomputedTextCompat create(@NonNull CharSequence text, @NonNull Params params) {
    Preconditions.checkNotNull(text);
    Preconditions.checkNotNull(params);

    try {
        TraceCompat.beginSection("PrecomputedText");

        //  Omit the code here 

        // No framework support for PrecomputedText
        // Compute text layout and throw away StaticLayout for the purpose of warming up the
        // internal text layout cache.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            StaticLayout.Builder.obtain(text, 0, text.length(), params.getTextPaint(),
                    Integer.MAX_VALUE)
                    .setBreakStrategy(params.getBreakStrategy())
                    .setHyphenationFrequency(params.getHyphenationFrequency())
                    .setTextDirection(params.getTextDirection())
                    .build();
        } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            new StaticLayout(text, params.getTextPaint(), Integer.MAX_VALUE,
                    Layout.Alignment.ALIGN_NORMAL, 1.0f, 0.0f, false);
        } else {
            // There is no way of precomputing text layout on API 20 or before
            // Do nothing
        }

        return new PrecomputedTextCompat(text, params, result);
    } finally {
        TraceCompat.endSection();
    }
}

// null on API 27 or before. Non-null on API 29 or later
private final @Nullable PrecomputedText mWrapped;