void SurfaceComposerClient::Transaction::cacheBuffers() {
    if (!mContainsBuffer) {
        return;
    }

    size_t count = 0;
    for (auto& [handle, cs] : mComposerStates) {
        layer_state_t* s = &(mComposerStates[handle].state);
        if (!(s->what & layer_state_t::eBufferChanged)) {
            continue;
        } else if (s->what & layer_state_t::eCachedBufferChanged) {
            // If eBufferChanged and eCachedBufferChanged are both trued then that means
            // we already cached the buffer in a previous call to cacheBuffers, perhaps
            // from writeToParcel on a Transaction that was merged in to this one.
            continue;
        }

        // Don't try to cache a null buffer. Sending null buffers is cheap so we shouldn't waste
        // time trying to cache them.
        if (!s->buffer) {
            continue;
        }

• mContainsBuffer 表示是否调用 setBuffer()。
• 遍历 mComposerStates，当满足如下 3 个条件中一个则跳过该 layer_state_t：
  • Buffer 没有变化。
  • 已经缓存过。
  • buffer 是 null。因为 null 没有传递成本，所以没必要缓存，缓存反而有成本。

        uint64_t cacheId = 0;
        status_t ret = BufferCache::getInstance().getCacheId(s->buffer, &cacheId);
        if (ret == NO_ERROR) {
            // Cache-hit. Strip the buffer and send only the id.
            s->what &= ~static_cast<uint64_t>(layer_state_t::eBufferChanged);
            s->buffer = nullptr;
        } else {
            // Cache-miss. Include the buffer and send the new cacheId.
            cacheId = BufferCache::getInstance().cache(s->buffer);
        }
        s->what |= layer_state_t::eCachedBufferChanged;
        s->cachedBuffer.token = BufferCache::getInstance().getToken();
        s->cachedBuffer.id = cacheId;

        // If we have more buffers than the size of the cache, we should stop caching so we don't
        // evict other buffers in this transaction
        count++;
        if (count >= BUFFER_CACHE_MAX_SIZE) {
            break;
        }
    }

• 调用 BufferCache::getCacheId() 获取 cacheId，如果返回 NO_ERROR 则说明已缓存，所以清除 eBufferChanged 和 buffer。
• 否则调用 BufferCache::cache() 缓存该 buffer。
• 给 cachedBuffer 设置 token 和 id。
• 缓存个数递增，如果超过了 BUFFER_CACHE_MAX_SIZE 则退出循环。